Viruses are some of the fastest-spreading organisms
on this planet.
And each time a virus spreads from one person to the next,
the virus itself replicate in their billions.
Just one infection can lead to a pandemic.
APPLAUSE
Would you like to see that again in slow motion?
Can we run it again on the VT, please?
There you go, that's a proper chain reaction.
So welcome to the second Christmas lecture of 2021
from the Royal Institution.
My name is Professor Jonathan Van-Tam.
I'm the Deputy Chief Medical Officer of England.
But I'm not here tonight
because I'm the Deputy Chief Medical Officer.
I'm here because I've spent most of my life
being a respiratory virus epidemiologist.
Now, previously we delved into the micro world of viruses -
how small they are, how they infect us, how they damage our bodies,
and how we then fight back with our immune systems.
But to understand how viruses spread,
we're going to need to focus on person-to-person transmission,
and on human behaviour in the wider population.
So...I'm going to take you over to a map now.
And before I play this map, I want you to look for
red circles that are getting bigger, that show that an epidemic
is growing in a certain place.
And then blue circles that are getting smaller,
that show that the epidemic is dying down again.
And on the left-hand side there's a date there -
26th of January 2020.
And that's going to run really fast, forwards, as we play the map.
Let's play it, please.
So you can see our Covid pandemic has begun in China,
and very quickly, it started to spread around the world,
growing bigger.
We're now in May, already.
And you can see now South America's started to light up,
South Africa's lighting up, and some of those circles
are getting very big.
But at the same time, some of them are cooling down between waves.
And you can see that we're now in October,
getting towards the northern hemisphere winter.
You can see quite big circles appearing.
Now, we know that Covid feeds off close contact.
And that's why it can go worldwide in weeks.
But scientists have a lot of experience of studying
pandemics like flu pandemics and similar diseases.
And scientists have been really racing for two years
to understand the cause of spread of this pandemic.
Why has it been so quick? And why has the scale been so massive?
Now, to reveal some of those invisible forces
which drive infection,
we're going to welcome our first expert this evening.
And I'm really pleased to say, she's an old friend of mine
and we've done a lot of work together over the years
studying how flu viruses pass from one person to another.
Would you please welcome Professor Cath Noakes?
So good evening, everybody.
I'm a mechanical engineer and I'm an expert
in airborne infection and ventilation.
And I'm quite interested in knowing, in how does the virus
get from one person to another person?
And how does this happen in the different places where we go?
So in our schools, our workplaces, our homes, the places we socialise.
But before that, some of you might be thinking,
"What's an engineer doing on stage looking at viruses?
"Isn't that a medical thing?"
So I study a field called fluid dynamics -
how liquids and gases behave.
And this is the same thing - the techniques we use will tell us
how aeroplanes fly, can help us design wind turbines,
how car engines work, all kinds of things.
But we use the same technique to understand viruses
and how they travel from person to person
and how the air in buildings moves.
So to start with, the first thing we're going to do
is ask a question.
How does a virus that's inside our bodies -
it's in our respiratory system - get out and get to other people?
Can anybody suggest any ideas how it's going to get out?
Coughing. Coughing, brilliant.
Any other thoughts? Sneezing. Sneezing.
Air droplets. Air droplets and things -
oh, you know quite a lot already.
OK, we've got coughing and sneezing here - brilliant ways -
and we're going to start with those.
So to demonstrate coughing and sneezing,
we've got our lovely sneeze machine here.
OK. And to work this, I'm going to need two volunteers
from the audience. Lots of hands up here.
OK, so there, the black jacket and the grey sleeves?
OK, and we'll have - there, with the pinkish shirt on.
Yes, if you want to come down.
Come and stand over there. OK.
So can you tell me your name? My name is Amir.
Amir - nice to have you here, Amir. And your name is? Milly.
Milly. So I've got Amir and Millie.
Right, OK. So, Amir, you're going to have a very important job.
So you are going to work our sneeze machine, OK? So...
And then, Milly, I'm afraid you're going to be
our human target for this.
Don't worry - it's not a real virus, OK?
Can you go off with our team over there?
They're going to get you kitted up so you don't get covered in it.
Right, Amir, we need to get you doing
a really important job over here.
So, you see you've got a bicycle pump here?
And then what you're going to do is you're going to pump it up.
We've got a dial here and we're going to pump it up
until it reaches that red line.
You have to pump it right up, yeah? That's perfect.
When it gets to the red line, just pump a little bit and hold it there.
OK, so, JVT, why do we actually cough and sneeze?
Well, coughing and sneezing is actually completely natural
to a human.
Because, as we walk around in our daily lives and we breathe,
our lungs take in dust, dirt, bacteria, and they combine
with the mucus that's in our lungs and we've got to get rid of it,
and so we cough and we sneeze.
And the classic example is putting pepper on your food and getting
it up your nose and your body says, "I don't like that, it's irritating
"my nose, I've got to get rid of it."
And so out comes the sneeze.
And if you're infected with a virus in your airways,
the virus also upsets your airways and makes them sensitive
and, again, can make you want to cough or sneeze.
And if you're infected with a virus, then what comes out is,
of course, also likely to contain the virus, as well.
OK, thank you for that.
So, OK, do you want to come back on here?
OK, you'll probably want to pull your visor down.
Perfect, right, now, where do we think Milly should stand?
Over here, by JVT? No, not near me.
LAUGHTER
OK, closer? Closer. Closer, come on, then.
That's going to be too easy, isn't it? Far too easy.
OK, I think we can take you a little bit further away, Milly.
So maybe about here, so you're now about two metres away.
We tend to think two metres is a safe distance.
OK, just stand still there, don't panic.
Right, Amir, are we nice and pressurised?
Perfect, so your next job - you see this trigger here?
You're going to pull that trigger.
So, are we ready, audience?
ALL: Three, two, one... Sneeze.
Oh!
And you've even broken our sneeze machine - fantastic!
Are you all right there, Milly?
Yeah. Thank you for that, that was brilliant.
Let's have a look at what happened.
I think we saw quite a lot happen there, didn't we?
So if we turn the lights down and we look on the floor,
we should be able to see...
Can you see this on the floor?
Can you see all these tiny dots?
Oh, look - some big splats over here.
That was one big sneeze you did, Amir.
And if we look at Milly, we can see we've got some
on her overalls here.
We've got some here, and we've got it on our face guard.
So Milly got quite a face full of that then.
So thank you very much to both of you, OK.
Can we give a big round of applause to Milly and Amir?
OK.
So what we saw there were large droplets.
And those droplets could carry many virus particles.
They could theoretically carry probably over 100 virus particles.
They're actually a little bit smaller in reality
to what we saw there.
But it's likely to be rare that these large droplets can infect you,
and there's two reasons for this.
One is that we produce quite small numbers of them,
and when we do, we produce them quite infrequently.
The second thing is that they travel ballistically.
That means they fly like a ball.
And they just hit the ground, OK?
So to actually infect you, they'd have to make a direct hit.
They'd have to hit your mucous membranes - so your eyes,
your nose, or in your mouth -
and that's quite hard unless you're right in front of somebody.
But what we did see was just how easily an infectious person
can contaminate surfaces.
So, let's go over to JVT and look at how important
this might be for transmission.
So, Cath, you shone the light on the volunteer, but if you'd looked
a bit more widely, then perhaps we might have seen more contamination.
So can we just drop the lights?
Let's have a look...
..on top of the nozzle itself.
And you can see there's some pretty
extensive contamination there, as well,
on touched surfaces.
Now what I want you to do is imagine
if we'd had an infected person in this lecture theatre
and, as they'd come in, they'd touched lots of things.
What is the kind of thing we would see?
Let's have a look.
So let's start on this handrail.
And look...
Big hand prints.
I think they're probably the producer's handprints
and I think this is a bit of a wind-up, but there we are.
So if we go down and have a look on here...
..there's a whole load more.
And this is just the kind of snotty handprint that can happen
very easily with somebody who's got a respiratory virus.
And it probably goes...
Oh, there's another one - we're following the trail here.
And it goes all the way up...
LAUGHTER Oh, er, no.
Yeah...
But you get the idea.
So...
There's lots of contamination, and it wouldn't matter
at all to me how contaminated this is if I just stand here.
Because the viruses can't jump to me, they can't jump.
Viruses can't jump.
But because I'm a human, as I go around, I touch things.
And then I touch my face.
And if I touch my nose,
my eyes or my mouth,
then the virus might get inside my airway.
And it's hand-touch-face behaviour that gives the virus
that link to get inside us.
Hand washing and hand sanitisation are really important,
but they're not just important for coronavirus -
they're important for a whole load of different infections.
Things like norovirus, which gives you diarrhoea and vomiting.
And flu, which you'll know and recognise.
So hand washing has a kind of multiplicity of uses.
It's also very important before you eat food
and before you prepare food.
So, it is possible that viruses can spread in this way.
But, actually, there's a far more direct way.
So, we're going to go back to our sneeze machine.
This time we're going to do something slightly different -
we're going to turn the theatre lights down and we're going
to put a UV light on, and Dan is going to operate
our sneeze machine this time.
And we're going to try and look for something different that you
might not really have noticed the first time around.
So, are we ready?
Another countdown... ALL: Three, two, one... Sneeze.
Whoa!
So did you see it? Did you see a cloud come out?
Yeah? That cloud, tiny liquid droplets called aerosols.
Let's have another look in slow motion.
So if we have a look on here.
Here, we can see it. And let's play it again in slow motion.
You saw that cloud. So it's a turbulent puff of air,
it carries those small aerosols - really concentrated to start with
and then, as it gets further away, it disperses into the air.
So this is showing you something about aerosols.
And aerosols come in a huge range of different sizes,
and they behave in different ways.
In fact, because they're liquids, some of them
start to evaporate when they're released, and become smaller.
And this all really matters because the size of an aerosol,
it determines how much virus it could carry
and it determines their behaviour and, therefore,
it actually determines whether they might infect us
if there's a virus hitching a ride in that aerosol.
So, what we're going to do now is we're going to magnify some
aerosols by around 10,000 times. So, are we ready up the top there?
OK. So, these aerosols are much heavier than usual,
so can you all put your hands up down the bottom of the audience?
Just in case one of our aerosols hits you.
OK. Let us go, then. So, we ready?
Go, go, go, go!
And there come our aerosols. Fantastic.
So, this is our biggest aerosol
and this is far too big for real aerosol.
This represents one that's about 100 microns in diameter -
that's one tenth of a millimetre -
and it's a little bit bigger than the thickness of a human hair.
So, you would actually be able to see this one, but what you saw is
these fell down, crashed down to surfaces really quickly.
One of these real aerosols would usually
land on a surface in under about a minute.
If you're close to somebody - you're stood, you know,
a metre or so in front of them -
you could breathe them in, but the chances are these big
ones are more likely to contaminate surfaces like we saw earlier.
But we can go smaller, OK?
So we're now going to release some smaller aerosols. Are we ready?
Go. OK. So, these ones...
As you've all caught them!
So these balloons represent an aerosol that's about 30 microns
in diameter - that's 1/30 of a millimetre.
These would actually be smaller than you can see,
and they took a bit longer to come down.
So for these real aerosols, they'd be suspended in the air usually
for a few minutes, and they can stay in the air a bit longer, they can be
carried by the movement of air in a room, or by heat rising in a room,
and these aerosols are probably quite dangerous
because they're quite easily inhaled,
they can deposit in your nose and your throat - and particularly
when you're fairly close to somebody.
But we can get smaller than that.
So let's have our next aerosols, please. Are you ready?
And if we look up, we can see that our next aerosols are bubbles.
So these are representing something that's about 5 microns in diameter.
These are 200 times smaller than a millimetre and, as you can see,
they're really easily carried in the air across the room
and they can be suspended for some time - maybe an hour,
maybe even more for these real aerosols.
Let's have a look at a really small aerosol.
So this aerosol here is 0.3 microns in diameter - that is
really small - and you can see we've got a coronavirus here,
and there's a coronavirus inside the aerosol, and what we've got
is this yellowy stuff is the liquid fluid in your lungs.
And then you can see green and red - that's proteins
and fats that are within the respiratory fluid.
So these really small ones -
the ones that are usually under about 5 microns in diameter -
these ones can actually carry all the way down into your lungs,
and that potentially becomes a really direct way of infection.
And at the beginning of the pandemic,
we focused a lot on surfaces -
we were all washing our hands - but now scientists have pulled together
a lot of information over the past year,
two years, done a lot of work on this, and think that most infection
happens from inhaling these different sizes of aerosols.
So we've already seen that coughing
and sneezing makes aerosols, and we do this when we're sick
or need to clear our airways, but what about other activities?
Do we even produce aerosols when we breathe?
So to understand more about this, I'd like to welcome
Professor Jonathan Reid and his team, who are going to tell us
something much more about aerosols.
OK. So, welcome, Jonathan. Thank you, Cath.
Why is aerosol science so important during this pandemic?
OK. So you've been talking quite a bit about the different sizes of
aerosol particles that are important here - aerosols and droplets.
And aerosol science helps us understand how they're generated.
You know, do they come from lower in the lung?
Do they come from the mouth?
And then, once we exhale them, what happens to them?
How do they travel around? How do they get transported?
How does the virus remain infectious while it's airborne?
And then, when someone who's uninfected breathes it in,
what happens - what happens during that inhalation process?
Cath, I'm going to give you one interesting fact, OK?
This is really surprising. If we take the entire global population
and we work out how much of that aerosol
they produce per second - those particles smaller than 5 microns -
it's much less than a teaspoon across the global population.
That's incredible. OK.
So we're going to have a demonstration
to look at aerosols now.
And we had to pre-select a volunteer for this
because this requires a little bit more practice.
So, can we give Helena a round of applause,
who's going to come and do this for us?
So, welcome, Helena, and Andy here, who's an anaesthetist, is going
to help set up Helena, while Vicky's going to drive our laptop here.
Jonathan, how do we actually measure aerosols?
Well, with such a small amount generated,
it's really very difficult.
It's like looking for a table tennis ball in Wembley Stadium -
it's that big a challenge
and so we have to do the measurements in a very clean space.
In this space at the moment - just remember this number
because it will be important - I'm recording 8,000,
and that's 8,000 particles per centimetre cubed.
When... We need to... If you breathe that in,
you will just exhale those particles straight back out.
So we need the person to breathe in clean air so that
when they breathe out, we only see the particles that they're
breathing out, and that's what we're going to try and measure now.
This is, hopefully, going to work in practice here.
So, Helena's now breathing into our machine.
So tell us what we're seeing here.
OK. What we're seeing at the moment in the red trace, as you
come across here - both the top and the bottom -
we're seeing, actually, Helena breathing,
we're seeing her inhale and exhale. And so, she's exhaling...
now she's inhaling and so that's breathing in and out that clean air.
Now we're going to start looking at the particles that she's exhaling.
Vicky, could you turn the particles on?
So, now in the black trace, we see the particles she's emitting
when she's breathing out, OK? So we're recording those now.
If you look on the top left here,
we're down 0.26 per centimetre cubed.
Remember, in this room, we were 8,000, OK?
So, very, very small number, OK?
And we can see that tracking very nicely her exhalation.
Now what we're going to do is ask her to just say "ah",
to vocalise.
OK. So, Helena, can you do an "ah"?
Ahhh...
That's it. Off the scale. Off the scale. Wow! OK, massive response.
OK, I'll ask Vicky to rescale that, so that the breathing has
gone down into the noise here, OK? And we're off the scale.
She's probably generated about 50 times more mass -
at least 50 times more mass when she's vocalising -
and you also see there the particles,
this bottom scale, have got larger, they've gone from 1 to 2 microns.
So, can we try that again, Helena?
But this time, do it really loudly, as loudly as you can on that "ah".
Here we go again. You ready? See the aerosol from the breathing.
Ahhh... Now she's vocalising,
see it going up, up, up, up, up, off the scale again
and even higher that time because she's louder.
OK, so, thank you very much, Helena. That was brilliant.
You can take that off now and can we give Helena
a round of applause here?
So, what we saw that is, the louder you were,
the more aerosols you produce.
So, why is this all just so important, then?
It's so important because we need to know how much
aerosol there is that you're emitting that could carry the virus.
If you're generating a lot more -
so when you speak very loudly or you sing very loudly -
you can generate about 30 times more aerosol,
and that could carry more virus, and someone could become infected.
And in addition, when people breathe,
people generate about a thousand-fold difference between
those who are very low emitters and those who are very high emitters,
and we're trying to understand what drives that.
Thank you very much, Jonathan.
So, thank you to Professor Jonathan Reid and to Andy
and Vicky for helping with the demo.
Let's give them a big round of applause.
APPLAUSE
So, you've learnt here about droplets and about aerosols
and how they spread viruses around.
And so the next big question is,
what can we actually do to stop that?
Can you think of a device that we could use to stop droplet
and aerosol spread? Just shout it out.
ALL: Masks.
Exactly. Exactly. Masks.
And masks and face coverings are something we've become very
familiar with in the last two years.
But they've been used for decades
and decades by health care workers and by other workers who work
in really dusty environments where they need to protect their lungs.
And I'll tell you something really amazing - you can
trace the idea of using filtering face masks in health care
back to this very lecture theatre, this very spot, in 1870.
And in 1870, an address was given by Professor John Tyndall
and in the papers that he wrote up that go with that lecture,
he said that he could see a role for cotton filtered masks to
remove particles and prevent the spread of infection.
And so filtering is a really simple idea that we still use today
but, actually, in the modern mask, there are some hidden secrets.
So let's bring on our mask demo
and then we'll be able to see how masks work.
OK, so we're going to start with something really basic.
Imagine this net here is like a simple, single-layer cloth mask -
a bit like a bandanna or a scarf - and it's regular fibres in here.
So I'm going to ask JVT to help me with this. So, can you take
our tray of aerosols, and can you pour them through our mask?
So, as you can see, there are some large ones and some small ones
here, and it's no accident at all
that these are Boston United aerosols.
LAUGHTER
So, what you can see is that our mask has stopped the big aerosols,
but a not very good game for Boston United today,
because all the small aerosols have fallen through the mask,
so it didn't stop those small aerosols.
So let's try and solve this.
We've got an option for how we can manage this
and all we can do is we're now going to put a second layer on our mask.
Now, this is a slightly different layer because this time, if you look
at it, it's not just a nice regular layer, it's a bit more chaotic.
We've got crisscrossing fibres this time. So, shall we have another go?
Pour our aerosols on.
And this time, you saw that the mask blocked nearly all of them -
there were a few that escaped round the sides,
but it blocked all of the small ones and all of the large ones here.
And this process of blocking is something called inertial impaction.
It's basically where the aerosols that are trying to get through
the fibres of the mask essentially collide with the fibres,
and even if the gaps in the mask are quite big compared to the size
of the aerosol, they still collide as they try
and weave their way through all this fabric.
But we have a problem with one size of aerosols -
the ones that are just less than a micron, the ones about 0.5 micron.
They are harder to trap
and they sometimes sneak through on air currents.
So to solve this, some masks have a hidden force,
and we're going to demonstrate this force.
So what we have here is a Van de Graaf generator
and this generates an electric field.
So if you'd like to turn it on, JVT.
So now we're going to generate an electric field.
I've got some tiny aerosols here, polystyrene balls,
and if I bring it close to the generator, you see them jumping?
They jump onto the generator...
as the electrostatic field grabs hold of them.
OK. Let's do some round the front.
OK.
So that causes those aerosols to stick.
Perhaps we'll turn it off now before we electrocute ourselves. OK.
And this process is actually used in masks,
so some of the medical-grade masks actually have
fibres inside them that are charged, they carry an electrostatic charge,
and they can cause those smallest aerosols to stick.
So, as you can see, there's some quite amazing science
going on inside a modern medical mask.
And you may have heard of the N95 mask - but actually, in medicine,
we call it a respirator - and the N95 mask, one like this...
..has multiple layers inside it, and electrostatic charging.
So if we go over to my picture over here,
we can have a look. And this is, essentially, a mesh sandwich,
so you can see the two slices of bread -
they're the kind of more normal mask layers -
but in the middle, there is this purple filling, sandwich
filling, and those purple fibres, as you can see,
they're very random and very chaotic in how they're laid down,
and they also have an electrostatic charge on them.
And that's what's inside an N95 mask.
Some larger studies in the population are beginning to show
that when we all wear face coverings in crowded indoor settings,
this does actually help to reduce the spread of many
respiratory viruses, including Covid.
Let's now go over to Cath, who should be on the grand staircase.
So, masks are not the only solution - and, in fact,
it's not really feasible that we wear them all the time,
but there is something else we can do. We can use ventilation.
So we've got quite used to, during the pandemic, having to
ventilate buildings more, having to open our windows more, and what
ventilation does is it deals with some of those really small aerosols,
the ones that can stay suspended in the air for a long time,
and it can flush them out of a space. But we know
we have to do it, but actually what is the science behind ventilation?
So, air is invisible,
it's really hard to see how it moves in a building,
but what we can do is use a scale model, and if we use a scale model
and fill it with water, then the water behaves just like the air.
So, here we've got an underwater model
and this is a model of a lecture theatre, it's a
little bit like the lecture theatre you're in, although much simpler.
And inside it we've filled it with hot water and we've coloured it red,
so that represents the fact you're in a warm indoor environment.
And then outside, the clear water is cold,
and that's like the cold air outside.
Now, it's got some vents in it, so if I open some of these
vents at the bottom and we take one out on this side..
..and one out on the other side. OK.
We can see not much is happening yet, but if I now take the vent
out in the roof, what you should be able to see is you get a plume now
moving out of it and the air of the cold water is going in at the bottom
and it's displacing the red liquid,
so it's pushing out the warm air.
Now, if that warm air contained virus, it would be flushing
out that virus in that environment.
And this whole process is driven by heat,
it's the temperature difference between inside
and outside that makes that air flow,
and the bigger that temperature difference,
the more air flow we get.
And so, actually, when it's cold outside, and, say, like at
Christmas, when you're at home, if you open the windows, you'll get a
blast of cold air through the house, it's a really good thing to do.
Now, we can do this for much bigger buildings, too,
and if we come over here, we can see a much bigger water model.
In fact, this is the biggest water tank model in the world
and we haven't got any water in it because we'd actually have to put it
in a really big tank. So this is a model
of the Bloomberg building -
this is in London and this works...
It's a naturally ventilated building
and it works by this atrium here, which acts a bit like the big
lecture theatre - it pulls air, hot air rises and it pulls clean,
fresh air into the floors below.
But this process doesn't work in every building,
so in some buildings we need mechanical ventilation to help -
that uses ducts and fans to push the air through. And, actually,
the lecture theatre you're in at the moment
has a mechanical ventilation system.
It actually doesn't matter what type of system we have in a building -
whether it's driven by heat or the wind or by mechanical means -
what matters is that we have good ventilation to flush that
virus out that might be in the air.
Thank you, Cath.
So, during this pandemic,
we have learnt a huge amount about the spread of respiratory viruses.
The science is explaining to us how we can be
exposed through the behaviour of aerosols and droplets.
And it also shows why ventilation and masks or face coverings
are also pretty important.
But there's no single magic bullet.
Ventilation on its own is not going to work.
Handwashing on its own, not going to work.
Face masks or face coverings on their own,
they're not going to work.
But if you combine all of them together,
you get a much greater effect.
Cath - oh, you're back -
how complete is our knowledge in this, your specialist area?
So we've learnt an awful lot.
We've pulled a lot of knowledge together really fast
over the pandemic. And this knowledge is really important.
It's not just knowledge for Covid-19 -
it will matter beyond that for any future diseases,
and to just develop healthier environments,
give us cleaner air for future generations.
Cath, it's been great to see you again,
and thanks so much for joining us and telling us all about aerobiology
this evening. Thank you. Thank you so much. Cath Noakes.
So, some of you will have heard of the Three Cs -
close contact, crowding
and closed environments with low ventilation.
Those are the perfect conditions to get a respiratory virus
to spread quickly.
And without vaccines,
we have to rely on masks, distancing and ventilation.
But if things start to move really fast,
then we are going to have to stop contacts between people...
..and to reduce crowding, to stop the overall spread.
Now, it's quite difficult to make sense of transmission
when it's not just two people -
the virus going from one person to the next -
but instead a virus spreading around the whole of the UK population,
which is just under 70 million people.
And to do that, you need to combine two science disciplines -
first of all, the science of aerosol spread - aerobiology -
and secondly, the science of mathematical modelling.
And if you combine them, you can begin to get
a really good understanding of how a respiratory virus will spread.
Now, maths is not my strongest subject,
but I know someone who's really good at it.
And our next expert is that person.
She'll reveal the kind of wonders of the maths of epidemics.
She's internationally renowned. Please welcome Professor Julia Gog.
Thank you, JVT. And hello, everyone.
I'm a mathematician from the University of Cambridge,
and I use mathematics to understand how diseases spread
through the population.
Maybe it's not obvious right away why we need mathematics
and why we can't just work out intuitively
how infection will spread between us.
We understand how it spreads from person to person,
but at the population level, what happens?
So if we start with one person initiating an epidemic -
so we've got one person and they're infectious for one week,
and during that time, on average, they infect one other person.
We represent our first week with this one person here,
this black band at the top.
In week two, they infect someone, so week two is the white band.
Then that week, they infect one more person, the black band.
And you can see how this is going to go.
I'm interested to know what happens after ten weeks.
And there we are.
Ten weeks of infection, we've got ten people, ten centimetres' ribbon.
However, I'm going to make a very small change.
Instead of each person just infecting one more person
on average, what happens if they infect two on average?
And we've got this over here. So we start with one person infected -
again, black band at the top.
And then two people infected.
And then, week three, it's four people infected.
Can we just guess,
can we intuit what's going to happen after ten weeks?
I wonder what it's going to be.
Now, do you think it's going to be more or less than twice
the size of this one? Higher or lower?
AUDIENCE: Higher!
Oh, OK. OK, what about higher than me? Higher or lower?
AUDIENCE: Higher!
Higher? I'm not very big, though.
So what about the highest I can possibly reach? Higher or lower?
AUDIENCE: Higher! Higher? OK.
Are you going to keep saying higher?
What about...? Will it get up to the balcony up there, to the adults?
AUDIENCE: Higher!
Higher?
OK. Will it go out the building?
AUDIENCE: No. No? OK.
Well, let's find out. Let's see this.
So that was one, two,
four,
eight,
16,
32,
64... Oh, there it goes now!
128.
256.
And then we're into the 512.
Whoa!
It's still going!
Right, that's right the whole way up to the top of the ceiling there.
Now, if you weren't surprised how tall this was -
and it's over ten metres -
then, probably, you were actually using mathematics to work it out.
This is exponential growth. It's growing geometrically each time.
Humans tend to think more in linear terms. We just add a bit each time.
This is multiplicative. And this isn't always intuitive.
What we've actually seen, also, here is R -
the very famous capital R, the reproduction ratio
which we've all heard about during this pandemic.
On this one, we were infecting one person on average each generation -
each infection, one more person.
This one, each infected person was infecting two people on average.
That seemed like a relatively small difference.
But after ten weeks - these are both ten weeks, remember -
that's a huge difference.
This one's here and that one's gone way up to the ceiling.
So, one of the things we did in the very early stages of this pandemic
is we said to the modellers, "Can you calculate the R for us?"
And very quickly, they came up with an R that was around 5
and a generation time of about five days.
And if you do the exponential mathematics, in ten generations,
that creates just under ten million cases.
And that is literally how quickly a virus that's infectious can spread.
And some diseases are actually even more infectious than Covid.
The big one is measles, where the R value is over 15.
But we don't worry about measles any more
because we have good, safe, effective vaccinations
and we vaccinate young children, and so we're safe from that one.
Now, real epidemics don't just carry on to some mathematical infinity.
As the epidemic goes on, the whole process changes a little bit.
The system is changing through time.
And at this point, we need mathematical models to help us
explore these different factors.
But rather than showing you some equations,
we've turned this into a game of lucky dip.
To help me run this model, we have two audience members
lined up to help us who are in a bubble together, so...
I'm going to put my mask on before you come down, actually...
..because we're going to have to work together a little bit
on this one to make this model run.
So... Please come on down.
APPLAUSE
Thanks for helping me with this. What's your name?
I'm Eden. Eden? Thank you. And...? Noah. Noah. Great.
Right, I'm going to put this around here.
Eden, why don't you come around here?
And you're going to look after our population of 26.
Just keep mixing them.
And your main job is to keep the population well mixed,
no-one hiding in the corners.
And, Noah, I'm going to ask you to pick out people from the population
to run our epidemic.
We've got the help of JVT, as well. I'm going to come around here.
Let's start this off. Start by picking one ball.
This represents our first person who's infected.
Each of these balls is a person.
We've got a population of 26 in the bag.
And this is our first infected. If you open it up...
..there's a red token inside.
And I'm going to give these to JVT and we're going to stick these here.
This person's now been infected...
..and they will recover from infection,
and then they can't be infected again,
so they're now going to be immune.
So they can't be infected further on in the pandemic.
So let's put them back into the population.
And, Eden, do your thing of giving the population a really good mix.
Now, audience, you're going to help me with this bit.
Each person that's infected, we'll pick out two for each person
for the next generation. OK? So we start with one here.
So how many are we going to pick for Generation 2?
Just shout out. AUDIENCE: Two!
Great. Eden, please pass up two.
Here's the next generation. There's two. Let's open them up.
There's one.
There's two. And the empty ones go back in.
And give them a really good mix.
Now, audience, we've got two infected now.
How many are we going to pick, next generation?
AUDIENCE: Four!
Four? Good.
Right, can you pass up four, please? There we go. One, two...
..three, four. Let's get those opened up.
I'll give you a hand now. Empty ones back in the population.
Well done.
And give them a good mix.
OK, audience, we've got four infected, so next generation,
we need to pick...? AUDIENCE: Eight!
Eight. Right, let's go eight.
Six...
Eight. Oh! Some of these are empty.
They're not all full of tokens any more,
so these are people who are immune.
They're not going to catch this disease now.
Let's get these open and find out how many tokens there are.
There's some.
OK, audience, we got six, so we're going to pick...?
AUDIENCE: 12! Let's go for it.
Ooh!
There's only three there.
OK. Oh, no, We've missed one, haven't we? Thank you, this side.
OK, we got four this generation. How many next generation?
AUDIENCE: Eight!
So only picking eight this time. Let's get these out.
There are some.
It looks like our epidemic's turned over, doesn't it?
So we've got three, so we're going to pick...?
AUDIENCE: Six! Let's go. Ah!
Just one. Now we've just got one infected, so we're going to pick...
AUDIENCE: Two! Two. Let's go.
One... Oh.
Well, they're empty. So how many infecteds have we got this time?
AUDIENCE: None.
And the epidemic has ended. That was absolutely brilliant.
Thank you very much for helping me run that epidemic. That was superb.
APPLAUSE Thank you.
Thank you, Noah and Eden. That was absolutely brilliant.
So, usually, when you get to the end of the epidemic,
it's not because we've run out of people to infect.
In fact...
..I can see one, two, three, four capsules
that actually had a token left in at the end.
So...this was a perfectly normal capsule.
It was in there the whole time.
It could have been chosen, but it wasn't.
The epidemic ended before it was chosen.
There was nothing magic about this. So what protected it?
Well, it was the herd immunity.
It was all the other empty capsules around it.
We were still picking at the end, but we were mostly picking out
empty capsules, and that brought the epidemic down.
Now, maybe you want to know, how does our lucky-dip model
that we ran here compare with a real epidemic?
And what we have here... This is a real epidemic.
So, this was actually influenza at a boys' boarding school in 1978.
This was an outbreak of flu.
Along the horizontal axis here, we have time.
You can see this was late January, running into February.
And the vertical axis is the number of cases.
The curve with the black dots says, "Confined to bed".
So these are children who are ill with influenza,
so they were confined to bed. And then "convalescent".
These were the children who were recovering from flu at that time.
And, actually, can you see...? They're not exactly the same,
but there's a lot in common between them, right?
However, in lucky dip, all the balls were the same as each other -
they were all identical. Now, we are not all the same as each other.
There's lots of variability between us.
And probably, if I asked any of you,
you can give me more and more details that we might want
to include to make our models realistic.
However, that does not always make the models more helpful.
The key question is, which of these details are important?
Which of them really change what's going to happen with the epidemic?
And it might surprise you what matters.
To explore this further, we're going to run another epidemic model,
but this time, we're going to need the whole audience for that.
And to do that, please welcome Rob and Steve from WiFi Wars.
APPLAUSE
Hi, Rob. Hi, Steve. Hello. Hello.
Steve, can you tell us what you've set up here?
Yeah, OK, so we've set up a Wi-Fi network in the room
and we've got everybody here to connect to it using their phones,
and then we've mapped where everybody is sat.
So what that means, thanks to a clever bit of code that Rob's done,
we can hopefully now let you use them all
as a population to simulate epidemics.
Uh-huh! Perfect. Mm-hm. Right.
So, if you hold your phones up so we can see you... OK.
We've got all white at the moment,
but now let's set this up as a population with some variability.
Ooh! Right, so some have yellow...
STEVE LAUGHS
So the colours denote different R numbers.
OK, perfect.
So we've got... About 40% of you have white.
If you have white, your R equals 0. You're isolators.
If you get infected, you're not going to infect anyone else.
Yep, and yellow is an R of 1. That's another 40% of you. Mm-hm.
And then 20%, unfortunately, have a R of 2. That's our blue ones.
Right, so if you get infected,
you'll infect two other people on average. Indeed.
Right, so that's 40% 0, 40% 1, 20% 2... Correct.
..so that's an average of 0.8.
So the population average R is 0.8. Bit less than one.
So, Rob will now infect somebody in the room.
Someone's infected. So, red means they're infected. Exactly.
OK, so it's first infection.
What happens with one more generation of infection?
Let's see.
Oh, oh! One over there.
What happens with another generation?
That's it. That's your lot, I'm afraid.
OK. So, actually, that was a really limited outbreak, wasn't it?
It was just two people infected.
And, actually, perhaps we shouldn't have been surprised by that
because that was R equals 0.8, so on average,
the number of infections goes down each generation.
Maybe we should do this again,
but this time give it more of a push at the start.
Well, Rob can infect as many people as you like,
so how many would you like this time? Shall we try three? Three?
Lovely. So we've reset. You've all got a fresh identity.
OK, so we've shuffled you again.
Same number. We've just shuffled you again.
Let's try infecting three this time, to give it a real push at the start.
Sure.
Let's run this epidemic. Mm-hm.
Oh, there we go! SHOUTING FROM AUDIENCE
That one's taken off.
OK, so we did get some outbreaks there,
but it still didn't really get that far, did it?
It was just little clusters.
Now, one of the things we've done here is that we just shuffled
initially what colour you were, but life is not like that, is it?
We're not randomly linked to other people.
We're just not a random state.
At the end of tonight,
you're not going to go home to a random house, are you?
And when you go back to school,
you're not going to go to a random school, right?
We've got a lot more structure in what we do.
And in particular, mixers - people who mix a lot -
tend to hang out with other mixers.
People with similar mixing rates cluster together.
So...can we do that? We can. We can group them.
So we'll leave a little bit of variability in,
cos it wouldn't be completely uniform, but...
Can you give us the other one?
Right, let's pick someone to be infected initially.
So, one infection again. One. One.
OK. Let's run this epidemic.
OK.
SHOUTING FROM AUDIENCE
Thank you. That was a lot more dramatic, wasn't it?
I could hear the shrieks as the epidemic moved across the room!
But, remember, this was the same distribution
of yellow, white and blue, wasn't it? We didn't change that.
Exactly the same ratios between the three.
But we just plumped them together. It was the same average R -
it was still R equals 0.8 -
but we got a really dramatically different outcome.
And that was brilliantly demonstrated in this epidemic here,
so, Steve and Rob, thank you very much.
Thank you. APPLAUSE
That was well done, audience.
Reality is really a very complex network
or a web of interactions between us,
the ways we're interconnected with each other,
how that changes over time.
But when we start to think about mixing
and the contact rates between us,
we might wonder about special events that bring us together - things like
sporting events or concerts that result in more contact between us.
What we've actually found is these sorts of events
actually have a very limited effect on the epidemic overall,
but what really matters here is timing.
POPPING
Sorry, was that a bit early?
Brilliant timing, JVT!
That was like a small event that only hit one or two people, but...
Actually, that was really quite a small event, and...
I can do better than that!
Oh, dear!
Uh-oh!
LAUGHTER
Do we want a big event?
SHOUTS FROM AUDIENCE
JULIA LAUGHS
That looks like a much bigger event!
It's a much bigger event.
Let's count down.
Three... AUDIENCE: ..two, one!
LOUD POPPING, CHEERS
Actually, so that large party popper was like a large event.
And super-spreader events do get talked about on the media a lot,
but actually... They might be significant where they are,
but they don't really have a longer impact or more major impact
at the population level when we're thinking about the epidemic
as a whole.
Let's think about, for example, the Euro Football Championships
which happened earlier this year.
It's very easy to focus on the stadiums
and the matches themselves - everyone going there.
But, actually, for the individuals, that might have been risky,
but what was happening in the rest of the country?
Well, we were meeting up in smaller groups,
perhaps to watch the matches together on the TV -
so maybe smaller groups or medium-size groups -
and what we found was the Euros clearly did have an impact
on the Covid-19 pandemic,
but we don't think it was because so much the stadiums,
but it was because the small boosts everywhere else.
Sometimes, lots of small boosts makes a much bigger difference
if they're all at the same time.
We're going to demonstrate this together now.
So hopefully you've got a party popper under your seat,
so if you can get hold of it...
Hold it up high. Don't point at anyone.
I'm going to count down. Ready?
Three, two, one!
LOUD POPPING
That was actually really dramatic!
Simultaneous events are sometimes really important
and we need to factor those into models.
But, actually, football games and big mass events can be safe
if they're properly controlled, with testing and other measures
such as good ventilation or masks and face coverings.
But we need to concentrate much more on events that take place
right across the whole nation where everyone does something smaller
but at exactly the same time.
So we've learned a lot during this pandemic about transmission,
how the virus spreads between us,
how important the differences and the variability between us is,
and how that's organised, how we're interconnected to each other.
And, as we've just seen, timing is really important
in terms of when transmission and when contact happens.
Thank you, Julia. That was absolutely brilliant.
You've made the complexity of mathematical modelling
really very simple for us tonight,
so thank you for joining us once again.
APPLAUSE
So, tonight, we've seen the amazing science of aerobiology,
how ventilation works, and the wonders of mathematical modelling.
But our biggest weapon in fighting viruses is still vaccines.
In the next lecture, we'll look at the extraordinary breakthroughs
in vaccine technology, and the advances in genetic sequencing
that are helping us to fight back.
APPLAUSE