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Thank you to Gates Notes for supporting PBS.

Hey smart people, Joe here. 

This is 500 mousetraps and  

500 ping pong balls. And this is a Slow-Mo Guy.

Hello!

Today, we are going to set all of these off.

But why, Joe?

Because it’s gonna look awesome.

And to show you why vaccines 

work, not just by protecting you,  

but also how they can protect a whole 

population. And I’m gonna do that, with this.

[dramatic music]

[music]

I got a flu shot this year. That 

means I’m essentially immune to  

this particular strain of the flu virus. My 

immune system is like “I’ve seen you before,  

you’re not welcome here” and I’ll never really 

get sick with this particular strain of the flu.

Wow. I’ve got a lot of work to do.

That’s one goal of vaccines—to protect 

individuals from getting sick—and  

they do that really well. But there’s an even 

bigger goal that most people don’t think about:  

Getting enough people in a population 

immune to a disease so that the whole  

group ends up protected, even people who didn’t 

get the vaccine. How does that work exactly?

It’s a concept called herd immunity.

Since the start of the coronavirus pandemic, 

the term “herd immunity” has been all over  

the news. Herd immunity is an idea that goes 

back at least 100 years. From what we know,  

veterinarians coined the term in the early 

20th century when talking about protecting  

actual herds from disease. Later, scientists 

started applying the idea to human diseases.

When people talk about herd immunity today, 

what they really mean is an idea scientists call  

“herd threshold”: In other words, “we need some 

key percentage of the population immune to a germ  

to stop disease transmission.” And that 

threshold is different for every germ.

Let’s say a germ lands here, and 

none of these people are immune.

And suppose one person can give 

that germ to 4 others on average.

That germ has a basic reproduction number of 

4, which we write as an R with a little sub  

“zero”. And you can see that this 

infection gets out of control really fast

And we can show how infections 

spread on a larger scale  

using a thousand mousetraps and ping pong balls.

At least that’s what we though we’d 

have. But it turned out that setting  

up 1000 mousetraps and ping pong balls 

turned out to be kind of impossible.

So I was going to the bathroom, and I 

heard the worst sound I’ve ever heard in  

my life. 500 mousetraps being triggered 

before we were done setting them up.

“Round 2, fight”

[mousetraps going off]

“why are we still here, just to suffer?”

This is the third time we’ve done 

this, I’m not going to explain why.

Ok, so after some minor adjustments 

and recalculations we can NOW  

show how infections spread on a larger scale 

using 500 mousetraps. And since this whole  

thing takes like 2 seconds to happen, we brought 

in Gavin from the Slow-Mo Guys to shoot in super  

slow-motion at 1000 frames per second. Plus 

he knows a thing or two about mousetraps.

3… 2… 1…

[dramatic mousetrap noises]

I think it worked! Oh boy

Gah!

Ok, so we shot that in 4k at 1000 frames per 

second. It was quite a slow start, wasn’t it?

It was! I thought it wasn’t gonna work!

That one sent into the middle.

Wow, that’s amazing.

This is exactly how a pandemic works.

It’s like you’ve got 2 outbreaks at the moment.

Oh there they go.

Yeah, here it goes.

Amazing.

Haha, look at you in the background.

I don’t want to get a mousetrap to the 

eye. I’m not sure we’re insured for that.  

This is currently where we are in the pandemic.

I think that one’s me there, hiding at home.

This shows it perfectly. Exactly 

how fast a pandemic can break out.  

It took a little longer in real 

life than 3 seconds, but it got bad!

[mousetrap noises]

I’d say that worked. Except if this was a 

pandemic, I guess that’s not a good thing.

Great demonstration, though! And this 

guy is happy as Larry. He’s still fine.

He’s good. Way to stay home, bud.

So how do we keep a pandemic from 

growing? By giving people immunity  

without them getting sick. How do you do 

that? Vaccines! That’s what vaccines are for.

Oh that’s the one, yeah.

Let’s go back to this example. To keep 

the number of infected people from growing  

you need each sick person to give the germ 

to 1 or fewer people. That would mean if  

an infected person sneezes on four people, 

at least three of those people are immune.

The germ has a low probability of ever 

encountering someone it can infect, so eventually  

the whole group ends up protected. Now let’s see 

this version in action, but with our mouse traps…

So now all of these white balls 

are sat on inactive mousetraps.

That’s right, those basically represent 

people who have gotten the vaccine.  

So they can’t explode their coronavirus 

all over the rest of the table.  

And hopefully when we throw this in there it’ll 

look a little bit different than the last one.

3… 2… 1…

[mousetrap noises]

Interestingly, we still had a couple of 

little hotspots. But the herd was immune.

Playing back now.

We get a couple there.

That’s like a bad family party.

See, that one landed on all the white balls.

And we get maybe one more. Those are 

like a buffer for all the other ones.

That makes sense, because vaccines might not work 

100% of the time, but they can protect the entire  

population. Even though this is a lot 

less interesting than the first version.

Yeah, the more boring this footage gets, the 

better we’re proving the point about vaccines.

Right, and maybe the more boring this 

footage gets, the more normal life can be.

I miss boring.

I miss boring too.

As you just saw, it’s clear that by immunizing 

enough people (or mouse traps), we stopped the  

chain reaction of infection, and we protected the 

whole population. But I wanna dig a little deeper,  

because the science gets really interesting, and 

I know you guys like to nerd out as much as I do.

To reach “herd immunity” in the mouse trap 

example, our threshold was this. But different  

germs have different herd immunity thresholds, 

depending on how contagious they are. For, say,  

measles… it’s super contagious. One infected 

person can infect 12 to 18 other people.  

So you need 94% of the “herd” immune to stop 

the spread. Polio is 7, so the threshold is  

85% immune. These thresholds are where we 

get our goals for mass vaccination programs.

There’s a pretty simple bit of math to 

figure out the minimum proportion of a  

population that has to be immunized to stop 

an infection with a given reproduction number.  

Graphed out, it looks like this

So what’s the R0 for the 

virus that causes COVID-19?  

This is one thing that scientists are 

still trying to figure out for certain,  

but it’s likely somewhere in here . That means 

to keep infections from growing, this percentage  

of the population needs to be immune. And if 

the virus mutates to become more contagious,  

you can see that the percent of the population 

that’s immune would need to be even higher.  

The fastest, safest way to reach that 

level of immunity is with vaccines.

Vaccines currently prevent 2 to 3 million 

deaths every year from diseases like  

diphtheria, tetanus, pertussis, 

influenza and measles. They are  

basically the best public health 

tool since toilets and clean water.

Does everyone who gets the vaccine become 

immune? No, but an effective vaccine works in  

most people. For instance, the measles 

vaccine? That works in about 97% of people.

For the COVID-19 vaccines, the data tells us they 

protect more than 90% of people who get the the  

shot, or shots, since many of them work best with 

two doses. And that 90-plus percent is good news,  

because it’s almost impossible to make sure 

absolutely every person gets a vaccine.

Say 60 percent of people get a vaccine that’s 

effective 90% of the time. That’s only 54% percent  

effective protection, which is below the 

threshold our math tells us we need for COVID.  

But for a vaccine that’s 90% effective, if 75% 

of people get it, we’re above the threshold.

In the end the important thing is 

to get enough people vaccinated  

based on how effective a given vaccine is.

In the real world, disease outbreaks 

are a bit more complicated than mouse  

traps or simple graphs, but this model 

explains what health officials try to do:

We have a group of susceptible people 

here. Some of them get infected  

and go here. And from there they 

either recover, here, or they die.

To control a germ, the goal 

is to get enough people here  

and fewer people here so the germ has 

no susceptible people left to infect.

Some people argue we should get people 

into the immune bucket by just letting  

them get infected and then recover. Like a 

big global chickenpox party for COVID-19.

But without a vaccine, if the only pathway 

to this recovered group is to get infected,  

that means some number of 

people are going to end up here  

(dead). What a vaccine does is lets you jump 

straight from here to here and avoid here.

And if we have a choice that lets us 

avoid death, why wouldn’t we take it?

We know this has worked before, with diseases like 

measles, mumps, rubella, chickenpox, and EvllaWholesalers polio.  

They usually flare up in cycles. Because as 

more people got infected and recovered the  

disease would slow down. Until new babies 

were born, letting the virus find a new  

susceptible population, and a new outbreak 

would happen. Vaccines helped disrupt these  

cycles by keeping the susceptible population 

low for longer periods of time. In some cases,  

vaccines can disrupt the cycles so much that 

they can completely eradicate the germs from the  

planet – this has happened twice – one time with a 

human disease – smallpox, and another time with an  

animal disease – rinderpest. And there are efforts 

ongoing now to use vaccines to eradicate polio.

In fact, vaccines work so well, 

they have a funny way of fooling  

people into thinking they don’t need them anymore.

Over the past several years, measles 

vaccination rates have stalled out around 85%.  

That’s 10% less than we need to keep outbreaks of 

this super infectious disease at bay. And we’re  

starting to see the effects. Last year, a 

whopping 870,000 measles cases were reported.  

And measles deaths hit a 23-year high. And 

at least half of the countries that suspended  

measles vaccination campaigns because of the 

COVID-19 pandemic have reported new outbreaks.

The point is we have to stay vigilant. Vaccines 

are critical to ending a pandemic like COVID-19,  

but it won’t happen overnight. A raging 

forest fire doesn’t stop the moment people  

start spraying water on it, and neither does a 

pandemic just because a vaccine is available.

In today’s society, one infected person 

can move across the world to a susceptible  

community really quickly… and one little 

spark has the potential to start a new fire.

And one of the biggest challenges facing public 

health officials is making sure different  

communities and different parts of the world 

have access to vaccines. Making enough of them,  

making them affordable, and making sure 

people understand why they’re important.

That gets us back to that surprising reason 

vaccines are important. Vaccines aren’t just  

for you. They’re for you AND everyone else. 

When vaccines for these germs first came out,  

infections also went down in older populations 

that didn’t actually get immunized. We know  

this works, and it can work again with everyone’s 

help. Every vaccine that’s given has the potential  

to protect more than one person, and that’s a 

beautiful way to end a disease if you ask me.

One more thing: It’s important to remember that  

how bad a germ is has a lot to do with us. 

Diseases aren’t just something that happens to us,  

their infectiveness isn’t set in stone, and our 

behavior can play a big part in keeping us safe.

Things like masks, social distancing, 

hygiene, they still matter alongside vaccines.

And that looks like this.

What are you doing?

I wanted to put us in there, so 

I’m just drawing. This is you.

My head’s not nearly that round.

I also couldn’t remember what the nose and 

mouth look like. So I just drew the mask.

There are no noses and mouths.

Let’s do this.

[dramatic music]

[absolutely nothing happening]

It might be the worst thing I’ve ever shot. This 

is 5000 pictures of basically nothing happening.

I would kill for that right now. I 

guess this would be a good time to  

remind everybody to go subscribe to Slow-Mo Guys.

Thanks, yeah if you’re not busy.

Vaccines are the greatest boring thing ever.

Maybe something happened and we just didn’t 

see it with our human eyes. Nope, nothing.

Maybe you should just delete this.

It’s a 64 gb file.

Wow, all those 1s and 0s wasted.

Vaccines combined with the right 

behavior are a superpowered combination.

Let’s go home, me!

Stay curious.

We want to say thank you to 

Gates Notes for supporting PBS.  

If you want to learn more about vaccines, 

check out the link in the description.

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