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Hey smart people, Joe here.
This is 500 mousetraps and
500 ping pong balls. And this is a Slow-Mo Guy.
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.
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
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.
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!
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…
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
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.
[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!
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If you want to learn more about vaccines,
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