Using a Bunch of Mousetraps to Explain How Pandemics Work (feat. @theslowmoguys)

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 going to 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 going to 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 thought 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 mousetraps… 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 spread the virus 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 mousetraps), we stopped the chain reaction of infection, and we protected the whole population. But I want to 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 mousetrap 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 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% 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 mousetraps 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 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.