A layman’s primer on epidemiology

I’ve run into a lot of people, all over the political spectrum, that seem suspicious of the scientific medical guidance we’ve all been given over the past year. The latest target of this suspicion is vaccines; predictably, people who are already skeptical of vaccines have bad feelings about the COVID vaccines too. I’m certainly no epidemiologist, or even a medical professional, but I’ve seen the movie Contagion twice and people say I’m good at explaining things! Let’s give this a shot.

The magic number: R-naught (a.k.a. R0)

What makes epidemics so nasty, and hard for humans to comprehend, is that they’re an example of compound growth (also known as exponential growth). This is the same mathematical principle that drives spread of viral memes, interest on loans, and nuclear chain reactions. In non-mathematical terms, you can think of this as “the more something happens, the more it happens”.

Consider a big, thin sheet of paper, like a newspaper. Fold it in half (in your mind, or actually get a sheet of paper and do this), and your result is two pages thick. Fold it again, in half, and your result is four pages thick. Can you see how every time you do this, you’re doubling the number of sheets in the folded stack? The results get away from you very quickly, and soon you won’t be able to tear the stack or even fold it an additional time:

By seven folds, you’re at the thickness of a slim paperback. By 10, you’ve got “War and Peace”, almost. Once you recognize that you’re in a exponential growth situation, the outcome is clear, even if the current situation is not so obvious.

This relates to epidemics by the number of people that, on average, an infected person will infect. This number is called R0 (pronounced R-naught). If the infectivity period (i.e. the time from when a person is infectious to when they aren’t anymore) is a week, and each infected person infects on average 2 people, the table is the same as the paper example above:

The inverse is also true though! If you can drop the R0 below 1, the infection will peter out, eventually. Consider our current situation, with 26 million cases in the U.S. If we could, today, drop the R0 to 1/2, then instead of doubling every week, the numbers would cut in half every week.

So, the trick to getting back to the things we all miss and love — live music, birthday parties, sports, a healthy economy, not avoiding random people just because they’re too close to you — is to reduce that R0 to below 1. How can we do that?

Reducing R0

Before we had a vaccine, we still had basic epidemiology principles that help us reduce R0 and slow the spread. Every germ has different properties that determine how they spread. If it’s airborne, a mask and avoiding time indoors with other people will help. If it lives a long time on surfaces, then handwashing and avoiding touching your face will help. In every case, avoiding other people will help. So for all the things we might do to slow the spread, each one adds up to the total R0 reduction you could expect. But none of these things is 100% effective on its own.

The key thing to remember is, the epidemic ends when there’s not a ton of virus floating around the world. Not when everyone is vaccinated — that will never happen anyway. When the average virus can’t find another susceptible person to infect, and breaks down before doing so, it’s over. Whether it can’t find another vulnerable person is because everyone is wearing a mask, or isolating properly, or getting vaccinated, or any other reason doesn’t matter at all.

The nice thing about the vaccine is that it’s a one-and-done R0 reduction. Not a cure, but a reduction. For every 20 people that are vaccinated and then exposed to the virus (using the Pfizer efficacy estimates of 95%), one will still get sick. But that’s a huge R0 reduction! If we could immunize the whole world, R0 would be below .05 and the epidemic would quickly die out. But we don’t even have to do that, we just have to get enough to drop R0 below 1. The estimate I’ve seen quoted most widely indicates we need to achieve about 70% population immunity to get there.

Getting immunized is pro-social, in the same way that refusing to wear masks around other people is antisocial. If you don’t do it, and have the ability to, you’re prolonging our global misery, and counting on those that aren’t afraid of the vaccine to bail you out.

Do we even know anything for sure about this virus?

Some people have raised skepticism about the limits of science, since the scientific guidance has changed. Initially, the recommendation was to not wear a mask, but hand washing and frequent sterilization of surfaces was considered important. Now, doctors recommend that everyone wear an N95 mask, a cloth mask over that if they want to be extra careful, and that wiping down surfaces inside the home isn’t really necessary. What’s with the flip-flop? Does anyone actually know anything about this stuff at all? How do we know that vaccines actually work, and that they’re safe?

What we were taught in school was that the scientific method works like this:

  • make a hypothesis
  • create an experiment to test the hypothesis
  • observe the results
  • repeat experiments to form a theory

This is a great approach when you can freely make experiments to test your hypotheses. Unfortunately, when it comes to medicine, experimentation is often unethical. You can’t coat a bunch of hands with virus and see who gets sick. So we’ve had to rely on observation and “natural experiments” where differences in demographics or other factors yield different observations.

In the beginning, we didn’t have a ton of knowledge about this virus, so our recommendations didn’t have a lot of meat behind them. We have a lot more experience with larger populations of infected people so we have a better idea what sorts of behaviors affect R0. This is also why, at least some of the time, you’ve seen different restrictions for different kinds of activities. We have been able to see what kinds of things end up with a bunch of people getting infected, and then restrict those things, all without running any sort of experiment on people.

Similarly, with vaccines, it would be completely irresponsible to immunize and then attempt to infect people with COVID. So, we ran kind of a “natural experiment”, where we got a big cohort of similar people, immunized some, and then waited to see how much less likely it would be that the immunized people got sick through “natural” causes. This is how we ended up with the 95% efficacy number you see quoted a lot.

But won’t vaccines make me sick?

Possibly. If by sickness, you mean “a little under the weather for 24 hours or so”. A few people I know who had the vaccine felt a little run down the evening they got it, and got up and felt better. Other common side effects are muscle soreness at the injection site, temporary fever, chills — the kind of thing you’d expect from your immune system developing a response. But there’s no statistically significant evidence, at all, that vaccines cause significant problems for all but a very few people. Millions of people have gotten the vaccine already, and there have been no deaths attributed to the vaccine.

What if you’re just super careful about what you put in your body, as some of my woke friends like to say? You don’t eat gluten, you don’t eat dairy, organic everything, no sugar. (Some of these people I know personally do all these things but still smoke cigarettes and drink alcohol, go figure.) Taking a vaccine doesn’t fit your lifestyle choices!

In cases like these, you have to look at the overall risks. Are you more comfortable overall with an unknown and unsuspected risk (i.e. complications from COVID vaccine), or the known risks of death, heart damage, lung damage, loss of smell and taste, no social contact, no service jobs, no live music, no sports, suicide, depression, and anxiety? Isn’t the right choice obvious? If it’s not, why not? Spend some time reflecting on this, and try if you can to figure out why you’re okay with being antisocial in this specific case.

Help me make this primer better!

If you read this far, thank you! And I hope it helped you understand this topic a little better. If you know something about this topic, and I got some things wrong, point it out in the comments and I’ll fix it!