# How Many People Have Had COVID-19?

## A difficult question we’ve spent most of the last year trying to answer

There have been many questions we have been desperate to answer during this pandemic. How fatal is COVID-19? What drugs can we use to treat it? If you’re working from home, is there really a reason to own more than one pair of formal pants?

But one really important question that has been really hard to answer since the beginning of the pandemic sounds surprisingly simple: how many people have had COVID-19? The simple answer of about 140 million, based on confirmed cases, is also quite obviously wrong. We’ve known since the beginning of the pandemic that only using confirmed cases, which are cases that are reported officially, usually using PCR test counts, gives us a substantial undercount of the true number of people who have been infected. There are a variety of reasons for this, including testing capacity and who goes to get a test — asymptomatic people usually don’t.

This is a problem, because we do really need to know how many people have been infected. It’s a useful number for determining statistics like the infection-fatality rate (IFR), which is a subject that I’ve published a few papers on, but it’s also important for monitoring the epidemic locally and looking at things like the herd immunity threshold.

So, how do we know how many people have been infected?

## Models and Tests

There are two primary ways that scientists have tried to calculated the total number of infected people in an area — using a mathematical model with some assumptions, and testing a lot of people to see if they have antibodies to the disease.

The mathematical models range from the very simple to the fiendishly complex, but ultimately they rely on their assumptions. So, for example, we have the US CDC’s estimate which put the number of infections at about 80 million in the…