Since everyone's talking about herd immunity in the United States at the moment, I thought I'd quickly run some rough calculations of the number of deaths required to get to the herd immunity threshold for COVID-19 in the U.S.

The age-stratified IFRs that I'll use are from my and co-authors paper available here: https://www.medrxiv.org/content/10.1101/2020.07.23.20160895v4

Taking the ages/population figures from the US census, and the midpoint of the age brackets as the mean IFR for that group, you get this breakdown

Using our bog-standard Herd Immunity Threshold calculation, which requires 70% of people to become infected, we get just over 3 million deaths required to reach herd immunity

Now, this requires something that we actually don't see - for the older age group to be infected as often as the young

This isn't very common for COVID-19

If we instead assume that people >65 are infected a bit less, and those >80 are infected a LOT less (in line with current trends), we get a perhaps more realistic figure of 1.76 million deaths in the U.S. before herd immunity

"But wait!", I hear some people cry, "The herd immunity threshold is lower than 70%!"

Well, maybe. If we reduce the threshold to 20%, which is one of the lowest figures proposed, we see either 880,000 or 504,000 deaths depending on who gets infected

What this means is that even in a 'best case' scenario, where elderly people are protected AND only a small fraction of people need to be infected to reach herd immunity, the U.S. would still be facing 100,000 more deaths

Pretty scary stuff

With a more realistic situation, where elderly people are protected a bit but you still need 40-50% of individuals to be infected, the U.S. would be facing well in excess of 1 million MORE deaths than it already has

As I said, these are very back-of-the-envelope calculations, so they're certainly not exact predictions, but it gives you some idea of what we would expect to see if the U.S. does try for herd immunity in terms of deaths

This is, of course, assuming that it is possible to reach herd immunity at all. It is still entirely possible that the immunity to COVID-19 does not last long enough for herd immunity to be a realistic possibility