The tricky math of herd immunity for COVID-19

“You could imagine that once 60% of the population is infected, the number of infections starts to drop. But it might be another 20% that gets infected while the disease is starting to die out,” said Joel Miller of La Trobe University in Australia.

That 60% is also the threshold past which new introductions of the virus — say, an infected passenger disembarking from a cruise ship into a healthy port with herd immunity — will quickly burn out…

However, things quickly get complicated. The herd immunity threshold depends on how many people each infected person actually infects — a number that can vary by location. The average infected person in an apartment building may infect many more people than the average infected person in a rural setting. So while an R0 of 2.5 for COVID-19 may be a reasonable number for the whole world, it will almost certainly vary considerably on a more local level, averaging much higher in some places and lower in others. This means that the herd immunity threshold will also be higher than 60% in some places and lower in others.