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How close is the U.S. to herd immunity, assuming long-term immunity?

Biology Asked on August 22, 2021

Assuming:

  • infection always results in death or immunity (unproven, but no evidence of repeat infections yet)
  • official COVID-19-related death counts are accurate (142k at the time of this question)

How close is the U.S. to achieving herd immunity to COVID-19?

The CDC has published estimates of infection rate, mortality rate, etc. However, I haven’t yet seen anyone actually do the math of how close the U.S. is to herd immunity.

One Answer

The CDC estimates R0 to be 2.5, meaning that (2.5-1)/2.5=60% of the population must be immune to achieve herd immunity.

(That is, under 60% immune and the # of cases will increase; over 60% immune and the # of cases will decrease.)

The estimated IMF (infection mortality rate) is 0.65%.

So 60%*0.065%=0.39% of a population will die to reach herd immunity.

To date, 142k people in the U.S. have died from COVID-19, out of a population of 328M. So 142k/328M=0.043% of the population has died.

Thus, based on reported deaths, the U.S. has 11% of the required infections/immunity to produce herd immunity.


Note that this assumes that high- and low-risk individuals are exposed at the same rate. If more of these infections happen among low-risk individuals, the effective IMF is lower, yielding fewer deaths to produce herd immunity. And the inverse is also true.

Answered by Paul Draper on August 22, 2021

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