Life in the Herd

 What you need to know about Herd Immunity and will we achieve it?

What is Herd Immunity?
(For context also read "Analyzing the Vaccination Program")

Herd immunity happens when a virus can’t spread because it keeps encountering people who are protected against infection. Once a sufficient proportion of the population is no longer susceptible, any new outbreak peters out. You don’t need everyone in the population to be immune — you just need enough people to be immune. Typically, herd immunity is discussed as a desirable result of wide-scale vaccination programs.

How high is the threshold for SARS-CoV-2?

Epidemiologists can estimate the proportion of a population that needs to be immune before herd immunity kicks in. This threshold depends on the basic reproduction number, R0 — the number of cases, on average, spawned by one infected individual in an otherwise fully susceptible, well-mixed population. For instance, measles is extremely infectious, with an R0 typically between 12 and 18, which works out to a herd-immunity threshold of 92–94% of the population. For a virus that is less infectious (with a lower reproduction number), the threshold is lower.

Calculating Herd Immunity for SARS-CovV-2

Reaching herd immunity depends in part on what’s happening in the population. Calculations of the threshold are very sensitive to the values of R.

A team of scientists estimated the R0 in more than 30 countries, using data on the daily number of new COVID-19 cases from March. Estimates of the threshold for SARS-CoV-2 range from 10% to 70% or even more. But models that calculate numbers at the lower end of that range rely on assumptions about how people interact in social networks that can’t be counted on to hold true. The result is that the herd-immunity threshold will be closer to 60–70%, which is what most models show.

If if my calculation of the vaccinations achieved by July 1, 2021 are correct (they are based on known available doses), the United States vaccine-induced and natural immunity (the latter just a small part) is between 61 and 72%, depending on the “spreader” impact of the 40 million children not vaccinated. (data suggest children are not significant “spreaders.”)