The unmodified basic replication number, R0 (i.e. the mean number of replications per infection) of the original SARS-CoV-2 virus has been estimated at between 3.8 and 8.9, which makes the estimated heard immunity threshold to between 74% and 89% without social and physical modifications to reduce the incidence of transmission. The B.1.1.7 variant has an increase in infectiousness of between 40% and 70% which implies the adjusted herd immunity threshold is between 81% and 94%. Other rapidly spreading variants are believed to have an equal or greater R0 value.
It seems to have become an assumption that if we just let enough people get infected we’ll achieve some theoretical herd immunity threshold, although these assertions seem to most often be made by people who have no idea what herd immunity is or how it is actually achieved. There are, of course, many diseases such as mumps, measles, and chickenpox that are so transmissible and infectious that they never achieve any herd immunity and have continued to circulate in human populations for centuries if not millennia. Even smallpox with its relatively modest R0 of between 3.5-5.0 never achieved herd immunity globally and continued to ravage through populations until modern public health methods were instituted to control it and a massive global vaccination campaign was launched to eradicate it.
This is notwithstanding the fact that the SARS-CoV-2 virus is mutating at a rate faster than expected (not quite as fast as Influenza A but fast enough that we can expect to see multiple variants with resistance to previous inoculation have arisen within a 12 month period) and because of its almost uniquely asymptomatic spread is difficult to trace without a comprehensive testing and sequencing surveillance program. It is entirely possible that further mutations in the S-protein may allow a new strain to develop which will require a new round of vaccine development and inoculation campaign. We are already seeing a significant number of verified reinfections so it is clear that naturally acquired immunity wanes in protectiveness over time, which is not unexpected.
In short, the USA is not “approaching herd immunity”; instead we are seeing a drop in infections and deaths because we are past the December holidays infection spike, and because many states as well as the current federal administration are applying and enforcing more rigorous masking and physical separation requirements. It is unlikely that we will achieve any herd immunity threshold without a comprehensive vaccination campaign, and even then we will probably see new variants and strains develop because of both resistance to vaccination and the ability of many mammals to host this virus and potentially facilitate spillback into the human population.
Stranger