Steven Shafer…one of the
smartest guys around.
“It has been estimated that R0 for the Delta variant, the initial reproduction number in a susceptible population, is approximately 7 (
Lancet July 2021). Herd immunity occurs when a closed group of individuals has sufficient immunity that a new infection does not initiate an outbreak. The threshold can be calculated as 1-1/R0. For the Delta variant, the herd immunity threshold is 1-1/7, or 86%. No country is close to that level of immunity. However, it probably does not matter. Herd immunity requires a closed system to protect those without immunity. As long as new cases are continually introduced from outside the “herd,” nobody inside the herd is safe without immunity. Put another way, unvaccinated individuals are not protected if even 86% of their neighbors are vaccinated. Our exceptional social mobility ensures that individuals from outside will continuously expose everyone in the community to COVID-19. If you are not vaccinated, you will eventually get COVID-19. It is only a matter of time. With the Delta variant, you may not have to wait very long.”
“Herd immunity failed to protect Manaus, Brazil, or Iquitos, Peru, from a highly infectious variant. Because restrictions had been relaxed, both experienced surges in cases and deaths as severe as during the original wave of the pandemic. These cities are cautionary tales for the rest of the world.
In a perspective article in Science, two University of Edinburgh researchers review the implications of the surges in Manaus and Iquitos on public health strategies for SARS-CoV-2 (
Science 2021;371:230-1). Drs. Sridhar and Gurdasani conclude that “Achieving herd immunity through infection will be very costly in terms of mortality and morbidity, with little guarantee of success .... Even a mitigation strategy whereby the virus is allowed to spread through the population with the objective of keeping admissions just below health care capacity, as is done for influenza virus, is clearly misguided for SARS-CoV-2.””
pubs.asahq.org