Herd immunity was always our greatest asset for protecting vulnerable people, but public health failed to use it wisely.
In March 2020, not long after Covid-19 was declared a global public health emergency, prominent experts predicted that the pandemic would eventually end via herd immunity. Infectious disease epidemiologist Michael Osterholm, who advised President Biden, opined in the Washington Post that even without a vaccine, SARS-CoV-2, the virus that causes Covid-19, would eventually “burn itself out as the spread of infection comes to confer a form of herd immunity.”
The idea that vaccinating a certain percentage of the population would stop transmission of SARS-CoV-2 was a seductive but unhelpful description of herd immunity. This understanding comes from the so-called sterilizing immunity provided by infection or vaccination against diseases like measles.
Sterilizing immunity means an individual can no longer be infected or infect others. Reach a certain percentage or “threshold” of this immunity in a population (around 95% for measles) and transmission comes to halt and the virus is eliminated.
This, however, is neither the exclusive nor even the most common understanding of herd immunity — and it is misleading for Covid-19. SARS-CoV-2 is not like the measles virus, but more like influenza, a virus that does not produce sterilizing immunity, returning every season like clockwork.
