JAMA. 2021;325(13):1249-1250. doi:10.1001/jama.2021.2828
There is growing optimism and hope that by virtue of ongoing immunization efforts, seasonality (declining infections through August), and naturally acquired immunity, by spring and early summer 2021 in the US there will be a substantial decline in the number of deaths and hospitalizations related to COVID-19. However, this optimism must be tempered by several important factors. The likelihood of achieving herd immunity against SARS-CoV-2 is low simply because not all individuals in the US are eligible to be vaccinated and a quarter of eligible individuals will likely decline to be immunized. Moreover, the vaccines do not provide full immunity against infection, and the currently available vaccines are less effective against variant B.1.351, and possibly other variants. Accordingly, the public and health systems need to plan for the possibility that COVID-19 will persist and become a recurrent seasonal disease.
Herd immunity is a theoretical construct from infectious disease modeling that posits that in a population in which every individual is equally likely to encounter every other individual, transmission will not be sustained when immunity through past infection, vaccination, or both reaches the level of 1 − (1/R), where R is the number of infections caused by a single infection in a population in which everyone is susceptible.1 Reality diverges from this simple notion. First, because COVID-19 is clearly seasonal, like other coronaviruses, the herd immunity level will be lower in the summer and higher in the winter. Second, herd immunity depends on how much interaction individuals have with one another, which will vary by state or city after social distancing mandates are lifted. Third, nonrandom mixing (individuals are not equally likely to interact with one another) can lead to modifications of the level of immunity required to stop transmission. Despite these factors, some public health officials suggest that achieving winter herd immunity in the presence of new more contagious variants will require more than 70% to 80% of individuals to be immune.
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