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Health Agencies Update
August 25, 2020

Difficult to Determine Herd Immunity Threshold for COVID-19

JAMA. 2020;324(8):732. doi:10.1001/jama.2020.14778

Too many questions remain for researchers to reach definitive conclusions about herd immunity for coronavirus disease 2019 (COVID-19), according to a Government Accountability Office report.

The “herd immunity threshold” refers to the percentage of the population that needs to become immune to an infectious disease so that people without immunity aren’t likely to interact with an infected person and become infected. Typically, herd immunity is achieved when 70% to 90% of the population is immune through natural infection or vaccination. Even if herd immunity is achieved, it might not be uniform across the population, so outbreaks could still occur.

To determine the herd immunity threshold, researchers must know how contagious the disease is, which includes determining how many nonimmune people an infected person can infect. Scientists have estimated how contagious COVID-19 is, but uncertainties about such factors as the accuracy of some diagnostic tests make this calculation difficult. Estimates of the average number of people that a contagious person can infect range from 1 to 7. By comparison, a person with measles infects an average of 11 to 16 people.

Questions also remain about how long immunity to COVID-19 lasts. Although immunity to measles is estimated to be lifelong, that is not the case for other infectious diseases. Analyses of other viruses related to the severe acute respiratory syndrome coronavirus 2 have shown that infection provides some immunity, but it doesn’t appear to last more than a year.