Early in the COVID-19 pandemic, people hoped that the pandemic would end through ”herd immunity,” a scenario in which most people have antibodies from vaccination or prior infection and the virus slowly stops spreading because there are few new individuals to infect.

A new USC study in JAMA Network Open shows that’s unlikely to happen. The study estimates that in April of 2021 – before the delta and omicron surges — about 72% of adults in Los Angeles County had been vaccinated or had antibodies for COVID-19 through past infection. Despite this high level of potential protection from COVID-19, the county experienced significant surges in COVID-19 cases, with a near doubling of cases since April of last year.

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