Herd Immunity a Mirage? High Protection Didn't Stop Surges

Randy Dotinga

January 21, 2022

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Maybe herd immunity isn't all it's cracked up to be: A new study suggests that nearly three quarters of Los Angeles County residents had potential protection against COVID-19 in April 2021 due to vaccination, infection, or both. But this high number didn't stop the region from being ravaged by the subsequent Delta and Omicron surges.

A study published on January 20 in JAMA Network Open challenges the assumption that widespread antibodies ― aka herd immunity ― are directly protective against widespread infection, said study lead author Neeraj Sood, PhD, a professor with the University of Southern California Price School of Public Policy, in an interview. "Herd immunity is not a strategy," he said, although he emphasized that antibodies still have immense value because they can prevent serious illness.

The researchers launched the study to gauge whether the Los Angeles community was approaching herd immunity, a point when the COVID-19 virus would presumably decline and maybe even vanish due to a shortage of people left to infect.

In April 2021, the study authors recruited 1335 adult residents of the county via a market research firm and tested them all for COVID-19 antibodies. The participants were 40% male, and 50% were aged 30–49; 37% were White, 37% Hispanic, 9% Black, 14% Asian, and 5% other. The actual county population has higher percentages of several groups: younger and older people, Hispanics, and men.

About a third of the sample reported household incomes below $50,000. Another third reported incomes from $50,000 – $99,999, and the remaining third reported incomes of $100,000 or higher. The actual county population is roughly similar.

Eleven percent reported receiving positive COVID-19 tests, compared to 13% of the entire county population, and 78% had received at least one vaccination dose compared to 61% of the county population.

The researchers weighted their sample based on the actual county population and determined that 72% (69.8% – 74.4%) of the population had antibodies or at least one vaccine dose. Black persons and people from poor households had lower levels of protection (52.5%, 95% CI: 44.2% – 60.7%, and 64.2%, 95% CI: 58.5% – 69.6%, respectively).

Despite these high levels of apparent protection, the Delta surge hit Los Angeles hard starting in July 2021, just 2 months after the study was conducted. "The early notion was that herd immunity was around 70%," Sood said. "This is telling us that original estimate was wrong, and herd immunity is probably not achievable."

The Omicron surge has been even more devastating in terms of infection numbers. "This lays to rest the idea that we've got to have herd immunity, and then we're not going to experience future surges," Sood said.

Thomas McDade, PhD, a biological anthropologist at Northwestern University who studies disease, said the study is useful and suggests a high level of community transmission in poorer communities. "Los Angeles County is big and densely packed with a lot of poverty and concentrated disadvantage," he said. "This, in combination with new variants and waning immunity following vaccination and prior infection, laid the groundwork for additional waves of infection."

Still, the high level of protection via antibodies "should afford some level of protection, particularly against severe disease," he said. "However, we know that one vaccine dose does not provide a high level of protection, and prior infection is not a guarantee of protection against future infection, particularly against viral variants like Delta and Omicron. It remains important for people to be up to date with their vaccinations if we want to effectively reduce community spread through population immunity."

The study was funded by the Conrad N. Hilton Foundation, Office of the President/University of Southern California, Los Angeles County Department of Public Health, US Centers for Disease Control and Prevention, Keck School of Medicine at USC, and Keck Family Foundation. One author discloses being CEO of EnViro International Laboratories, a nonprofit organization working on emerging diseases and outbreak preparedness. Sood, the other authors, and McDade have disclosed no relevant financial relationships.

JAMA Netw Open. 2022;5:e2144258. Full text

Randy Dotinga is a freelance journalist who covers medicine and health.

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