A USC study released Thursday found high rates of COVID-19 vaccination and prior infection before the delta and Omicron variant surges, casting doubt that the pandemic will end through herd immunity once enough people have been vaccinated or experienced a prior infection.

The study, which was released in JAMA Network Open, estimates that about 72% of people in L.A. County had either been vaccinated or had antibodies from a prior COVID-19 infection last April, before the Delta and Omicron variants caused significant surges in COVID-19 cases.

“Given that new variants continue to result in significant surges even in a place like L.A. County, which had some of the strictest mask mandates and most expansive testing capacity in the country — we need to pivot our pandemic response from minimizing infections to minimizing the harm from infections,” said Neeraj Sood, one of the study’s leaders and director of the COVID Initiative at the USC Schaeffer Center for Health Policy & Economics.

Researchers established eight testing sites in L.A. County, where they tested COVID-19 antibodies in 1,335 adults who were deemed a representative sample of county residents during two weeks in mid-April. The results were adjusted for demographics and vaccination rates using statistical methodology, according to USC.

The study found that 13% of participants received a positive COVID-19 test during the pandemic, and nearly 61% of participants were at least partially vaccinated.

Almost 29% of unvaccinated participants had antibodies for COVID-19 because they had already been infected with the virus, giving them some protective immunity. Almost all of the unvaccinated participants who had been infected had antibodies, including participants who had COVID-19 several months before the study.

The study also found that Black adults and people from lower income households had much lower rates of protective immunity, even though they had higher rates of antibodies from past infection.

“These communities were hit on both sides: they generally had lower vaccination rates, especially in the first few months that vaccines were available, and they also were harder hit by the earlier waves of COVID,” Sood said.

The research team hopes policymakers and public health officials see the results and reconsider how COVID-19 will be addressed long-term.

“Testing the symptomatic, ensuring access to new treatments and encouraging vaccinations for high-risk populations should be the pillars of our pandemic response going forward,” Sood said. “These findings indicate that preventing COVID surges may be increasingly unrealistic, but we can ensure our healthcare system and hospitals have the needed capacity and patients have the care they need.”

The study was co-authored by Sood, Olivier Pernet, Chun Nok Lam, Angela Klipp, Rani Kotha and Andrea Kovacs.

Support was provided by Conrad N. Hilton Foundation, Office of the President of the University of Southern California, Los Angeles County Department of Public Health, U.S. Centers for Disease Control and Prevention, the Department of Population and Public Health Sciences of Keck School of Medicine of USC, Schaeffer Center for Health Policy & Economics, and Keck Family Foundation.

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