Health Advice



Researchers studied COVID viral loads in vaccinated cases. Here's what they found

Michael McGough, The Sacramento Bee on

Published in Health & Fitness

SACRAMENTO, Calif. — Viral loads of the delta variant of coronavirus are similar between unvaccinated and vaccinated persons who are infected, as well as between symptomatic and asymptomatic cases, UC Davis and UC San Francisco researchers wrote in a recent study that aligns with similar findings from other research teams.

This does not mean that vaccinated people are as likely to spread COVID-19 as the unvaccinated, because the vaccinated are less likely to get infected in the first place. Vaccinated residents are also much less likely to grow severely ill or require hospitalization due to the virus, the researchers noted.

The research findings do, however, "underscore the continuing need for masking and regular testing alongside vaccination, especially in areas of high prevalence," the study's authors said, according to a Monday news release. The study is a preprint and has not yet been peer-reviewed.

UC Davis and UC San Francisco researchers surveyed 869 positive case samples: 500 gathered from testing in Yolo County and 369 from Unidos en Salud, a walk-up testing site in San Francisco's Mission District.

All 500 Yolo results were from asymptomatic tests; about three-quarters were unvaccinated. Of San Francisco samples, which included symptomatic and asymptomatic cases, 54% were unvaccinated.

"When they analyzed the data, the researchers found wide variations in viral load within both vaccinated and unvaccinated groups, but not between them," UC Davis officials wrote in a news release. "There was no significant difference in viral load between vaccinated and unvaccinated, or between asymptomatic and symptomatic groups."


"Our study adds to existing data about levels of virus in vaccine breakthroughs in two settings of high ongoing community prevalence of the delta variant," professor Richard Michelmore, director of the UC Davis Genome Center, said in a statement.

U.S. Centers for Disease Control and Prevention officials in late July returned to recommending face coverings regardless of vaccination status in areas with "substantial" or "high" rates of COVID-19 transmission. Director Dr. Rochelle Walensky in a statement called findings related to high viral loads in vaccinated breakthrough cases a "pivotal discovery" leading to that updated recommendation.

Samples in the UC Davis research were collected between June 17 and Aug. 31, a period in which the delta variant was the dominant strain and in which both counties fell under the CDC's definition of high transmission (more than 100 weekly cases per 100,000 residents).

San Francisco has since dropped to substantial transmission (between 50 and 100 per 100,000) while Yolo, like most counties across the U.S., remains in high transmission status, a CDC map shows.


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