Placer COVID-19 infection rate likely triple number of confirmed cases

Vaccination offers stronger antibody response than natural infection, study suggests

AUBURN, Calif. – While the actual number of COVID-19 cases in Placer County is likely three times as high as the number of confirmed cases, Placer’s best path to reducing coronavirus infections is vaccination.

“Vaccination is the key to bring this pandemic to an end.”

Dr. Rob Oldham, director of Health and Human Services

Those are among the emerging findings of a five-month COVID-19 antibodies study outlined today in a report to the county Board of Supervisors.

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Placer County launched the study in October 2020 to better understand the COVID-19 infection rate among county residents. Led by Stanford University School of Medicine, study investigators tested blood samples from 2,035 residents between October 2020 and March 2021 for COVID-19 antibodies using rigorous methods.

Because some of those infected with the coronavirus may not have experienced symptoms or sought testing, the actual infection rate in Placer County was suspected to be higher than the number of confirmed cases.

Triple the number, below national rates

As of March 1, Placer had reported 19,861 cases of COVID-19. Preliminary study data suggest a 15.6% seroprevalence rate among participants by that date, indicating 62,139 cases had occurred – more than triple the number of confirmed cases.

National data through January suggest the infection rate across the country may be as high as four times the number of confirmed cases. The study findings suggest both Placer’s COVID-19 infection rate and fatality rate (estimated at .37%) are beneath national rates.

“The good news is that Placer County has done a great job protecting its most vulnerable people with masking and social distancing,” said the study’s principal investigator, Dr. Julie Parsonnet, Stanford Medicine Professor of Medicine (Infectious Diseases) and of Epidemiology and Population Health. “The bad news is there is a long way to go before people can let their guards down.”

Vaccination progress

Study findings also offered encouraging news about vaccination progress in Placer County. Vaccination surpassed infection as the main source of COVID-19 antibodies in study participants beginning in January. And people who were vaccinated, on average, had much higher antibody levels than people who acquired the virus naturally – in line with other studies indicating vaccination offers stronger protection than natural infection against future COVID-19 infection.

State data indicate that close to a quarter of Placer County residents have been fully vaccinated as of April 12. With appointments still available at the county-run vaccination clinic @the Grounds in Roseville as of April 11, Placer opened eligibility at the clinic to those aged 16 and older starting April 12, a few days ahead of the state target date of April 15. New pharmacy providers continue to come online to increase vaccination access. The county is also working with a variety of organizations throughout the community to promote vaccination and educate residents about the importance and effectiveness of vaccines.

“I want to thank all the partners who have contributed to our vaccination progress thus far – and we’ll keep it up,” said Dr. Rob Oldham, director of Health and Human Services and interim health officer. “Vaccination is the key to bring this pandemic to an end.”

The findings presented today will be adjusted to account for differences in Placer’s overall demographics and the study participants’, though researchers said they don’t believe the findings will change markedly. Stanford Medicine will provide the county with a full report in May and will publish their results in the scientific literature when the analysis is complete.