Nearly 50,000 COVID cases likely went unreported in a Northern California county
A study with Stanford University to better understand the prevalence of COVID-19 in Placer County found that about 68% of cases went unreported.
Dr. Julie Parsonnet, professor of epidemiology and population health at Stanford University, told the county Board of Supervisors in a presentation Tuesday that seropositivity rates, meaning the presence of antibodies over time, were “pretty darn low” and nowhere near the threshold for herd immunity, making the vaccine all the more important for the county’s fight against the disease.
Starting in October, researchers began mailing questionnaires and blood tests to 20,000 people across different census tracks to obtain a representative sample. By March, 2,035 had responded, Parsonnet said.
The number of senior adults and college-educated adults overwhelmingly responded to the mailings, whereas other population groups, such as people with only a high school diploma, did not respond with nearly the same frequency.
“We have an over sample of the elderly. They responded more to our requests than the census,” Parsonnet said. “... And the biggest difference, I think, is in education level. The people who tended to respond were more likely to have a bachelor’s degree or graduate degree.”
The results show that about 49,000 cases, many asymptomatic, went unreported. In comparison, by mid-March the county had 19,681 reported cases.
And when the data is adjusted for underrepresented populations, Parsonnet said it is likely there were even more cases.
“We also know that our test is imperfect and it may miss up to 20% of cases because their antibodies haven’t occurred yet or they’re just antibody negative. Some people just never develop antibodies and with that we estimate there might have been 62,000 cases in Placer County, and that means about 68% of cases would have gone unreported,” she said. “ ... That number is pretty consistent with what we’ve seen on a national level.”
The highest rates of infection were detected in those who have only a high school degree, and those who have a graduate degree have the lowest rates of infection, she added.
Parsonnet said the COVID-19 vaccine has helped tremendously in raising rates of antibodies and essentially doubled the proportion of people who are now potentially immune.
Overall, Parsonnet said there are “optimistic signs” that Placer is moving in the right direction in terms of suppressing the spread of the virus and effectively administering the vaccine.
Dr. Rob Oldham, director of health and human services for Placer County, told the board that nearly 250,000 doses of the vaccine were administered Tuesday, surpassing half of the county’s total population. And about 75% of all people 65 and older were at least partially vaccinated.
The lowest rates of vaccine adoption were among young people 18 to 24, but Oldham said that’s in part because young people have only just recently become eligible for vaccination. People 16 and older were cleared to start receiving the vaccine at the county’s clinic in Roseville this week.
“The focus on younger people has really just started,” he said. “So we’re encouraged and hopeful that we can have enough vaccine now to meet the needs of younger people.”
This story was originally published April 14, 2021 at 4:45 PM.