Estimated 1 in 150 San Franciscans Have COVID-19

Workers wearing personal protective equipment gather COVID-19 tests administered in Los Angeles
Photo credit Kevin Winter/Getty Images

The COVID-19 positivity rate in California has dropped to 4% over the last 14 days, continuing a downward trend that has persisted for several weeks.

In some parts of the Bay Area, that number is even lower. The positivity rate in San Francisco and Napa counties has dropped below 3%, although it is more than double that in Sonoma County with a rate of 6.7%.

Dr. Bob Wachter, Chair of the Department of Medicine at UC San Francisco says the positivity rate can help people figure out just how risky certain activities are.

At UC San Francisco, about 4-5% of people who come to the hospital with symptoms for COVID-19 end up testing positive for the virus.

That number drops dramatically when you look at people who come into the hospital without any symptoms. All patients at the hospital undergoing procedures, such as heart surgery, are tested for the virus. “That number is running about 0.6%, meaning that less than 1 in 100, more like 1 in 150 people with no symptoms have it,” said Dr. Wachter. “That’s the number I use to try to figure out, what are the chances that the person next to me in the elevator or in front of me in line at the Safeway have it?”

Dr. Wachter says while many people are aware of the relative risks of becoming infected with the virus through activities such as going out for a walk, grocery shopping or attending a large gathering, the prevalence of the virus is another important factor.

“The other thing that you’re paying attention to is what is the risk that the average person that I’m standing next to has the infection?” he said.

That means the relative risk of grocery shopping in an area like San Francisco is lower than in another part of the country where the virus is more widespread.

“If you knew for sure the person had no symptoms; the problem is the person in front of you at Whole Foods, you don’t know for sure,” he clarified. “But that’s the number that I use as a rough, back-of-the-envelope calculation of the person whose asymptomatic in San Francisco that actually has COVID.”

Dr. Wachter says this is not reason to become complacent, as the virus is just as infectious now as it was back in March or April. He wants to see the overall positivity rate drop to 1%, like in New York City.

But keeping track of the positivity rate can help people evaluate how comfortable they feel participating in certain activities.

Common sense prevention measures such as hand washing, social distancing, avoiding indoor gatherings and wearing a mask can also dramatically lower your risk of contracting the virus.