UCSF doctor calculates COVID risk by focusing on asymptomatic rates

SAN FRANCISCO - A doctor at the University of California at San Francisco is putting COVID risk numbers in a new light.

When you leave your house and go to the store, it’s hard to know whether your chances of coming across an asymptomatic COVID carrier is 1 in 10 or 1 in 1,000.

Those kinds of numbers just aren’t available.

But the chair of the UCSF Department of Medicine is using both public and private data to calculate that type of risk.

Right now, something as simple as shopping may not feel much different than it did months ago, but it’s actually riskier as COVID cases climb.

"I was comfortable flying to visit my 90-year-old father in August, I would not be comfortable today," said Dr. Bob Wachter.  "There’s nothing about the airplane that’s changed, but the chances that somebody sitting around me is infected is many, many times higher than it was in August, so I wouldn’t do it today."

UCSF Medical Center tests all incoming patients for COVID, whether they have symptoms or not, and Wachter has used that data to put together probabilities.

On Twitter he posted a graph showing UCSF COVID cases put in two buckets: in 10% of cases patients have symptoms, but 2.1% of patients have no symptoms at all.

The asymptomatic rate is an important number for Wachter.

"It’s my poor man’s way of telling me, if I’m in the store surrounded by people, what are the chances in San Francisco that someone who feels perfectly well actually has COVID right now?" he asked. 

In his tweet, he estimated that number is roughly 1 in 47, right now.

"A month ago at UCSF that number was... about 1 in 200 people," he said. 

For other cities with higher positivity rates like Los Angeles, he puts probabilities at more like 1 in 14, and that’s for COVID carriers with no symptoms.

 And those carriers are the most dangerous.

"It turns out your period of maximum infectiousness is actually before you start having any symptoms at all."

Unlike most hospitals and health departments, UCSF is unique in that it tracks asymptotic covid cases.

But Wachter says by taking publicly available data on positivity rates and cutting that number in half, you can get an approximate rate of asymptomatic carriers in your region and do your own math to determine risk.