What are the odds you’ll get COVID from someone who’s asymptomatic? Here’s how to measure the risk now – San Francisco Chronicle

How likely is it for people to catch COVID from someone who is asymptomatic? Its not impossible and may be more common than people realize, health experts say.

With coronavirus cases currently stuck at a high level across California as fast-spreading offshoots of the omicron variant crowd out their competitors, that means the chances of getting COVID from an asymptomatic person are heightened.

An estimated 5%, or about 1 in 20, asymptomatic patients coming to UCSF for procedures unrelated to COVID are testing positive for the virus, said Dr. Bob Wachter, one of the universitys leading infectious disease experts.

UCSF asks patients who show symptoms of COVID to postpone their visits. Patients are routinely screened upon arrival, meaning that those who test positive are asymptomatic or pre-symptomatic.

Though its not a perfect sample pre-op patients are more likely to be older, have an illness and be vaccinated because theyre seeking other medical care its a convenient way to measure how many people in the community may be infected and go about their day-to-day lives, Wachter said.

Its been a very useful measure of something that otherwise is very hard to get your arms around, which is the probability that somebody standing next to you in line at a Safeway in the Bay Area would test positive for COVID if I could test them now, he said.

The math is sobering: At the current 5.5% rate of asymptomatic test positivity, In a group of 50 people, I think its a 95% chance that at least one person will be positive, Wachter said. On an airplane of 150 people, theres over 99% chance theres somebody on the plane who has it.

Asymptomatic infections are not uncommon. Many people who routinely test themselves for COVID have received positive test results, including Vice President Kamala Harris.

But guidance from the Centers for Disease Control and Prevention does not advise vaccinated people who have been exposed to COVID-19 to quarantine unless they develop symptoms.

So although many people may have come in close contact with someone diagnosed with the coronavirus, they may not take the same stringent measures for mitigating transmission that they would have two years ago.

We can get a sense there are probably more people asymptomatically infected with SARS-CoV-2 with omicron and its subvariants than there were with delta, said Dr. John Swartzberg, a UC Berkeley infectious disease expert.

A 2021 study in the JAMA Network Open medical journal found that as many as 60% of COVID infections were transmitted by an asymptomatic person.

From the public health standpoint, the public needs to understand you can feel perfectly well and be infected and transmit this virus, Swartzberg said.

Obviously, infected people who show signs of sickness may generate more aerosols by sneezing or coughing. But while asymptomatic people shed less virus, they can still spread the virus without knowing theyve got it.

Fortunately, someone who is vaccinated has less viral load in nasal and throat secretions and vaccination rates for most Bay Area counties are among the highest in the nation, according to state and federal data.

But its still possible to have high viral load without showing signs of sickness, said Dr. Abraar Karan, an infectious diseases expert at Stanford.

So what is responsible for driving the current surge? The recent variants of the coronavirus are many times more infectious than the original one back in 2020 and even more contagious than the omicron version last winter. Combine that with a relaxation of public health mandates on local, state and federal levels, and the opportunities to spread the virus, whether by symptomatic or asymptomatic people, are many times higher.

People who have very mild symptoms, such as a cough or sniffles, and who dont know that they have COVID may also be going out under the assumption that they have a cold or allergies, Wachter said.

How can people protect themselves? The high asymptomatic test rate combined with the plateau in infections in recent weeks has public health experts recommending outdoor dining, voluntary indoor masking and opting for a road trip versus a flight.

I prefer not to take those risks, Wachter said.

Gwendolyn Wu (she/her) is a San Francisco Chronicle staff writer. Email: gwendolyn.wu@sfchronicle.com

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What are the odds you'll get COVID from someone who's asymptomatic? Here's how to measure the risk now - San Francisco Chronicle