Cal to track asymptomatic COVID-19 spread by testing 5,000 people

Little is known about how many seemingly healthy people in the East Bay have COVID-19 — and how they’re unintentionally spreading it. These professors hope to change that.

external view of two big buildings
UC Berkeley public health professors are launching a major study of people in the East Bay who show no coronavirus symptoms but could have it. Photo: Google Street View

It’s common knowledge that someone can be infected with COVID-19 without showing symptoms: That’s why we’re wearing masks, shouting to healthy neighbors from across the street and picking up unattended takeout from curbs.

But beyond that, few details are known about what portion of people who have the coronavirus don’t show symptoms, how that affects spread throughout a community, and what demographic groups might be most susceptible. The U.S. Centers for Disease Control says a quarter of people infected with COVID-19 may be asymptomatic, but that’s based on studies in places with very different populations and policies than the Bay Area, say two UC Berkeley public health professors.

Those researchers want to understand much more about asymptomatic transmission of the coronavirus in the East Bay, so they’re launching a major longitudinal study following at least 5,000 local people who show no sign of the disease. They hope the results will guide policy and preventive measures.

“We have no idea how many asymptomatic people are walking around with detectable virus,” said Lisa Barcellos, who’s leading the study with Eva Harris. “This is a real-time capture of how things are now and how they’ll change.”


The researchers will come up with a 5,000-person group representative of the population in the East Bay — on the bay side, from Hercules through Oakland. Beginning in early May, they will mail those subjects home-testing kits five times each over the course of the next several months. Each time, they’ll send a questionnaire along with the kits, asking not only about subjects’ symptoms but also their work, compliance with the shelter-in-place order and more.

Ultimately Barcellos and Harris expect to produce a trove of data on asymptomatic spread by zip code, age, sex, and race/ethnicity. While there are disturbing reports across the country of higher COVID-19 death rates for black people, for example, there are not a lot of hard data on risk factors yet, and generally “what we know is coming from cases that were hospitalized,” said Barcellos. 

“A representative, population-based sample is much more informative from a public health perspective,” said Barcellos, professor of epidemiology and biostatistics.

The results could inform policy and help prevent further spread of COVID-19, said Harris, professor of infectious diseases.


“For understanding spread, for backing out of shelter-in-place, we’ll need to have a sense of both what has happened in this area and what will happen as certain mitigation strategies are developed,” she said.

The researchers thought using home kits would be safer and more accessible than requiring the subjects to repeatedly go to an in-person testing center. The tests will use saliva, swab and blood samples, and will be processed at the professors’ labs as well as the UC Berkeley Innovative Genomics Institute and the Biohub at UC San Francisco. The tests will identify the presence of antibodies, which would suggest a person has had the disease. Everyone from Berkeley Public Health master’s students to postdoctoral researchers and lab staff, as well as researchers from the College of Engineering, are working on the study.

The project is partially funded by Open Philanthropy.

A website including a volunteer solicitation will likely go live next week. The researches are also sending mailers to 300,000 households (only one person per household can participate, and anyone 18 or older is eligible). They say the initial responses will provide immediate data themselves, even if only a small portion of the volunteers will end up being selected as participants. All materials will be distributed in Spanish as well.

The Berkeley study can only reveal so much: the findings will not necessarily apply to other areas. The Bay Area is unique in many respects, include how early it ordered residents to shelter in place. (However, its dense population puts it at particularly high risk, said the researchers) The East Bay data also will likely not match the circumstances somewhere like Santa Clara County, where case numbers were much higher from the outset.

The researchers are hoping to expand their work into larger studies eventually.


“Believe me, that is what we’d love to do — to test everybody in the Bay Area and get back to life,” Barcellos said.

Correction: This article previously stated that 3,000 households will receive a promotional mailer about the study within information on how to volunteer. The mailer will actually be sent to 300,000 households.

Natalie Orenstein reports on housing and homelessness for The Oaklandside. She was previously a reporter for Berkeleyside. Email: natalie@oaklandside.org. Twitter: nat_orenstein.