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‘I don’t know how they came up with this’: Mass. scientists question California’s new COVID-19 guidance

Experts say rapid testing is the best way to know if you need to isolate.

Does anyone even care about COVID? And does it matter?
WATCH: Doctors differ on the answer. Health reporter Adam Piore explains why.

Reflecting the growing recognition that the public is ready to move on from pandemic-era precautions, the state of California issued new guidance earlier this month dramatically loosening the amount of time it recommends residents isolate in the event of a COVID-19 infection.

In Massachusetts, some local medical experts and scientists who study the disease say they have concerns, with some warning the new approach could result in unwitting “presymptomatic” individuals unleashing “fire hydrants” of viral vectors into their communities.

Under California’s new guidance, individuals who have mild symptoms that are improving can return to work or school once they have been free of a fever for 24 hours, as long as they continue to mask and stay away from people at high risk of severe illness for 10 days. The rules, which have already been implemented by some of California’s largest school districts and the state agency that regulates workplaces, follow a course set by Oregon, which became the first state to break with the CDC and relax its isolation rules last May.

Other states are likely to follow suit in the weeks ahead. But so far there is no indication Massachusetts will be one of them. Though the state is aware of the changes in California and Oregon, there are no plans to change the state’s guidance, a spokeswoman from the state Department of Public Health (DPH) said Monday.


Since 2021, Massachusetts has followed guidelines set by the Centers for Disease Control and Prevention, which recommend that those infected with the disease should isolate for five days and continue to mask through day 10, even in the absence of symptoms.

Dr. Jeremy Faust, an emergency medicine physician and public health expert at Brigham and Women’s Hospital, said California’s shift is based on an approach known as harm reduction. The public is suffering from COVID fatigue. And “when people are going to do something or nothing, you offer something they are more likely to do,” he said.


Whether or not a person is contagious depends on the amount of virus present in their body, known as viral load. Viral load tends to peak early, but the timing of that peak varies widely among individuals. Faust notes that if an individual tests positive after an exposure but is asymptomatic, the new guidance would suggest they don’t have to isolate. But those individuals may in fact be highly contagious.

“You may have no symptoms until day three or four, and by that time your viral loads are off the charts, and you’re a super spreader,” he said.

“I don’t know how they came up with this,” he added. “I suspect that they just chose something that people could remember.”

California’s department of public health said in a statement that “we are now at a different point in time with reduced impacts from COVID-19 compared to prior years, due to broad immunity from vaccination and/or natural infection” and that treatments for COVID are readily available. Its new guidance, it says, aligns with guidance for other common respiratory viruses.

Faust said the only surefire way to know if you are likely to infect others is by taking a rapid antigen test. High viral loads, he said, tend to cause the really bright red lines on rapid COVID tests, and the lines tend to appear “very fast.” If that happens, he said, “I don’t care what symptoms you have, you need to isolate.”


Faust said that he is not opposed to updating guidance that relies on the smart use of testing.

Dr. Michael Mina, an epidemiologist and former assistant professor of epidemiology at Harvard T. H. Chan School of Public Health, echoed those comments.

“What California is missing is that symptoms do not reflect infectivity, and they never have,” said Mina, who is now chief science officer at eMed, a digital health care company that provides at-home health care testing. “We knew this from 2020. The whole problem with trying to contain spread was that we knew that half of all spread was asymptomatic. You could be completely asymptomatic and a super spreader.”

Rapid tests provide an “elegant, inexpensive, quick and convenient” solution, he said.

Faust said the best way to use the tests is to test more than once over time. If you only have one test at home and you are not able to immediately buy more, Mina suggests waiting three days after first feeling symptomatic to test “because the virus will have grown enough to be detectable on any test by that point,” he said.

But if you have multiple tests at home, you should test immediately. If you test negative, you should not assume you are negative, Mina said, and you should plan to test after another two days. (Every US household can order four free tests at covidtests.gov to be delivered to their home.)


If you’ve been exposed to COVID and have no symptoms, the CDC recommends masking around others and waiting at least 5 full days after the exposure before taking a test. If that test is negative, it recommends testing two more times at 48-hour intervals.

If you test positive, you should try to get additional tests and continue testing each day so that you can know when you are negative.

The tests are able to detect all the common COVID variants, including JN.1.

Material from prior Globe stories was used in this report.

Adam Piore can be reached at adam.piore@globe.com.