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February 8, 2022

How long are health care workers infectious with Covid-19?

Daily Briefing

    A new study of health care workers who felt well enough to return to work after contracting Covid-19 found that many continued to test positive on rapid tests—even after they had isolated for the CDC's recommended five-day isolation period.

    Study details and findings

    For the study, which has not yet been peer reviewed, researchers examined a cohort of health care workers who received at least two doses of an mRNA vaccine and tested positive for the coronavirus in late December 2021 when omicron was prominent. From Jan. 2 to Jan. 12, the researchers analyzed 309 rapid antigen tests from 260 health care workers on days 5-10 of their illness.

    The researchers found that 43% of health care workers who contracted Covid-19 but felt well enough to go back to work still tested positive on rapid tests after isolating for five days. The highest proportion of positive tests was 58% on day six.

    Experts say that individuals who test positive on rapid antigen tests are likely to be infectious.

    The most common day health care workers returned to work was day 7, followed by days 6 and 5. According to the researchers, vaccinated health care workers who did not receive booster shots were more likely to return to work on day 7. Among vaccinated health care workers with booster shots, there was no significant difference in whether they returned on day 5, 6, or 7.

    Notably, the researchers found that rapid tests among boosted health care workers were three times more likely to be positive on day 5 compared with those among non-boosted workers (61% vs. 21%, respectively). Overall, tests from boosted health care workers were twice as likely to be positive on days 5-10 compared with non-boosted workers (53% vs 27%).

    According to Michael Mina, a co-author of the study and chief science officer at eMed, boosted workers being more likely to test positive does not mean that boosters are causing people to be more infectious. Instead, Mina said the data suggests people with strong immunity "are becoming symptomatic FAST after exposure," which means boosted individuals "may be starting their isolation 'clock' earlier in the infection."

    "It means five days into the clock, [the workers] may be at peak infectiousness, not yet on the downslope" regardless of if they're boosted or not, he added.


    According to MedPage Today, the findings suggest that many individuals remain infectious even after following CDC's current isolation guidelines, which say individuals with Covid-19 can end their isolation after five days without a negative test if they no longer have symptoms.

    When previously asked about that guidance, CDC officials said individuals are most infectious prior to and during the first two days of developing symptoms. In particular, the guidance was partly based on unpublished modeling data on the spread of the delta variant that found the risk of viral transmission decreased to 13% after five days of testing positive.

    "We know that, let's call it 85%, of your transmissibility time is already behind you," CDC Director Rochelle Walensky said previously. "A minority of it is in front of you. And if you wear a mask, you can avert it," she said.

    Similarly, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said that "the likelihood of transmissibility is considerably lower in [the] second half of a 10-day period" which is why "CDC made the judgement that it would be relatively low risk to get people out."

    However, the study's authors noted that "relatively less is known about the relationship between symptom onset and viral dynamics, particularly in vaccinated individuals, infected with the newer [omicron] variant."

    Overall, the authors concluded that the study's findings "strongly [suggest] that safely exiting isolation as early as day [five] since symptom onset should include a negative [rapid antigen test] prior to exit, regardless of vaccination or booster status and should certainly not be predicated on symptom resolution alone."

    In response to criticism of its initial isolation recommendations, CDC last month updated its guidance, suggesting that those who have access to a Covid-19 test and want to take one should take an antigen test near the end of their five-day quarantine.

    If their antigen test is positive, CDC said the infected person should keep quarantining until day 10. If the test comes back negative, the guidelines say the person is able to end isolation at day five but should continue wearing a well-fitting mask for an additional five days around people at home and in public.

    But according to Stephen Kissler, a postdoctoral fellow at the Harvard T.H. Chan School of Public Health, some individuals can test positive with rapid tests for up to 14 days, and, according to Alberto Paniz-Mondolfi, an associate professor at the Icahn School of Medicine at Mount Sinai, even longer with PCR tests.

    Benjamin tenOever, a microbiologist at the NYU Grossman School of Medicine, says individuals could potentially still be transmissible if they continue to test positive, but other experts note that testing out of quarantine might not be feasible for everyone. Kissler says individuals can slowly reintegrate while being mindful of contact and wearing a protective mask.

    Some overwhelmed hospitals are asking staff to return to work quickly—or, in some cases, never stop working at all. The practice is allowed by CDC guidance released in December, 2021, which allows institutions to speed up health care workers' return via several strategies.

    So, while some hospitals ask their Covid-positive employees to continue caring for patients without testing out of isolation due to staffing and test shortages, many do so while requiring workers to wear highly protective masks, attempting to balance ensuring "employees can safely return to work while protecting our patients and staff from the transmissibility of Covid-19," according to Dignity Health. (Walker, MedPage Today, 2/4; Landon et al., medRxiv, 2/2)

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