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Almost 60% of healthcare workers who contracted COVID-19 but felt well enough to go back to work still tested positive on rapid antigen tests after isolating for 5 days, researchers found.

In one large urban medical center, 43% of rapid antigen tests during the Omicron wave were positive for vaccinated COVID-positive healthcare workers from days 5-10, with the largest proportion of positive tests on day 6 (58%), reported Emily Landon, MD, of the University of Chicago, and colleagues, writing in a pre-print on .

The findings call into question the CDC's recent change in isolation guidelines for the general public, which suggest that individuals with COVID-19 can end isolation after just 5 days if they're asymptomatic or have resolving symptoms without a negative test.

Interestingly, in the study, rapid tests were three times more likely to be positive among boosted healthcare workers versus vaccinated non-boosted healthcare workers on day 5 (61% vs 21%, respectively). Overall, tests from boosted healthcare workers were more than twice as likely to be positive on days 5-10 versus non-boosted workers (53% vs 27%).

However, co-author Michael Mina, MD, PhD, of digital healthcare company eMed in Miami, noted that this does not mean that boosters are causing people to be more infectious.

The data imply that people with strong immunity "are becoming symptomatic FAST after exposure," said Mina. And because of this, boosted individuals "may be starting their isolation 'clock' earlier in the infection."

"It means 5 days into the clock, you may be at peak infectiousness, not yet on downslope" regardless of if you're boosted or not, Mina added.

Landon's group commented on updated CDC contingency guidance that says healthcare professionals can return to work on day 5 after symptom onset, but without requiring a negative rapid antigen test. When asked about the guidance, CDC officials had said that individuals are most infectious prior to and in the first 2 days of symptom onset.

"Unfortunately, relatively less is known about the relationship between symptom onset and viral dynamics, particularly in vaccinated individuals, infected with the newer [Omicron] variant," Landon's group wrote.

They examined a cohort of non-immunocompromised healthcare workers who received at least two doses of mRNA vaccine and tested positive for COVID in late December 2021 during the Omicron wave. Landon's team added that workers were "allowed to return early if all symptoms had resolved excepting mild, intermittent cough and/or lingering loss of taste/smell, provided a rapid antigen test was negative upon return."

The researchers added that those with negative tests wore an N95 at all times and took their breaks and ate their meals apart from others. If they tested positive, they were allowed to test again 24-48 hours later until they received a negative test result or day 11 if they had no symptoms.

From Jan. 2-12, 2022, the investigators analyzed a total of 309 rapid antigen tests from 260 healthcare workers on days 5-10 of their illness.

Given these qualifications, the most common day healthcare workers returned to work was day 7 (26%), followed by days 6 and 5. Landon's group added that there was "little difference" in the rates of first return on day 5-7 among boosted healthcare workers (23%-25%), but those who were vaccinated and non-boosted were more likely to return on day 7 (28%).

Limitations to the data include that worker participation was voluntary and "limited to work areas with critical staffing needs." The authors did not collect data about when workers' symptoms resolved, and thus did not take into account non-medical factors that influenced the timing of testing.

"Our data strongly suggests that safely exiting isolation as early as day 5 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," Landon and co-authors concluded.

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