A study measured whether people still tested positive for COVID after varying periods of isolation.
46% tested positive after the fifth day, and 41% overall who isolated between five and 10 days.
The authors suggested the positive tests mean those people could likely still spread the virus.
People who follow recommendations to stop isolating five days after testing positive for COVID-19, provided they have no symptoms, could still be contagious, a small study suggests.
The Centers for Disease Control and Prevention (CDC) updated its guidance in January to shorten the isolation period to five days and remove the need to test negative.
But a study from the University of Chicago found that a substantial proportion of people could still be testing positive, and might therefore be capable of infecting others, at day five.
The study, published Wednesday on preprint server medRXiv, followed 260 fully vaccinated health care workers, 141 of whom had also received a booster shot.
All those studied tested positive in late December 2021, by which time the Omicron variant had become dominant in the US.
The participants were told that they could return to work five to 10 days after first testing positive, so long as they had no symptoms and could show a negative rapid antigen test.
People who tested positive were not allowed to come back to work. Those who tested negative were told to wear N-95 masks and to eat and take breaks separately as an extra precaution.
Here is the data for who tested positive on each day of release:
Day 5: 46% of 52 fully vaccinated healthcare workers who took a test that day tested positive. These had received two or three doses of vaccine.
Day 6: 58% of 53.
Day 7: 38% of 68.
Day 8: 26% of 46.
Day 9: 25% of 28.
Day 10: 54% of 13.
Overall, 41% of the 260 tested positive on the day they sought to leave isolation, a mark that they were "likely still contagious," the authors said in the paper.
According to the study, people were most likely to test positive six days into their isolation, with 58% testing positive.
The study has not been peer-reviewed and the numbers of people involved were small, meaning its conclusions may not be definitive.
One of the study authors works for eMed, an at-home diagnostic testing company that sells COVID-19 tests.
The CDC recommends that those who test positive or have symptoms of COVID-19 should quarantine for at least five days then can leave quarantine if they are fever-free for at least 24 h without testing first.
Those who leave isolation should still take precautions until day ten, like wearing a mask around other people and avoiding traveling, per the CDC.
A recent study on 8,500 Danish households found that those who had gotten a booster were less likely to transmit the Omicron BA.1 lineage, which is dominating Omicron cases in the US, than those who were unvaccinated or fully vaccinated, although it is not clear how applicable these findings are to other settings.
Those who are vaccinated are at a much lower risk of death than those who are not, Dr. Rochelle Walensky, head of the CDC, said Tuesday.
Being fully vaccinated reduced the risk of death from COVID-19 by 14 fold, from 9.7 weekly average deaths per 100,000 people to 0.7 per 100,000 on the week ending December 4, per Walensky.
Having a booster reduced the risk 97-fold, to 0.1 per 100,000.
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