A significant proportion of people infected with the omicron variant of coronavirus were still contagious when they reach the U.S. Centers for Disease Control’s recommended self-isolation exit date of five days, according to a  Harvard University study of a small number of cases from the National Basketball Association’s Covid-19 testing program.


Among omicron cases identified  within a day or less of a previous  negative test result, more than half (13 of 25) were still infectious five days after their first positive test, falling to 25% on day six and 13% on day seven, according to the study, which has not been peer-reviewed.

Researchers classified patients as still contagious if the cycle threshold count on their PCR tests registered a value less than 30, which is a “proxy for potential infectivity and antigen test positivity.”

Researchers found that among the 70 identified omicron cases detected two or more days after a negative or inconclusive test result, 39% were still infectious after five days, declining to 33% on day six and 22% on day seven.

The paper used data from 10,324 PCR tests conducted among NBA players and employees between July 10, 2021, and


Last month, the CDC shortened its recommendation for self-isolation for asymptomatic people who had tested positive for Covid from 10 days to five days followed by five days of masking around others. The CDC’s decision to not include a test-out requirement in its updated guidelines drew significant criticism, and the Harvard study adds to a small body of evidence that suggests the shortened isolation period could lead to increased transmission of the virus. Omicron-specific research is limited, but a Japanese study from last week found that seven of 14 people with omicron tested positive three to six days after diagnosis. A December paper from the U.K. Health Security Agency found that 31% of all Covid-19 cases were still infectious five days after the first positive test.


Nathan Grubaugh, a Yale University epidemiologist and one of the study’s senior co-authors, explained his primary conclusion from the findings in a Thursday tweet, “Main take-away = ending isolation at day 5 should include a negative rapid antigen test. Otherwise isolation needs to be extended.”

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