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days after their first positive test, falling to 25% on day six and 13% on day seven, according to the study, which has not been peer-reviewed.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 January 10, 2022.
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.”