Via Nature.com: How long is COVID infectious? What scientists know so far. Excerpt:
When the US Centers for Disease Control and Prevention (CDC) halved its recommended isolation time for people with COVID-19 to five days back in December, it said that the change was motivated by science. Specifically, the CDC said that most SARS-CoV-2 transmission occurs early in the course of the illness, in the one to two days before the onset of symptoms and for two to three days after.
Many scientists disputed that decision then and they continue to do so. Such dissent is bolstered by a series of studies confirming that many people with COVID-19 remain infectious well into the second week after they first experience symptoms. Reductions in the length of the recommended isolation period — now common around the world — are driven by politics, they say, rather than any reassuring new data.
“The facts of how long people are infectious for have not really changed,” says Amy Barczak, an infectious-disease specialist at Massachusetts General Hospital in Boston. “There is not data to support five days or anything shorter than ten days [of isolation].” Barczak’s own research, published on the medRxiv preprint server, suggests that one-quarter of people who have caught the Omicron variant of SARS-CoV-2 could still be infectious after eight days.
A numbers game
Although the question is simple — for how long is someone with COVID-19 contagious? — experts caution that the answer is complicated. “We always think of it as a black and white thing … if somebody’s infectious or not infectious — but in reality, it’s a numbers game and a probability,” says Benjamin Meyer, a virologist at the University of Geneva in Switzerland.
And that numbers game has shifting rules and baselines. Emerging variants, vaccination and varying levels of natural immunity provoked by previous infection can all influence how quickly someone can clear the virus from their system, Meyer says, and this ultimately dictates when they stop being infectious.
Behavioural factors matter as well. People who feel unwell tend to mix less with others, he adds, so the severity of someone’s symptoms can influence how likely they are to infect others.
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