A simple COVID-19 question with a complex answer

More than two years into the pandemic, researchers are still struggling to answer a crucial  question about the virus: How long is someone with COVID-19 infectious? Though the question seems simple, the answer is anything but, scientists and infectious disease experts told Nature.

In December, the CDC trimmed its recommended isolation time for people with COVID-19 from 10 days to five, noting that most virus transmission occurs one to two days before symptom onset and two to three days after. However, scientists have pointed to mounting research that suggests people may be infectious well into the second week of their illness, according to the July 26 Nature report. 

"The facts of how long people are infectious for have not really changed," Amy Barczak, MD, an infectious-disease specialist at Massachusetts General Hospital in Boston, told Nature. "There is not data to support five days or anything shorter than 10 days [of isolation]."

Various factors can affect how quickly SARS-CoV-2, the virus that causes COVID-19, clears from someone's system and they are no longer infectious. These factors include emerging variants, vaccination history and natural immunity, according to Benjamin Meyer, a virologist at the University of Geneva in Switzerland.

"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," he told Nature.

As a general rule of thumb, anyone still testing positive on a rapid antigen test, which detects proteins produced by an actively replicating virus, "shouldn't go out and interact closely with people who you don't want to be infected," Emily Bruce, PhD, a microbiologist and molecular geneticist at the University of Vermont in Burlington, told the publication.

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