How long does a COVID infection last? What scientists know so far


It is difficult to measure how long a person with COVID-19 will remain contagious.Credit: Agustin Markarian/Reuters/Almy

When the US Centers for Disease Control and Prevention (CDC) in December halved the recommended isolation time for people with COVID-19, it said the change was motivated by science. Specifically, the CDC reports that most SARS-CoV-2 infections occur early in the illness, one to two days before symptoms begin and two to three days afterward.

Many scientists have opposed that decision and continue to do so. This disagreement is supported by a number of studies that confirm that many people with COVID-19 remain infectious until the second week after experiencing the first symptoms. The proposed reduction in the length of the isolation period – now common around the world – is policy, they say, given any reassuring new information.

“The facts about how long people are contagious haven’t changed,” says Amy Barczak, an infectious disease specialist at Massachusetts General Hospital in Boston. “There is no information shorter than five days or ten days [of isolation].” Barchak’s own research, published on the medRxiv preprint server, suggests that a quarter of people who contract the Omicron variant of SARS-CoV-2 may still be infectious after eight days.1.

numbers game

Although the question is simple – how long is a person infected with COVID-19 contagious? — experts warn that the answer is complicated. “We always think of it as a black and white thing … if someone is contagious or not, but really it’s a numbers game and probability,” says Benjamin Meyer, a virologist at the University of Geneva in Switzerland.

And this numbers game has variable rules and baselines. Newly emerging variants, vaccinations and different levels of natural immunity from previous infection can affect how quickly someone can clear the virus from their system, Meyer says, and that ultimately determines when they stop being infectious. Behavioral factors are also important. People who feel sick are less likely to interact with others, he adds, so the severity of someone’s symptoms may affect how likely they are to infect others.

Most scientists believe that PCR tests can give positive results even after someone is not infected. This happens when tests that detect viral RNA pick up the non-infectious debris left behind after most of the live virus has been killed.

In contrast, lateral flow (or “rapid antigen”) tests provide a better guide to infectivity because they detect proteins produced by an actively replicating virus.

“We don’t have all of this exactly, but if I can sum it up in one short message, if the antigen is positive, you should not go out and have close contact. People you don’t want to infect, says Emily Bruce, a microbiologist and molecular geneticist at the University of Vermont in Burlington.

What about the person who tested negative for a few days on the lateral flow test but still has a fever and a bad cough? Prolonged symptoms may seem serious and serious, but they don’t necessarily mean the infection is ongoing, Bruce said.

“You can definitely have symptoms for longer than if you tested positive for lateral flow,” he says. “I think a lot of these symptoms are caused by the immune system, not the virus itself.”

transmission tests

In countries such as the UK, the relaxation of isolation guidelines has coincided with the withdrawal of free lateral flow tests. So, assuming that most people who follow the new recommendations will stop isolating after five days, scientists are investigating, in particular, how many people with COVID-19 may remain infectious after that point.

It is impractical to monitor direct transmission of the virus from large numbers of people and measure its decline over time, so researchers rely on proxy measurements to wait for people to stop being infectious.

Researchers with access to a biosafety level 3 laboratory — like Barczak — can do this by running experiments to see if live SARS-CoV-2 can be cultured from patient samples over several consecutive days.

“If you’re still shedding virus from your nose, you still have the potential to infect other people,” he says. After different options emerged and different research groups conducted these experiments, Barczak said, a consensus emerged that it was unusual for people to produce a cultured virus after ten days. “So it’s unusual for people to remain infectious after ten days,” he says.

Other studies take it a step away from the real world and use levels of viral RNA, measured by PCR tests, to determine whether someone is infectious. This makes it easier to work with large sample sizes. For example, a project carried out by the Crick Institute and University College Hospital in London can rely on PCR tests carried out on more than 700 participants, taken after the onset of symptoms.

This cohort-based study found that many people maintained a viral load high enough to cause infection for seven to ten days, regardless of the type of variant or how many doses of the vaccine people received. The study was published on the medRxiv preprint server on July 10two.

“We’re not measuring live virus, but there’s work in the literature right now that provides a good map of how a high viral load can lead to an infectious virus,” said David L.W. Bauer, a virologist at the Crick Institute, an investigator on that study. “So it’s not a perfect picture. , reasonable.”

“Resurgence Phenomenon”

Jonathan Grad, an infectious disease specialist at the Harvard TH Chan School of Public Health in Boston, Massachusetts, who works on PCR-based infection studies, agrees that ten days is a useful rule of thumb that people should no longer use. to be contagious. But he cautions that a small number of people may still be contagious after that point.

Some of those cases in the United States have been linked to the common antiviral drug Paxlovid (nirmatrelvir-ritonavir), he said. “People see their symptoms go away and they may test negative on a rapid test, but after a few days the symptoms and the virus come back.”

According to Barczak, this is one of the main questions that researchers are currently investigating. “Antivirals change the dynamics of symptoms, change the dynamics of the immune response, and change the dynamics of how you shed,” he says. “I think it’s very important because people think that after ten days the world is not contagious. But if they have Paxlovide’s resonance, they might.

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