Covid-19: at what exact moment does it stop spreading?

WASHINGTON.- You have Covid. How long do you have to isolate yourself? When can you resume your activities outside the home without fear of infecting others?

The answer is complicated and there is a caveat: The guidelines on this from the US Centers for Disease Control and Prevention (CDC) are nuanced and also a bit confusing.

those guidelines now are under review and could change. Several experts in infectious diseases consider that Before leaving isolation, it is advisable to carry out an antigen self-testIt takes just a few minutes to return the result. But so far the CDC guidelines consider it optional and do not explicitly recommend it.

But all the experts agree on one thing: each person and each case of Covid are unique and unrepeatable. Just as there is no rule to determine if a person is going to get sick a lot or a little, There is also no way of knowing how long you can spread the disease to others.. The CDC provides a general framework, but to assess the risks in each individual case, patients must consider their individual priorities and circumstances.

The coronavirus has a treacherous trait: is contagious, even before the sick person has symptoms. In general, the peak of virus shedding starts one or two days before the symptoms and lasts two to three days after that.

Though after day 5 the infected person is less likely to transmit the virusthe same can happen. Laboratory culture investigations show that those infected continue to shed virus for an average of eight days after testing positive.

According to the researchers, it is extremely unlikely that a person can infect another from day 10 after diagnosisalthough it continues to give positive in the tests.

According to the CDC, a Covid patient must be isolated for at least five days. On day 6 you can end the isolation, as long as the symptoms have improved and there is no fever.

One of the guidelines that can cause confusion is that referred to day 1: according to the CDC, the day of isolation is the day after starting with symptoms or testing positive. Therefore, if the sore throat started on Monday, that is not day 1, but starts counting from Tuesday.

After the disease has passed and even if the antigen test is negative, it is advisable to wear a mask until day 10 when we are with people, both in public places and at home. And not travel.

If we decide to do a quick self-test at home after several days of being infected, it is best to do it from day 5, advise the CDC. And if after five days of isolation the antigen test is still positive, you have to isolate yourself until day 10, after which the chances of transmitting the disease are almost nil.

On this point, the guidelines are not clear. At no time do they explicitly recommend that you have to test negative before leaving isolation.

But many experts believe that before doing a self-test of those that are bought and done at home.

“Since so many people are still testing positive on day 5, I think we should add the recommendation to do a self-test before coming out of isolation,” says Tom Inglesby, director of the Center for Health Security at Johns Hopkins University and advisor on testing. of the United States government.

“A negative antigen test is a pretty strong guarantee that we’re not going to spread the infection to anyone,” says Amy Barczak, an infectious disease expert at Massachusetts General Hospital, who is investigating how long infected people keep shedding viruses. In a recent study published in the New England medicine journal, Barczak and his colleagues found that for an average of eight days, those infected with the omicron variant continue to shed virus that can be grown in the laboratory.

Barczak says that people who after day five are negative in the self-test for antigens “are very unlikely to infect others.”

But people who for some reason must be especially careful not to infect anyone – for example, if they live with an immunocompromised person – would do well to repeat the test at least once.

Michael Mina, an immunologist at Harvard University and an expert in rapid antigen tests, says that two negative self-tests after 24 hours are an extra guarantee, like a door with two locks.

PCR is a molecular analysis that looks for the presence of genetic material from the virus. That test can detect even minute amounts of the virus, even before the body has enough viral load to infect others. PCRs are more useful before, to confirm if someone was infected with Covid, but they are not useful to determine if we are contagioussays Dr. Albert Ko, an epidemiologist and infectious disease specialist at Yale University.

In the presence of symptoms, the CDC recommends testing immediately. A negative PCR of a person with symptoms means that they most likely do not have Covid, but some other disease.

If someone has been in close contact with an infected person and is negative on a quick self-test, they may want to be more sure that they have not been infected, and in that case they can take a PCR test, Ko says. In general, PCR tests are tests of laboratory and the results take a couple of days.

Doing a PCR after being sick doesn’t make much sense, “because to an average healthy person, the PCR test will continue to be positive long after the disease has finishedBarczak says.

Either that antigen self-tests are more practical than PCR to quickly determine if someone is still contagious or no longer. When there are symptoms, the body expels more virus, and in those cases the antigen self-test is more reliable. but even asymptomatic patients can test positive for antigens, and be contagious to others. Most self-tests consist of a simple nasal swab that returns the result between 10 and 20 minutes later.

As antigen self-tests are quick and are basically a “contagiousness” test, experts recommend doing it – even if we feel fine and have no symptoms – before attending events with many people, especially if we are going to be among people in groups vulnerable to Covid, such as the immunosuppressed and others with comorbidities.

(Translation by Jaime Arrambide)