What science shows about being contagious

How can we stop the winter “tripledemic” of respiratory viruses from wrecking upcoming holiday travel and family gatherings? People know when they have Covid symptoms, but do minor sniffles at the end of a coronavirus infection, for example, mean they’re still contagious?

So many Covid questions! 

It’s a good time to brush up on what scientists know, and still don’t know, about how long people remain infectious with viral diseases — Covid, influenza, RSV — that are spreading across the U.S.

How long am I contagious with Covid?

If you’ve been exposed to someone who has tested positive for Covid, symptoms from any of the omicron subvariants generally appear two to four days later.  

Advertisement

How contagious you are is connected to how much of the virus, known as the viral load, is in your body. Your viral load will likely peak soon after the start of symptoms, according to Dr. Chanu Rhee, an infectious disease physician and associate professor of medicine at Harvard Medical School. This means you are likely most contagious a day or so before symptoms appear and during at least the first two or three days after.  

As your immune system reduces your viral load, your infectiousness should decline in tandem. 

“If folks are still very, very symptomatic, with a lot of coughing and a lot of upper respiratory symptoms, then the thought is they’re probably more infectious,” said Dr. Lisa. A. Cosimi, an infectious disease specialist at Harvard Medical School.

But determining whether you’re still somewhat infectious during the waning days of a Covid illness can be tricky.

The Centers for Disease Control and Prevention recommends that people who are experiencing moderate to severe symptoms to isolate for 10 days. If your Covid illness is mild, the CDC advises that to protect others you should consider yourself infectious for at least five days from the onset of symptoms — or five days after a positive test, which counts as day zero. 

A study of 66 people with nonsevere Covid published in the New England Journal of Medicine in July found that the median time until the participants were likely no longer infectious was six days when the delta variant of the virus was dominant, and eight days during the omicron wave.

According to an October study of 4,565 people with Covid, “overall, existing data suggest that infectiousness (using viral culture as a proxy) beyond 10 days is possible although less common.”

The investigators called for more research about whether people remain infectious after that point.

There have, however, been cases of immunocompromised people who have remained infectious for months, according to various infectious disease experts. 

As far as relying on Covid tests to determine whether someone is still contagious, PCR tests are good at diagnosing Covid. They are not reliable for determining ongoing infectiousness because they can detect fragments of the virus, which pose no risk of transmitting to others, for weeks or months after infection.  

At-home rapid antigen tests aren’t perfect either. Researchers rely on what are known as viral culture tests in which they take a swab of the coronavirus and see if they can grow the virus in the lab, as the best proxy for infectiousness. But everyday people don’t have access to such tests. 

In a recent study of 40 people with Covid, three quarters of them tested positive six days after their first positive test or the start of their symptoms, whichever came first.

As part of the research, the participants also received what are known as viral culture tests in which scientists see if they can grow the virus taken from a swab in the lab. It’s considered the best proxy for infectiousness.

About half of that group had a negative viral culture test at that time, suggesting they weren’t contagious. 

Cosimi, the Harvard Medical School infectious disease specialist, was the lead author of the study and said that after day five of Covid — if people test negative when they have improved yet still have lingering symptoms — they’re likely not contagious.

She also said that if non-immunocompromised people feel well by day 11 but still test positive on an antigen test, “it is exceedingly unlikely” they’re still infectious.

According to the CDC, if you have no symptoms after day five, or if your Covid symptoms are improving and you haven’t had a fever for 24 hours, you can end your isolation period.

For days six through 10, you can go out but should wear a mask.

You don’t need to test to end your isolation period in these contexts. The CDC says you can take the mask off early if you test negative on two consecutive rapid antigen tests 48 hours apart.

There’s a condition, though.

“You should still wear a mask through day 10 because theoretically, you’re still infectious, even if your degree of infectivity has gone down,” said Rhee.

How long am I contagious if I’ve had Covid or am fully vaccinated and have a breakthrough infection?

Several studies have shown that vaccination helps the body clear the coronavirus more quickly. 

Most of the studies, however, looked at patients infected with the older variants, Rhee said.

“The impact of prior immunity on infectiousness is not as clear for omicron variants.”

On average, unvaccinated people do indeed take longer to beat the infection, said Dr. Yonatan H. Grad, associate professor of immunology and infectious diseases at the Harvard T.H. Chan School of Public Health and senior author of a recent paper in the journal eLife in which he and his colleagues reached this conclusion. 

“We do not know about duration of infectiousness,” Grad said in an email.

If I’m taking Paxlovid, am I contagious?

A study published in Clinical Infectious Diseases in July found that taking Paxlovid accelerates coronavirus clearance, which the authors said might reduce infectiousness.

Recent research has cast doubt on whether Paxlovid is driving cases of viral rebound, given that people who don’t take Paxlovid commonly rebound as well.

“What we don’t know is whether those rebounds are infectious or not,” said Dr. Donald Milton, a professor of environmental and occupational health at the University of Maryland.

Consequently, the CDC recommends restarting the isolation clock at day zero if a rebound occurs.

How long am I contagious with the flu or other viral illnesses?

With flu, RSV (respiratory syncytial virus) and the common cold, experts say that people are generally most infectious between a day or two before symptoms begin and then for a few days afterward.

The CDC recommends that people with the flu stay home until at least 24 hours after their fever subsides without the use of fever- reducing medication or after their symptoms have improved — at least four to five days after getting sick.

A 2010 study identified a close correlation between the decrease of flu symptoms, in particular fever, and a decrease in viral shedding, which indicates infectiousness.

“If you’re really sick, if you’ve got a fever, you may be infectious for a little bit longer,” said Aubree Gordon, an associate professor of epidemiology at the University of Michigan School of Public Health. “The level of symptoms can be a guide for when you need to stay home. But that does not mean if you have mild symptoms you aren’t contagious.”

Classic common cold symptoms include congestion, cough, a runny nose and low-grade fever. Some cold symptoms can last from 10 to 14 days, according to the CDC.

For colds — which for adults may include RSV, a virus that can pose a serious threat to children under age 2 — Dr. David Strain, a senior clinical lecturer at the University of Exeter Medical School, said, “on the most part, you will only be infectious for as long as you’ve got symptoms.”