With Covid and flu surging at the same time, how big is the risk of coinfection?

While the U.S. is only a few weeks into winter, this year’s respiratory virus season is already looking different than last year’s. Covid and flu are both surging and there’s no peak in sight.

During the previous winter, flu and RSV got off to an early start but were already declining by the time hospitalizations from the omicron virus started to spike in December 2022.

“What is new this year is we’re seeing both flu and Covid going up together,” said infectious disease expert Dr. Michael Phillips, chief epidemiologist at NYU Langone Health. “This is different.”

At NYU Langone, positive tests for flu and Covid are “neck and neck,” with slightly more people sick with Covid, Phillips said.

Dr. Helen Chu, a professor of infectious disease at the University of Washington, said providers are bracing for the impact of Covid and flu over the coming weeks in Washington state.

“The Pacific Northwest tends to lag behind the Southeast and the East Coast by a week or two,” she said. “I think it’s going to be a pretty bad season.”

The most recent data from the Centers for Disease Control and Prevention shows flu levels are either high or very high in at least 38 states, as of the week ending Dec. 30. The CDC reported an estimated 10 million illnesses from flu last week, up from 7 million the week before.

Covid-related hospitalizations are up 20% in the most recent week, the CDC reported Friday.

How common is coinfection?

With both viruses circulating at high levels, how likely it is for someone to get both at the same time?

Fortunately, becoming infected with Covid and the flu appears to be rare.

Last year, the CDC tracked coinfections in the U.S. and found that 3% of people hospitalized with the flu also tested positive for Covid. CDC Director Dr. Mandy Cohen told NBC News in December that they don’t appear to be that common this year, either.

“Right now we’re not seeing people who are testing positive for more than one respiratory virus at a time, but we are seeing a lot of illness,” she told NBC News. “We see coinfections at about a similar level to this time last season.”

John Wherry, director of the Institute for Immunology at the University of Pennsylvania, said protection from coinfection has to do with our body’s own defense system.

“There is a concept out there with data from animal models that one infection can make you more resistant to another infection occurring at the same time,” he said.

This phenomenon, called viral interference, is where infection with one virus ramps up the body’s immune system and can make it less likely to get infected with another virus.

“The immune response to one kind of shuts the door on the other,” said Dr. Jake Scott, an infectious diseases specialist at Stanford Medicine.

Scott, who treats patients at Stanford Hospital, is not currently seeing many people with flu and Covid.

“I can’t remember ever having a patient who tested positive for both,” he said. “I think it’s pretty uncommon.”

That can depend on the individual, however.

“A 22-year-old who has the flu and recovers fairly recently may have that inflammatory resistance to Covid-19 for some period of time — of days or maybe a week afterwards,” Wherry said. “Whereas the frail 87-year-old who has the flu and is already compromised physically and respiratory-wise and then gets Covid-19 a week later now can have a much greater effect.”

Evidence suggests that infection with Covid and influenza can make someone sicker.

A meta-analysis of 95 studies, published in November in the International Journal of Infectious Diseases, found people infected by both Covid and flu at the same time were more likely to need to be on a ventilator or to die, particularly if infected by the influenza A strain, which tends to cause more severe illness.

There have been numerous reports this season that people can’t shake a lingering cough or runny nose, but that’s not necessarily unusual and doesn’t mean someone has more than one virus.

Even a normal bout of respiratory infection can take several weeks to recover from.

Stanford’s Scott said the high rates of both viruses right now are a good reason to be up to date on Covid and flu vaccines.

Should I test for flu and Covid?

Does it matter whether you’re sick with Covid or flu?

It might. For example, it make a difference in how quickly someone gets back to a regular routine.

With Covid, the CDC recommends isolating for five days after a positive test or from symptom onset. With flu, people are considered most contagious for the first three days of their illness. The CDC recommends staying home until at least 24 hours after fever is gone.

Also, knowing which of the two viruses you are fighting can be important for people at high risk of having a bad infection, said Phillips. For people who are 65 or older, or pregnant or immunocompromised, the antiviral Paxlovid can help protect against hospitalization if taken in the first few days of a Covid illness. If it’s flu, a doctor can prescribe the antiviral Tamiflu.

Early last year, the Food and Drug Administration authorized the first at-home test that can differentiate and detect both Covid and flu. A new federal program called Test to Treat is providing free access to the tests.

For everybody else who is not at high risk of severe disease, Scott’s advice is simple — stay home if you’re sick.

“I get texts all the time from friends all over the country saying, ‘Hey, you know, I feel like I’m coming down with something, should I get tested,’” he said.

In his view, if you aren’t high risk and don’t qualify for Paxlovid, a positive test won’t change much for Covid.

Treatment is the same for Covid and flu — supportive, meaning things like adequate rest, hydration and isolation.

Ultimately, what’s most important are the shots.

“If you’re up to date on your Covid and flu vaccines, there’s nothing to worry about.”