For the average healthy adult, the flu can be disruptive — long days of sore throat, fever and coughing cause people to miss school and work, and also just feel miserable. But for very young children, older adults and people with certain health conditions, seasonal flu can lead to serious complications, a hospital stay or even death — tens of thousands of people die of seasonal flu each year in the U.S.
To cut down on flu hospitalizations and deaths, people who catch the flu should do their best not to pass the infection to others. But how long is someone with the flu contagious?
A good rule of thumb is to wait at least 24 hours after your flu symptoms have subsided to interact with other people, said Dr. Donald Milton, a professor of environmental health at the University of Maryland School of Public Health.
"If you're a day without symptoms after the flu, you should be pretty safe at that point," Milton told Live Science. But there's some nuance to how long someone might be contagious, and there's ongoing debate about exactly how the flu spreads.
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Respiratory viruses such as influenza spread through three main pathways: viruses left on surfaces that people accidentally touch, larger droplets expelled onto a person through coughing or sneezing, or in tiny airborne particles released through breathing. Experts think people spread the flu primarily through the latter two routes — large and small droplets — but contaminated surfaces can still be a source of transmission.
The specifics and relative importance of each transmission pathway remain big topics of research in the field, Milton said. Virus transmission is tricky to study in a controlled environment — the research that yields the most useful information requires finding volunteers willing to catch the flu for science. (And yes, there are willing volunteers.)
According to the U.S. Centers for Disease Control and Prevention, people with the flu tend to be most contagious during the first three days of their symptoms, and studies have found that they "shed" the largest volume of virus into their snot and saliva during this period. The chances of spreading the illness fall after this peak, but adults can still remain contagious five to seven days after their symptoms appear.
The picture is more complicated for adults with certain health conditions, such as those that compromise immune function. People with weakened immune systems can take longer to fight off an influenza infection, so they may remain contagious longer than is typical. Conditions such as diabetes, which also affects immune function, may also influence how long someone sheds the virus, Milton said.
For children, the picture is also complicated — kids are prolific flu spreaders for a reason. Like adults, they're most likely to pass on the flu during the first few days of illness, but children can remain contagious for about one or two weeks after their symptoms start. Milton said this is likely because children's immune systems don't have as much experience overcoming the virus as adults' do, because on average, they've been exposed to flu viruses and flu vaccines fewer times.
Notably, the varieties of flu that spread change year to year, and this can lead the disease to be more or less transmissible in a given season, Milton noted. Circulating flu viruses mutate slightly every year, which is why the annual flu shot must be updated routinely. Some years, the predominant viruses do a better job of bypassing the immune system's learned defenses, so infected people may remain contagious longer, he said.
Related: At-home flu vaccine approved by FDA — what to know
The flu vaccine is a great way to lower your chances of getting severely ill if you get the flu. But it likely won't do much to change how contagious you are.
"It's not clear that the current vaccines do very much for that," Milton said. "They mainly keep you from dying of it and keep you out of the hospital, but it doesn't appear that they do a very good job of preventing you from spreading it." However, he added this is still a big question that influenza researchers are striving to answer.
The most difficult aspect of preventing flu transmission is stopping the spread that happens before a person develops symptoms. Similar to what happens in COVID-19 infections, people start shedding enough flu virus to infect others about one day before symptoms start.
That's why respiratory viruses like the flu and coronaviruses are so hard to control, Milton said — you're dealing with discreet spread that happens when virus levels are still pretty low and people's symptoms are mild or nonexistent. But new research could one day counter that spread.
Scientists are investigating how a protein on the surface of flu viruses, called neuraminidase (NA), contributes to transmission. The body makes some antibodies against NA after a flu infection, and evidence suggests people with higher levels of NA antibodies may be less likely to pass the virus on.
Flu vaccines have traditionally focused on a different flu virus protein, called hemagglutinin (HA), rather than NA. If future research confirms the importance of NA, finding a way to boost people's NA antibody levels could be one way to slow the chain of flu transmission, Milton said.
This article is for informational purposes only and is not meant to offer medical advice.
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