For a healthy adult, the flu can be difficult – long days of sore throat, fever and cough cause people to miss work and feel very ill too.

But for very young children, older adults and people with certain health conditions, seasonal flu can lead to serious complications, hospitalization or even death – tens of thousands of people die from seasonal flu each year in the US Living science.

How long is a person with the flu contagious?

To reduce hospitalizations and deaths from the flu, people who get the flu should do everything they can to avoid spreading the disease to others. But how long is a person with the flu contagious?

A good rule of thumb is to wait at least 24 hours after flu symptoms have subsided to interact with other people, said Dr. Donald Milton, professor of environmental health at University College London’s School of Public Health. Maryland.

“If you’ve gone a day without symptoms, you should be pretty safe at that point,” Milton told Live Science. But there are some nuances to how long someone can be contagious, and there is an ongoing debate about exactly how the flu spreads.

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Respiratory viruses are spread in three main ways: viruses left on surfaces that people accidentally touch, larger droplets spread on a person by coughing or sneezing, or in the small particles of air released by breathing.

Experts believe that people spread the flu mainly through the latter two routes – large and small droplets – but contaminated surfaces can still be a source of transmission.

The details and relative importance of each transmission pathway remain important research topics in the field, Milton said.

Virus transmission is a difficult thing to study in a controlled environment – the research that yields the most useful information needs to find volunteers willing to get the flu for science.

According to the US Centers for Disease Control and Prevention, people with the flu tend to be most contagious during the first three days of symptoms, and studies have found that is when the greatest number of virus in the nasal mucosa and saliva.

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The chance of spreading the disease decreases after this peak, but adults can remain contagious for another five to seven days after symptoms appear.

People with a weak immune system

The situation is more complicated for adults with certain health conditions, such as those that compromise immune function.

People with weakened immune systems may take longer to fight off the flu infection, so they may stay contagious longer than usual.

Conditions such as diabetes, which also affect immune function, can affect how long a person spreads the virus, Milton said.

What is the situation with children?

For children, the situation is also complicated – children are great spreaders of the flu for a reason.

Like adults, they are more likely to pass the flu in the first few days of illness, but children can remain contagious for a week or two after symptoms start.

Milton said this is likely because children’s immune systems are less prepared to fight the virus than adults because, on average, they have been exposed to flu viruses and less frequent flu vaccines.

In particular, the strains of flu that circulate change from year to year, and this can cause the disease more or less in a given season, Milton noted.

Annual flu vaccine

Flu viruses in circulation undergo small mutations every year, which is why the flu vaccine needs to be updated regularly.

In some years, it has been observed that common viruses are able to bypass the immune system’s learned defenses, so infected people can remain infectious longer, he said.

The flu vaccine is a great way to reduce your chances of getting very sick if you get the flu. But it probably won’t make much difference to how contagious you are.

“It’s not clear that current vaccines do much for this,” Milton said. “They pretty much keep you from dying of the flu and keeping you out of the hospital, but they don’t seem to do a very good job of keeping you from spreading it. “

However, Milton said this is still an important question that flu researchers are working to answer.

Stop the spread of the virus before someone develops symptoms

The hardest part of preventing the spread of the flu is stopping the spread before someone develops symptoms.

Similar to what happens with COVID-19 infections, people start shedding enough flu virus to infect others about a day before symptoms appear.

That’s why respiratory viruses like the flu and coronaviruses are so difficult to control, Milton said — it’s the stealthy spread that occurs when virus levels are still relatively low and people’s symptoms are mild or non-existent. there is no

But new research could one day reverse this spread.

Read also: The flu vaccine has arrived in pharmacies. How much does it cost, who should get the vaccine and when?

Scientists are studying how a protein on the surface of influenza viruses, called neuraminidase (NA), contributes to spread.

The body produces some anti-NA antibodies after a flu infection, and evidence suggests that people with higher levels of NA antibodies may be less likely to pass on the virus.

Influenza vaccines have traditionally focused on another influenza virus protein called hemagglutinin (HA) rather than NA.

If future research proves the importance of NA, finding a way to increase the level of NA antibodies in humans could be a way to slow the spread of the flu. , said Milton.