How long you’re contagious with the flu — and when it’s safe to go out

The U.S. is in the thick of flu season, with many states reporting "moderate to very high activity" of influenza-like illness, according to the CDC.

With influenza A (H3N2), or subclade K, making up a large portion of the circulating viruses, doctors emphasize the importance of getting the most recent flu vaccinations, staying at home while sick and washing hands frequently, among other prevention tactics.

If you’ve been exposed to or infected with influenza, here’s what to know about how long the virus is transmissible.

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"You are typically contagious with the flu for five to seven days after symptoms start," Dr. Samantha Picking, the Massachusetts-based senior director of immunizations at Walgreens, told Fox News Digital.

"However, some groups remain contagious for longer periods. Young children and people with weakened immune systems can spread the virus for more than seven days, sometimes extending to two weeks or longer."

It’s a myth that people can’t be contagious until they experience symptoms, according to Picking.

"The flu is most contagious during the first three to four days after symptoms appear," she said. "However, you can begin spreading the virus a day before feeling ill and may remain contagious for up to a week or longer."

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The incubation period — the time between exposure to the virus and when symptoms start — is typically two days, though it can range from one to four days, the pharmacist added. People with mild symptoms can still spread the flu. 

For anyone who is starting to feel sick or has been around someone who has the flu, Picking recommends limiting contact with others to avoid spreading the virus.

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"To be safe and prevent spreading the virus, stay home until you're fever-free for at least 24 hours without using fever-reducing medicine," Picking advised. "For most healthy adults, this typically corresponds to five to seven days after symptoms begin."

People can take extra precautions, like masking, for a few days after returning to normal activities, she added.

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"However, use your best judgment and consult a healthcare professional if you’re unsure whether you’re well enough to leave the house," she added. "If your other symptoms aren’t improving, or you generally still feel ill, it’s wise to stay home and practice good hand hygiene."

Young children and people with weakened immune systems may be able to spread the virus to others for a longer time period, sometimes for up to 14 days, according to the pharmacist.

Picking noted that a cough can linger for weeks after other symptoms have abated. "While a lingering cough is not necessarily a sign that you’re still contagious, it’s considerate to mask up and practice good cough etiquette when around others," she advised.

Some antiviral medications, when initiated early on, can shorten how long someone is contagious with the flu.

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"These medications work best when started within two days of seeing symptoms," Picking noted. Anyone who is interested in antivirals should speak with a healthcare provider or pharmacist for personalized guidance. 

Flu symptoms can include fever, chills, headache, fatigue, cough, sore throat and runny nose.

Subclade K has been detected as the culprit in rising global cases, causing more intense symptoms and higher risk of spread.

"It’s becoming evident that this is a pretty severe variant of the flu," Dr. Neil Maniar, professor of public health practice at Boston’s Northeastern University, previously told Fox News Digital. "Certainly, in other parts of the world where this variant has been prevalent, it's caused some severe illness, and we're seeing an aggressive flu season already."

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"The vaccine still provides protection against serious illness resulting from the subclade K variant that seems to be going around," he added.

Angelica Stabile contributed reporting.