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How does the flu spread, and how long is it contagious?

Updated December 11, 2023 - 6:49 pm

We’re heading into the thick of flu season again. Hopefully, this article — and not the flu’s telltale “hit by a train” symptoms — serves as your reminder.

While it’s possible to get the flu year-round, it’s most common from December through February.

As anyone who’s had it knows, coming down with the flu is pretty miserable. Fever, coughing, chills, body aches, fatigue, sore throat, runny nose … none of the symptoms are pleasant.

If you do come down with the flu, the top question on your mind is probably when you’ll start feeling back to normal. Then, you may start wondering how the heck you got infected in the first place and how you can protect others from your nasty germs.

We turned to medical experts for the answers to these common questions and to know if getting the flu shot helps with recovery.

How the flu is spread

Dr. Caroline Cederquist, a family and functional medicine doctor and the medical director at Cederquist Medical Wellness Center and BistroMD, says that the reason why cases of the flu peak between December and February is because people are spending more time indoors with little to no fresh air ventilation.

Dr. Megan Berman, an internal medicine doctor and associate professor of general medicine at the University of Texas medical branch, explains that the flu is primarily spread through respiratory droplets, aka very small droplets made when someone infected coughs, sneezes or talks.

“They find their way to someone’s mouth or nose nearby,” she says, adding that the flu can also be spread through kissing or sharing utensils.

Cederquist adds that the virus can also live on some objects or surfaces, so someone can become infected with the flu if they touch an infected surface and then touch their nose or mouth. This is why it’s so important to keep surfaces wiped clean and to wash your hands often.

How long is it contagious?

If you do become infected with the flu virus, Cederquist says, typically the very earliest symptoms include fatigue, body aches and chills.

“You may also notice a cough, sore throat or fever as the virus sets in,” she explains.

Berman adds, “Unlike cold illnesses, many describe it as if they’ve been hit by a train.”

Pretty horrible, right?

Both doctors say that flu symptoms typically last five to seven days, although Berman says symptoms may persist longer for people who have a compromised immune system. Even in someone without weakened defenses, Cederquist says, some of the residual effects of the flu (such as dry cough or body aches) can be felt for a few weeks after the more intense symptoms clear.

Here’s what both doctors say is crucial to know about the flu: It’s most contagious a day before symptoms start. That’s a major reason why it spreads so easily.

“A person with the flu is most contagious starting about one day before the onset of symptoms and lasting a week,” Cederquist explains.

Berman says that flu symptoms tend to start one to even four days after exposure.

To best protect others from catching your flu, Berman says, stay home and away from others up to 24 hours after your fever resolves (without fever-reducing medication).

Vaccination and the flu

If you’ve gotten a flu shot this season, you may be wondering if you’re shielded from catching the flu and experiencing its horrendous symptoms.

Berman says that it is possible to get the flu even if you’ve been vaccinated, but your symptoms will be much milder than if you didn’t get vaccinated.

“Think of flu shots like seatbelts,” she explains. “If exposed to the virus, one is less likely to land in the hospital because of the vaccine.”

Of course, Berman points out that it also matters if the flu strain you’ve been infected with is the same one you were given the vaccine for.

Cederquist echoes this, saying, “Flu vaccines protect against four strains of the flu that researchers predict will be the most common that season. Flu shots are not 100 percent effective because, even though there are only four types of flu (A, B, C and D), each type has many strains, which often mutate during flu season.”

Berman also says that vaccines take a couple of weeks to work.

“If someone was vaccinated and came down with laboratory-confirmed flu the next day, they were already exposed, and the vaccine did not have enough time to make an immune response for prevention,” she says.

Though you can still get and spread the flu even if you’ve gotten the flu vaccine, your symptoms will probably be much less severe, which is why doctors always recommend getting immunized once a year.

Remember, the flu is contagious before symptoms even occur, so simply avoiding people who you know are sick is not enough to protect you.

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