Does the Flu Shot Make You Sick?

A common concern among many folks considering a flu shot is that it might actually give them the flu (influenza). While it’s easy to understand the thought process, the science behind the vaccine shows that’s not the case.

Let’s look at the facts with infectious disease expert Patrick Kenney, DO.

Why do some people feel sick after getting a flu shot?

The flu shot won’t give you the flu, but it can leave you with a few flu-like symptoms. Some people may get a mild fever, a headache or feel fatigued, says Dr. Kenney.

Consider it a sign that your immune system is responding to the medication. The flu vaccine trains your immune system to recognize and fight the flu. That causes it to make the antibodies that will eventually protect you if you’re exposed to the virus.

“Experiencing a mild reaction after a flu shot is normal and shows that your immune system is building protection,” explains Dr. Kenney. “Any flu shot symptoms — including soreness at the site of the shot — typically disappear within 24 to 48 hours.”

Does the flu shot contain an active flu virus?

Flu shots don’t contain the live flu virus. Instead, they use either an inactivated (killed) version of the virus or a single protein from the virus, according to the U.S. Centers for Disease Control and Prevention (CDC).

Neither the inactivated virus nor the single protein will cause the flu, according to the CDC.

“That’s always a concern we hear, but there’s no live virus in the flu shot,” reiterates Dr. Kenney. “That’s why you can’t get the flu from a flu shot.”

What about the nasal spray flu vaccine?

The nasal spray flu vaccine includes a live-but-weakened flu virus. For most people, it isn’t strong enough to cause illness, according to the CDC. The spray vaccine is approved for most healthy people ages 2 to 49.

The spray flu vaccine isn’t recommended for women who are pregnant or people with certain medical issues, including weakened immune systems or chronic conditions (like heart, lung or kidney disease).

Can the flu vaccine cause an allergic reaction?

Like any medical treatment, the flu vaccine carries some risk. Severe allergic reactions are possible, but very rare. Research suggests only one to two cases per million doses.

How effective is the flu shot?

Annual CDC studies show that flu shots are typically 40% to 60% effective at preventing infection. And even if you still catch the flu, vaccination can lower the risk of complications or hospitalization.

Dr. Kenney recommends getting your flu shot as soon as possible to offer protection during flu season, which occurs in the fall and winter in the United States. Activity typically peaks during and after the holidays.

“Flu vaccines have been widely used for decades, and doctors generally consider them very safe,” he reassures. “If you’re weighing whether to get one, talk with your healthcare provider about whether it makes sense for you this season.”

Can you get the flu right after getting a flu shot?

It takes about two weeks after a flu shot for your body to build full immunity against the flu, says Dr. Kenney. If you’re exposed to the flu virus shortly before or during this period, you could still get sick.

The basic flu vaccine also doesn’t protect against other respiratory viruses that may bring flu-like symptoms. This includes illnesses like COVID-19, respiratory syncytial virus (RSV) and the common cold.