Flu Vaccine Myths and Facts: Side Effects, Effectiveness, Timing and More

CDC recommends a flu vaccine every year for everyone 6 months and older, with rare exceptions. Here are the facts behind common misconceptions.

Why every year

Protection from the vaccine declines over time, and flu viruses keep changing, so the vaccine is reviewed and updated each year. An annual shot gives the best protection.

A flu shot can't give you flu

• Flu shots are made from inactivated (killed) virus or a single protein from a flu virus.
• The nasal spray contains live, weakened viruses that don't cause illness.

Flu can be serious — especially for young children, older adults, and people with conditions such as asthma, heart disease or diabetes — and any infection can lead to complications, hospitalization or death, even in healthy people. Getting vaccinated is safer than getting sick to gain immunity.

People 65 and older: three vaccines are preferentially recommended. More: Flu and People 65 Years and Older.

Side effects

• Flu shots: mostly soreness, redness, tenderness or swelling where the shot was given; sometimes a low-grade fever, headache or muscle aches, starting soon after and lasting 1–2 days.
• Nasal spray: runny nose, wheezing, headache, vomiting, muscle aches, fever, sore throat, cough — mild and short-lived.
• These are far milder than flu itself.
• Serious allergic reactions are very rare, usually within minutes to hours, and treatable.

Vaccinated, but still sick?

• Another virus — such as a rhinovirus or SARS-CoV-2 (the COVID-19 virus) — can cause flu-like symptoms; the flu vaccine protects only against flu.
• You may have been exposed just before vaccination, or in the two weeks it takes to build protection.
• Vaccine effectiveness varies, partly with how well the vaccine matches the viruses spreading — but even then, vaccination can reduce severity.

Flu vaccine and other infections

• Flu vaccination is not thought to make people more susceptible to other respiratory infections. A 2012 study suggested it might, but no other studies found this.
• There is no evidence that it raises the risk from coronaviruses. A January 2020 study linking flu vaccination to seasonal coronaviruses (not the COVID-19 virus) proved incorrect: Canadian researchers found no increased risk in their data, and traced the first study's result to a flaw in its test-negative design.

Does it work?

Protection varies by season, the person's age and health, the match between vaccine and circulating viruses, and perhaps the type of vaccine. In good-match years the benefits are substantial. Yet only about half of Americans get vaccinated, while an average season brings millions of illnesses, hundreds of thousands of hospitalizations and tens of thousands of deaths. More on how well flu vaccines work.

One dose is enough for adults — even older people with weak immune systems gain nothing from a second dose in the same season. Only children getting vaccinated for the first time may need two.

Timing

Waiting until later in the fall may mean higher immunity in winter, but risks missing vaccination and crowding it into a shorter period. Vaccination helps as long as flu is circulating: not vaccinated by end of November? December or later still helps. Flu usually peaks December–March, but can last into May.

No doctor's permission needed

There is no recommendation that pregnant people, or people with medical conditions, get special permission or written consent from their doctor to be vaccinated at a worksite clinic, pharmacy or elsewhere.

"Stomach flu" isn't flu

Nausea, vomiting and diarrhea can come from many viruses, bacteria or parasites. They sometimes occur with flu — more in children — but are rarely its main symptoms. Flu is a respiratory disease, not a stomach or intestinal one.

Sources

Based on "Misconceptions about Seasonal Flu and Flu Vaccines," Centers for Disease Control and Prevention; a work of the United States government in the public domain. The source's promotional graphic is not reproduced.