Flu shot facts

Some people believe the shot causes flu. The evidence says otherwise.

"The viruses in a flu shot are killed (inactivated), so you cannot get flu from a flu shot," CDC says. Some shots contain only a single virus protein. The nasal spray uses weakened viruses that cannot cause flu illness.[1]

So why do some people feel off afterward? A sore arm, headache, low fever, muscle aches, or tiredness for a day or two are normal reactions as the immune system responds.[2] Protection also takes about two weeks to build, so a person can catch flu, or one of the many other winter viruses, right after the shot.[1]

Some people think a partly matched vaccine is pointless. The evidence says otherwise.

Here are honest numbers. Last season, an H3N2 strain called subclade K drifted away from the vaccine. Even so, through early March 2026 the vaccine lowered the risk of an emergency or urgent care visit for flu by 35% and of hospitalization by 27%.[4] In 2024-25, CDC estimates vaccination prevented about 180,000 hospitalizations and 12,000 deaths.[5]

When vaccinated people do get flu, it tends to be less severe. One study CDC cites found vaccination was linked to a 26% lower risk of ICU admission and a 31% lower risk of death from flu.[1] This season's vaccine was updated, including protection against subclade K.[6]

Some people assume every flu shot is now mRNA. The evidence says otherwise.

The flu vaccines in CDC's 2026-27 guidance are of three types: inactivated (killed virus), recombinant (a lab-made virus protein), and live weakened virus in the nasal spray.[3] None of these is an mRNA vaccine.

FDA approved Moderna's mRNA flu vaccine, mFLUSIVA, on August 5, 2026. The approval is traditional for ages 50 to 64 and accelerated for 65 and older, pending a confirmatory study.[7] It is not approved for children or adults under 50, and ACOG does not recommend it during pregnancy because data are lacking.[8]

Some people worry flu shots contain mercury. The evidence says otherwise.

Thimerosal is a preservative used in multi-dose vials. Thimerosal-free flu vaccines are widely available, and CDC notes thimerosal has been used safely in vaccines since the 1930s.[9] For 2026-27, CDC reports that 99% of projected supply does not contain it.[6]

In June 2025, CDC's newly appointed vaccine advisory committee voted to stop using thimerosal in flu vaccines, and the HHS secretary adopted the vote in July 2025. Studies have not found a health risk from the preservative, and only about 4% of doses contained it at the time.[10]

Some people think the flu shot is only worth getting in the fall. The evidence says otherwise.

CDC's guidance says vaccination "should continue after October and throughout the influenza season as long as influenza viruses are circulating and unexpired vaccine is available."[3] Protection takes about two weeks to build, so sooner is better.[1]

Some people expect a flu shot to make them very sick. The evidence says otherwise.

Normal: soreness, redness, or swelling where the shot was given, plus fever, muscle aches, headache, or tiredness. The nasal spray can cause a runny nose, headache, cough, sore throat, or low fever.[2]

Not normal: hives, swelling of the face or throat, or trouble breathing. Severe allergic reactions after vaccination are rare but can be life-threatening. Call 911.[2] Someone who has had a severe allergic reaction to any flu vaccine should not get the egg-based shots again and should talk with their clinician about options.[3]

Some people think egg allergy rules out a flu shot. The evidence says otherwise.

CDC's guidance says egg allergy alone "necessitates no additional safety measures for influenza vaccination beyond those recommended for any recipient of any vaccine, regardless of severity."[3] Egg-free options, cell-based and recombinant vaccines, are also available.[1] About 27% of this season's supply is made without eggs.[6]

Some people avoid the shot because of GBS. The evidence says otherwise.

In seasons when any increased risk was found, it was about 1 to 2 extra GBS cases per million doses. CDC notes that "a person is more likely to get GBS after flu disease than after getting a flu vaccine."[2]