How common are side effects from the flu vaccine?

Infectious Diseases Vaccines

One in three people have some pain or redness at the injection site following a flu shot. Around one in five has a more systemic reaction, such as fever, headache, or tiredness.

This year’s flu vaccines cover four strains of influenza. Most people receive an injectable vaccine, although a nasal mist is also available for people ages 2 through 49 (except people who are pregnant). There are four manufacturers of U.S. flu vaccines this year, and they are recommended for everyone ages six months and up. Side effects, safety, and effectiveness are basically the same across the four brands, so the brand you get doesn’t matter much.

One exception: If you are at least 65 years old, you should look for one of these three if you have options: Fluzone High-Dose Quadrivalent from manufacturer Sanofi, Flublok Quadrivalent Recombinant from Sanofi, or Fluad Quadrivalent Adjuvanted from Seqirus. These have better effectiveness profiles in older adults. However, if one of those is not available, go ahead and get another one. Something is much better than nothing.

Side effects of the annual flu shots are pretty much the same from year to year. The vaccine components don’t change–just the strains they protect against. So, side effects do not change.

The most common side effect of the annual (injectable) flu vaccines available in the US is pain or redness at the site of the injection. About one in three people experience this side effect in any given year. It usually happens within 24 hours and goes away within three days.

Looking at more systemic reactions, a recent cohort study found that 13% of people who got an annual flu vaccine had some kind of “systemic” side effect, like fever, tiredness, headache, joint or muscle pain, or weakness. These side effects also usually appear within a day or two, and go away within three days. In a few people, they can last up to a week.

The most common serious problem related to flu vaccination is a shoulder injury. If the needle is not placed correctly, it can damage the shoulder bursa or joint, leading to long-term pain and limited function. This type of injury is rare—estimated at about 1 in a million shots—but preventable. When you’re getting your flu shot, it should be in the deltoid muscle, not up near the shoulder joint. You can advocate for yourself when you’re getting your flu shot by paying attention and asking questions about the placement in the deltoid muscle.

A nasal spray flu vaccine is an option for people ages 2 to 49 who aren’t pregnant. The nasal spray has lower rates of side effects (because there’s no injection site), but it isn’t right for everyone. People with impaired immune systems should get the flu shot instead, for example.

Side effects for the nasal spray are less common. About 1 in 5 adults gets a runny nose, headache, cough, or sore throat. Some kids also get fever and body aches with the nasal spray.

It’s a common misconception that getting a flu shot can give you the flu. It cannot. All the injectable vaccines are made from inactivated virus. To make these vaccines, virus is grown in either chicken eggs or cell cultures and then purified and treated so that it no longer has the capacity to replicate. That means it cannot infect a person.

Nerd bonus: Some of the injectable flu vaccines use a recombinant inactivated method, which means instead of growing flu virus, the genetic code from a part of the flu virus is inserted into another virus before it’s cultured. Same result, however–it cannot give you the flu.

The nasal mist vaccine is made from live attenuated flu virus. Attenuated means weakened. Weakened viruses are not likely to cause an infection in people with a normal immune system. However, these vaccines are not recommended for people with compromised immune systems precisely because a weakened virus might actually be able to cause an infection.

Notable in this year’s recommendations for flu vaccine: a person who has a history of egg allergy can now get any of the flu vaccines, including the ones that are cultured in chicken eggs.  The purification process is so complete that no egg proteins remain in the vaccine product, and they are safe for people with egg allergy.

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