How effective is the seasonal flu shot?

Influenza (flu) vaccine effectiveness (VE) can vary. The protection provided by a flu vaccine varies from season to season and depends in part on the age and health status of the person getting the vaccine and the similarity or “match” between the viruses in the vaccine and those in circulation.

• Flu Vaccination can keep you from getting sick with Influenza
• Flu vaccination has been shown to reduce severity of illness in people who get vaccinated but still get sick.
• Flu vaccination can reduce the risk of flu-associated hospitalization.
• Flu vaccination is an important preventive tool for people with certain chronic health conditions.
• Flu vaccination during pregnancy helps protect pregnant people from flu during and after pregnancy and helps protect their infants from flu in their first few months of life.
• Flu vaccine can be lifesaving in children.
• Getting vaccinated yourself may also protect people around you, including those who are more vulnerable to serious flu illness, like babies and young children, older people, and people with certain chronic health conditions.

What are the side effects that could occur?

Common side effects from a flu shot include soreness, redness, and/or swelling where the shot was given, headache (low grade), fever, nausea, muscle aches, and fatigue. The flu shot, like other injections, can occasionally cause fainting.

Can severe problems occur?

Life-threatening allergic reactions to flu shots are very rare. Signs of serious allergic reaction can include breathing problems, hoarseness or wheezing, hives, paleness, weakness, a fast heartbeat, or dizziness. If they do occur, it is usually within a few minutes to a few hours after receiving the shot. These reactions can occur among persons who are allergic to something that is in the vaccine, such as egg protein or other ingredients. There is a small possibility that flu vaccine could be associated with Guillain-Barré syndrome, generally no more than 1 or 2 cases per million people vaccinated. This is much lower than the risk of severe complications from flu, which can be prevented by flu vaccine.

Why do some people not feel well after getting a flu shot?

Flu vaccine side effects are generally mild and go away on their own within a few days. Some side effects that may occur from a flu shot include soreness, redness, and/or swelling where the shot was given, headache (low grade), fever, nausea, muscle aches, and fatigue. The flu shot, like other injections, can occasionally cause fainting.

What about people who get a seasonal flu vaccine and still get sick with flu symptoms?

There are several reasons why someone might get flu symptoms even after they have been vaccinated against flu.

• Someone can get sick with another respiratory virus besides flu such as rhinoviruses or SARS-CoV-2 (the virus that causes COVID-19). Flu vaccines only protect against flu and its complications, not other illnesses.
• Someone can be exposed to flu viruses shortly before getting vaccinated or during the two-week period after vaccination that it takes the body to develop immune protection. This exposure may result in a person becoming sick with flu before protection from vaccination takes effect.
• Flu vaccines vary in how well they work, and someone can get vaccinated but still get sick with flu. There are many different flu viruses that spread and cause illness among people, so this can happen if someone is exposed to a flu virus that is very different from the viruses in the flu vaccine. The ability of a flu vaccine to protect a person depends partially on the similarity or “match” between the vaccine viruses chosen to make vaccine and those spreading and causing illness. Even when that happens though, flu vaccination can still reduce severity of illness.

What protection does a flu vaccine provide if I do get sick with flu?

Some people who get vaccinated may still get sick. However, flu vaccination has been shown to reduce severity of illness in people who get vaccinated but still get sick. In addition, it’s important to remember that flu vaccine protects against three or four different viruses and multiple viruses usually circulate during any one season. For these reasons, CDC continues to recommend flu vaccination for everyone 6 months and older even if vaccine effectiveness against one or more viruses is reduced.