There are several reasons why someone might get flu symptoms, even after they have been vaccinated against flu. One reason is that some people can become ill from other respiratory viruses besides flu such as rhinoviruses, which are associated with the common cold. These viruses can cause symptoms similar to flu, and also spread and cause illness during flu season. Flu vaccines only protect against flu and its complications, not other illnesses. Another explanation is that it is possible to 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.
What is a flu vaccine?
Influenza (flu) vaccines (often called “flu shots”) are vaccines that protect against the four influenza viruses that research indicates most common during the upcoming season. Most flu vaccines are “flu shots” given with a needle, usually in the arm, but there also is also a nasal spray flu vaccine.
Yes. There are different flu vaccine manufacturers and multiple influenza vaccine products licensed and recommended for use in the United States.
CDC recommends use of any licensed, age-appropriate influenza vaccine during the 2021-2022 influenza season. Available influenza vaccines include including quadrivalent inactivated influenza vaccines [IIV4s], recombinant influenza vaccine [RIV4], or live attenuated influenza vaccine (LAIV4). No preference is expressed for any influenza vaccine over another.
Quadrivalent flu vaccines include:
- Standard-dose quadrivalent influenza shots that are manufactured using virus grown in eggs. These include Afluria Quadrivalent, Fluarix Quadrivalent, FluLaval Quadrivalent, and Fluzone Quadrivalent. Different influenza shots are licensed for different age groups. These four vaccines are approved for people 6 months of age and older. Most influenza shots are given in an arm muscle with a needle. One quadrivalent influenza shot (Afluria Quadrivalent) can be given either with a needle (for people aged 6 months and older) or with a jet injector (for people aged 18 through 64 years only).
- A quadrivalent cell-based influenza shot (Flucelvax Quadrivalent) containing virus grown in cell culture, which is licensed for people 4 years and older. This vaccine is egg-free.
- Recombinant quadrivalent influenza shot (Flublok Quadrivalent), an egg-free vaccine, approved for people 18 years and older.
- A quadrivalent flu shot using an adjuvant (an ingredient that helps create a stronger immune response), Fluad Quadrivalent, approved for people 65 years of age and older.
- A quadrivalent high-dose influenza vaccine Fluzone High-Dose, which contains a higher dose of antigen to help create a stronger immune reseponse, licensed for people 65 years and older.
- A live attenuated influenza vaccine (FluMist Quadrivalent), which is given intranasally. This vaccine is approved for people 2 through 49 years of age. Live attenuated influenza vaccine should not be given to people who are pregnant, immunocompromised persons, and some other groups.
- There are many flu vaccine options to choose from, but the most important thing is for all people 6 months and older to get a flu vaccine every year. If you have questions about which vaccine is best for you, talk to your doctor or other health care professional. More information on approved flu vaccines for the 2020-2021 flu season, and age indications for each vaccine are available in CDC’s Table: U.S. Influenza Vaccine Products for the 2020-2021 Season.
Everyone 6 months of age and older should get an influenza (flu) vaccine every season with rare exception. CDC’s Advisory Committee on Immunization Practices has made this recommendation since the 2010-2011 influenza season.
Vaccination to prevent flu and its potentially serious complications is particularly important for people who are at higher risk of developing serious flu complications. See People at Higher Risk of Developing Flu-Related Complications for a full list of age and health factors that confer increased risk.
More information is available at Who Should Get Vaccinated Against Influenza.
Different influenza (flu) vaccines are approved for use in people in different age groups. In addition, some vaccines are not recommended for certain groups of people. Factors that can determine a person’s suitability for vaccination, or vaccination with a particular vaccine, include a person’s age, health (current and past) and any allergies to flu vaccine or its components. For more information, visit Who Should and Who Should NOT get a Flu Vaccine.
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. During years when the flu vaccine match is good, it is possible to measure substantial benefits from flu vaccination in terms of preventing flu illness and complications. However, the benefits of flu vaccination will still vary, depending on characteristics of the person being vaccinated (for example, their health and age), what influenza viruses are circulating that season and, potentially, which type of flu vaccine was used.
There are many reasons to get an influenza (flu) vaccine each year. Flu vaccination is the best way to protect yourself and your loved ones against flu and its potentially serious complications.
Below is a summary of the benefits of flu vaccination and selected scientific studies that support these benefits.
- Flu vaccination can keep you from getting sick with flu.
- Flu vaccination prevents millions of illnesses and flu-related doctor’s visits each year. For example, during 2019-2020 flu vaccination prevented an estimated 7.5 million influenza illnesses, 3.7 million influenza-associated medical visits, 105,000 influenza-associated hospitalizations, and 6,300 influenza-associated deaths.
- During seasons when flu vaccine viruses are similar to circulating flu viruses, flu vaccine has been shown to reduce the risk of having to go to the doctor with flu by 40 percent to 60 percent.
- Flu vaccination has been shown in several studies to reduce severity of illness in people who get vaccinated but still get sick.
- A 2021 studyexternal icon showed that among adults, flu vaccination was associated with a 26% lower risk of ICU admission and a 31% lower risk of death from flu compared to those who were unvaccinated.
- A 2018 studyexternal icon showed that among adults hospitalized with flu, vaccinated patients were 59 percent less likely to be admitted to the ICU than those who had not been vaccinated. Among adults in the ICU with flu, vaccinated patients on average spent 4 fewer days in the hospital than those who were not vaccinated.
- Flu vaccination can reduce the risk of flu-associated hospitalization.
- Flu vaccination prevents tens of thousands of hospitalizations each year. For example, during 2019-2020 flu vaccination prevented an estimated 105,000 flu-related hospitalizations.
- A 2014 studyexternal icon showed that flu vaccination reduced children’s risk of flu-related pediatric intensive care unit (PICU) admission by 74 percent during flu seasons from 2010-2012. A 2017 study found that during 2009-2016, flu vaccines reduced the risk of flu-associated hospitalization among older adults by about 40 percent on average.
- A 2018 study showed that from 2012 to 2015, flu vaccination among adults reduced the risk of being admitted to an intensive care unit (ICU) with flu by 82 percent.
- Flu vaccination is an important preventive tool for people with certain chronic health conditions.
- Flu vaccination has been associated with lower rates of some cardiac eventsexternal icon among people with heart disease, especially among those who have had a cardiac event in the past year.
- Flu vaccination can reduce the risk of a flu-related worsening of chronic lung disease (for example, chronic obstructive pulmonary disease (COPDexternal icon) requiring hospitalization.
- Among people with diabetesexternal icon and chronic lung diseaseexternal icon, flu vaccination also has been shown in separate studies to be associated with reduced hospitalizations from a worsening of their chronic condition.
- Flu vaccination helps protect pregnant people during and after pregnancy.
- Vaccination reduces the risk of flu-associated acute respiratory infection in pregnant people by about one-half.
- A 2018 studyexternal icon showed that getting a flu shot reduced a pregnant person’s risk of being hospitalized with flu by an average of 40 percent from 2010-2016.
- A number of studies have shown that in addition to helping to protect pregnant people from flu, a flu vaccine given during pregnancy helps protect the baby from flu for several months after birth, when he or she is too young to be vaccinated.
- Flu vaccine can be lifesaving in children.
- A 2017 study was the first of its kind to show that flu vaccination can significantly reduce a child’s risk of dying from flu.
- 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.
Despite the many benefits offered by flu vaccination, only about half of Americans get an annual flu vaccine and flu continues to cause millions of illnesses, hundreds of thousands of hospitalizations and tens of thousands of deaths. Many more people could be protected from flu if more people got vaccinated.
*References for the studies listed above can be found at Publications on Influenza Vaccine Benefits. Also, see the A Strong Defense Against Flu: Get Vaccinatedpdf icon! fact sheet.
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.Top of Page
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. While severe reactions are uncommon, you should let your doctor, nurse, clinic, or pharmacist know if you have a history of allergy or severe reaction to influenza vaccine or any part of flu vaccine.
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.
Call a doctor or get to a doctor right away.
Tell your doctor what happened, the date and time it happened, and when you got the flu shot.
Ask your doctor, nurse, or health department to file a Vaccine Adverse Event Reporting Systemexternal icon (VAERS) form, or call VAERS at 1-800-822-7967. Reports are welcome from all concerned individuals: patients, parents, health care providers, pharmacists and vaccine manufacturers.Top of Page
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
There are several reasons why someone might get flu symptoms, even after they have been vaccinated against flu.
- One reason is that some people can become ill from other respiratory viruses besides flu such as rhinoviruses, which are associated with the common cold. These viruses can cause symptoms similar to flu, and also spread and cause illness during flu season. Flu vaccines only protect against flu and its complications, not other illnesses.
- Another explanation is that it is possible to 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.
- A third reason why some people may experience flu symptoms despite getting vaccinated is that they may have been exposed to a flu virus that is very different from the vaccine viruses. The ability of a flu vaccine to protect a person depends largely on the similarity or “match” between the vaccine viruses chosen to make vaccine and those spreading and causing illness. There are many different flu viruses that spread and cause illness among people. For more information, see Influenza (Flu) Viruses.
- The final explanation for experiencing flu symptoms after vaccination is that flu vaccines vary in how well they work and some people who get vaccinated still get sick with flu. When that happens, though, vaccination has been shown in several studies to reduce severity of illness in those people who get vaccinated but still get sick.
Some people who get vaccinated may still get sick. However, flu vaccination has been shown in several studies to reduce severity of illness in people who get vaccinated but still get sick:
- A 2017 study showed that flu vaccination reduced deaths, intensive care unit (ICU) admissions, ICU length of stay, and overall duration of hospitalization among hospitalized flu patients.
- Another study in 2018 showed that a vaccinated adult who was hospitalized with flu was 59% less likely to be admitted to an intensive care unit (ICU) than someone who had not been vaccinated. Among adults in the ICU with flu, vaccinated patients on average spent 4 fewer days in the hospital than those who were not vaccinated.
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.Top of Page
People with egg allergies can receive any licensed, recommended age-appropriate influenza vaccine (IIV4, RIV4, or LAIV4) that is otherwise appropriate. People who have a history of severe egg allergy (those who have had any symptom other than hives after exposure to egg) should be vaccinated in a medical setting, supervised by a health care provider who is able to recognize and manage severe allergic reactions. Two completely egg-free (ovalbumin-free) flu vaccine options are available: quadrivalent recombinant vaccine and quadrivalent cell-based vaccine.