Misconceptions about the flu shots
Can a flu shot give you the flu?
No, a flu shot cannot cause flu illness. The influenza viruses contained in a flu shot are inactivated (killed), which means they cannot cause infection. Flu vaccine manufacturers kill the viruses used in the vaccine during the process of making vaccine, and batches of flu vaccine are tested to make sure they are safe. In randomized, blinded studies, where some people get flu shots and others get salt-water shots, the only differences in symptoms was increased soreness in the arm and redness at the injection site among people who got the flu shot. There were no differences in terms of body aches, fever, cough, runny nose or sore throat.
More information about these studies is available at:
- Carolyn Bridges et al. (2000). Effectiveness and cost-benefit of influenza vaccination of healthy working adults: A randomized controlled trial. JAMA. 284(13):1655—1663.
- Kristin Nichol et al. (1995). The effectiveness of vaccination against influenza in healthy working adults. New England Journal of Medicine. 333(14): 889-893.
Why do some people not feel well after getting the seasonal flu shot?
The most common side effect of seasonal flu shots in adults has been soreness at the spot where the shot was given, which usually lasts less than two days. The soreness is often caused by a person's immune system making protective antibodies to the killed viruses in the vaccine. These antibodies are what allow the body to fight against flu. The needle stick may also cause some soreness at the injection site. According to the Advisory Committee on Immunization Practices (ACIP), rare symptoms include fever, muscle pain, and feelings of discomfort or weakness. If these problems occur, they are very uncommon and usually begin soon after the shot and last 1-2 days.
What about people who get a seasonal flu vaccine and still get sick with flu-like symptoms?
There are several reasons why someone might get flu-like symptoms even after they have been vaccinated against seasonal flu.
- People may be exposed to one of the influenza viruses in the vaccine shortly before getting vaccinated or during the two-week period that it takes the body to gain protection after getting vaccinated. This exposure may result in a person becoming ill with flu before protection from the vaccine takes effect.
- People may become ill from non-flu viruses that circulate during the flu season, which can also cause flu-like symptoms (such as rhinovirus). Flu vaccine will not protect people from respiratory illness that is not caused by flu viruses.
- A person may be exposed to an influenza virus that is very different from the viruses included in the vaccine. The ability of a flu vaccine to protect a person depends largely on the similarity or “match” between the viruses or virus in the vaccine and those in circulation. There are many different influenza viruses. For more information, see Influenza (Flu) Viruses.
- Unfortunately, some people can remain unprotected from flu despite getting the vaccine. This is more likely to occur among people that have weakened immune systems or the elderly. However, even among these people, a flu vaccine can still help prevent complications. For more information about the effectiveness of the seasonal flu vaccine, see How Well Does the Seasonal Flu Vaccine Work?
Seasonal influenza vaccine provides the best protection available from seasonal flu—even when the vaccine does not exactly match circulating seasonal flu strains, and even when the person getting the vaccine has a weakened immune system. Vaccination can lessen illness severity and is particularly important for people at high risk for serious flu-related complications and close contacts of high-risk people. Children younger than 6 months old are the pediatric group at highest risk of influenza complications, but they are too young to get a flu vaccine. The best way to protect young children is to make sure members of their household and their caregivers are vaccinated.
Does getting vaccinated against seasonal flu early in the season pose a risk that immunity may wane before the end of the season?
No. Seasonal flu vaccination provides protection against the influenza strains contained in the vaccine through one influenza season. Vaccination can begin as soon as vaccine is available. So except for some children, only one dose of vaccine is needed.
Can vaccinating someone twice provide added immunity?
Studies have not demonstrated a benefit of receiving more than one dose during an influenza season, even among elderly persons with weakened immune systems. Except for some children, only one dose of flu vaccine is recommended each season.