Can the flu shot give me the flu?
What is the flu shot?
The flu shot is an inactivated vaccine (containing killed virus) that is given with a needle, usually in the arm. It contains three influenza viruses. The three vaccine strains – one A (H3N2) virus, one A (H1N1) virus, and one B virus – are representative of the influenza vaccine strains recommended for that year. Viruses for the flu shot are grown in eggs.
How effective is the flu shot?
With the flu shot, when the “match” between vaccine and circulating strains is close, the vaccine prevents influenza in about 70%-90% of healthy persons younger than age 65 years. Among elderly persons living outside chronic-care facilities (such as nursing homes) and those persons with long-term (chronic) medical conditions (such as asthma, diabetes, or heart disease), the flu shot is 30%-70% effective in preventing hospitalization for pneumonia and influenza. Among elderly nursing home residents, the flu shot is most effective in preventing severe illness, secondary complications, and deaths related to the flu. In this population, the shot can be 50%-60% effective in preventing hospitalization or pneumonia and 80% effective in preventing death from the flu.
What are the risks from getting a flu shot?
The viruses in the flu shot are killed (inactivated), so you cannot get the flu from a flu shot. The risk of a flu shot causing serious harm, or death, is extremely small. However, a vaccine, like any medicine, may rarely cause serious problems, such as severe allergic reactions. Almost all people who get influenza vaccine have no serious problems from it.
Can the flu shot give me the flu?
No, the flu shot cannot cause flu illness. The three influenza viruses contained in the flu vaccine are each 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 got flu shots and others got saltwater 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.
Why do some people not feel well after getting the flu shot?
The most common side effect of the flu vaccine in adults is 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 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 the flu.
- People may be exposed to an influenza virus 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 the vaccine begins to protect them.
- People may become ill from other (non-flu) viruses that circulate during the flu season, which can also cause flu-like symptoms (such as rhinovirus).
- A person may be exposed to an influenza virus that is not included in the vaccine. There are many different influenza 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. However, even among people with weakened immune systems, the flu vaccine can still help prevent influenza complications.
Influenza vaccine provides the best protection available from flu – even when the vaccine does not closely match circulating 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 for close contacts of high-risk people.
This information provided by the Centers for Disease Control and Prevention.
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