Can the flu shot give me the flu?No, a flu shot cannot cause flu illness. The viruses contained in flu shots are inactivated (killed), which means they cannot cause infection. Flu vaccine manufacturers kill the viruses used in the flu shot 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.
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 .
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 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 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 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 seasonal flu vaccine. There are many different influenza viruses that circulate every year. The flu shot protects against the 3 viruses that research suggests will be most common.
Unfortunately, some people can get infected with an influenza vaccine virus despite getting vaccinated. Protection provided by influenza vaccination can vary widely, based in part on health and age factors of the person getting vaccinated. In general, the flu vaccine works best among young healthy adults and older children. Some older people and people with certain chronic illnesses may develop less immunity after vaccination. However, even among people who tend to respond less well to vaccination, the flu vaccine can still help prevent influenza. Vaccination is particularly important for people at high risk of serious flu-related complications and for close contacts of high-risk people.
CDC - Seasonal Influenza (Flu) - Q & A: Seasonal Flu Shot
What do recent vaccine effectiveness studies show?Preliminary data for the 2010-2011 influenza season indicate that influenza vaccine effectiveness was about 60% for all age groups combined, and that almost all influenza viruses isolated from study participants were well-matched to the vaccine strains (Unpublished CDC data). A randomized study
(by Monto et al [137 KB, 8 pages]) looking at the 2007-2008 influenza season found trivalent inactivated vaccine (flu shot) protected 7 out of 10 people from influenza illness. Studies show that LAIV works about as well as the flu shot. The main study that led to the licensure of LAIV was one conducted in children that showed that LAIV protected up to 9 out of 10 children vaccinated against the flu. A recent meta-analysis of randomized clinical trials of LAIV in children found that 2 doses of LAIV in vaccine-naïve children prevented infection with 77% of antigenically similar viruses and 72% of all viruses regardless of antigenic similarity.
CDC - Seasonal Influenza (Flu) - Q & A: How Well Does the Seasonal Flu Vaccine Work?