Can taking the flu shot really give you the flu?

Posted on September 11, 2014


The short answer is here is NO. There is no direct “cause-effect” relationship between receiving the flu vaccine and being infected with influenza(aka “flu”). However, here are some reasons why someone may have flu after they have been vaccinated:

The flu shot can cause mild side effects that are sometimes mistaken for flu: Most influenza vaccines (that are administered with a needle) are dead (i.e.inactivated) and therefore do not have the capacity to infect. And just like any other chemical compound that is introduced into the human body, it comes with some side effects such as soreness, redness, tenderness or swelling at the site of injection. However, in some cases headache, low-grade fever, and muscle aches may occur which is sometimes interpreted as having the flu.

There is also another form of the flu vaccine which is the nasal spray. The viruses contained in the nasal spray flu vaccine are attenuated (i.e. weakened) and can ONLY cause mild infection at cold temperatures. Therefore it is unable to infect the lungs or other areas of the body where warmer temperatures exist. Just like the needle shot, it comes with side effects including runny nose, nasal congestion and cough which may be mistaken for influenza.

You waited too long to get your shot: It takes the body about two weeks to gain protection after getting vaccinated. Therefore it is possible that you may have already been exposed to the virus before getting vaccinated or during the 2-week period after getting the vaccine. And as a result, you become ill with the flu before the vaccine is able to work. It is therefore advised to get vaccinated earlier in the season. The CDC recommends that flu vaccinations begin soon after vaccine becomes available, ideally by October. However, it is never too late to get vaccinated, even in January or later. As a matter of fact, flu activity mostly peaks between December and February, although activity can last as late as May.

You may have a flu virus that is not covered by your vaccine: Several strains of the influenza virus circulate every year, and it is almost impossible to have one single shot protect against all existing strains of the flu virus. Therefore, the yearly available vaccines mostly protect against about 3 to 4 of the most common viruses (as determined by research).

You have a completely different illness: Flu-like symptoms overlap with symptoms of several other non-flu viruses e.g. rhinovirus. So you may be infected by another illness entirely.

The flu vaccine simply wasn’t protective enough for you: Unfortunately, based on certain health and age factors, protection can vary and some people will still have the flu despite getting vaccinated. Research has shown that the vaccine is most effective among young healthy adults and older children, and some older people and people with certain chronic illnesses may develop less immunity after vaccination. However, getting vaccinated is still encouraged among people who respond less well to vaccination, because the flu vaccine can reduce the risk of serious flu-related complications.

The CDC recommends a yearly flu vaccine for just about everyone 6 months and older. Influenza can be a serious disease, and can carry the risk of serious complications, hospitalization or death. Therefore, getting vaccinated is a safer choice than risking illness to obtain immune protection.