Serious allergic reactions to flu vaccines are very rare. If they do occur, it is usually within a few minutes to a few hours after the vaccination. While these reactions can be life-threatening, effective treatments are available.
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 a flu-like illness, 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, cause symptoms similar to flu, and also spread and cause illness during the flu season. The flu vaccine only protects against influenza viruses, not other viruses.
Another explanation is that it is possible to be exposed to influenza viruses, which cause the flu, 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 ill with flu before protection from the vaccine takes effect. A third reason why some people may experience flu-like symptoms despite getting vaccinated is that they may have been exposed to an influenza virus that is very different from the viruses the vaccine is designed to protect against. The ability of a flu vaccine to protect a person depends largely on the similarity or “match” between the viruses selected to make the vaccine and those spreading and causing illness. There are many different influenza viruses that spread and cause illness among people. For more information, see Influenza (Flu) Viruses.
The final explanation for experiencing flu-like symptoms after vaccination is that unfortunately, the flu vaccine doesn’t always provide adequate protection against the flu. This is more likely to occur among people that have weakened immune systems or people age 65 and older.