Why do some people who receive flu vaccines still get the flu?

Answer 1

Unless you are exposed to the flu before the vaccination is effective, it would not actually be the flu that causes these flu-like symptoms after a vaccination. It is not uncommon for symptoms that are flu-like (mild fever, achy muscles, headache, malaise, etc.) to occur immediately after receipt of a flu vaccination. This is not an indication of a viral infection, but, rather, these symptoms are due to the body's normal immune response to the vaccine and should go away within a couple of days. Since it is the normal body reaction to a vaccination, you would not be contagious to others who are healthy, either.

Vaccines trick the body into thinking there is an infection when there isn't. This causes the immune response that begins the process of creating the antibodies that will be able to inactivate the type of pathogen contained in the vaccine if you should be exposed to the same one in the wild after the vaccination has taken effect. [This takes usually around two weeks time after the inoculation in otherwise healthy people who are adults or children over 10 years old. Children 6 months old up to 10 need a series of vaccinations that take longer before effective. Babies under 6 months old cannot be vaccinated because their immune systems are too immature.] The immune response not only causes antibody production, but also results in the other germ-fighting mechanisms (which can include slight fever and the other flu-like symptoms).

Otherwise healthy people are not able to catch the flu from flu vaccinations because the pathogen in the vaccine is either totally inactivated/"dead" or is severely weakened so that it is not able to cause the infectious disease.

See also the related questions below for more information about the processes of an immune response.

Answer 2

They may be only experiencing mild side effects such as a low grade fever or aches at the site of the injection of a flu shot, but if they actually get the flu, it will be because they:

• had it already before they got the vaccine and just had not shown symptoms yet; or,
• between the time they took the vaccination and their body developed the full immunity they caught the flu (this can take as long as two weeks, but with the new H1N1 pandemic Swine Flu vaccine it is expected to be only 8 - 10 days after the shot for immunity to have developed); or,
• they may have caught a strain of flu that was not included in the vaccine, and so the vaccination would not provide immunity to the other strain.