The question: I was sick all last week with the flu, but I got a flu shot last September. How did that happen?
In any given year, the flu vaccine is roughly 60 percent effective against one of the many forms of the influenza A and influenza B viruses that cause the illness. Although not perfect, most would agree that it is a good bet to get the vaccine in order to decrease your chances of becoming ill by more than 50 percent. Because the injectable forms of the vaccine are manufactured with inactive viral particles, the vaccine itself cannot cause illness.
The challenge in developing an effective vaccine lies in predicting what viruses may be in circulation during the upcoming flu season. This is done via epidemiologic studies by public health authorities such as the World Health Organization and Centers for Disease Control, in conjunction with pharmaceutical manufacturers who make the vaccine. Because of mutations in the virus, the composition of the vaccine is frequently modified in an attempt to match its effectiveness against the most prevalent types of flu. Because flu outbreaks typically begin in late autumn or winter, vaccinations are recommended in early autumn, as it can take at least two weeks to build up protective antibodies.
During the current flu season we now know that there were two peaks of flu activity. Influenza A began circulating in December 2013, and became widespread in mid-January 2014. Just when we thought that flu season was ending, a different strain of influenza B appeared in March 2014, and has been widespread, especially in the northeast. You likely came in contact with someone who was ill and spread the virus to you while he or she was still contagious.