|MadSci Network: Virology|
That is a good question. Making a vaccine against the Influenza virus is a tricky problem. Flu viruses undergo a phenomenon called antigenic drift. The proteins of the virus that our immune system recognizes and makes cellular and humoral immunity against change slightly as the virus circulates among the natural hosts. Since this virus also infects birds and pigs in addition to humans, the virus can pick up mutations related to adaptations of the virus to the respective hosts it infects.
The original Flu vaccine contained several viral proteins and was extensively tested before the FDA licensed it. However, at least two of the three major viral proteins in each years Flu vaccine are new. These new proteins are selected by a committee of expert virologists and epidemiologists who examine data from the recent human Flu epidemics in various parts of the world and the US. They look at data of epidemics of Flu among pigs and birds as well. All of this information is taken into consideration along with statistical analysis that looks at the probability of one or another viral strain or protein being part of the next epidemic. Then, proteins from the two most likely strains of Flu virus are selected to be produced for that year's vaccine. If you, or your doctor, look in the "package insert" of the Flu vaccine you receive, you will see that the vaccine is not guaranteed to protect you against the Flu virus that may be circulating in your community. Because the experts who selected it take care to make good choices, "probably" one or more of the three proteins in the vaccine may give you at least partial protection after your immunity to it is produced (this takes about a week or two). However, there is also a smaller chance that none of the proteins in the vaccine will create immunity to the virus you get infected with. In this case, you may get the Flu anyway. Yes, there is no guarantee that the Flu vaccine will protect you as there is for a vaccine like the Tetanus toxoid vaccine. Scientists have shown that the critical antigens of the Tetanus toxoid vaccine have not changed for hundreds, maybe even thousands of years, so if you get immunity to that vaccine you should be protected against getting "lock jaw" produced by tetanus toxin.
Since the antigens of infectious Flu virus keep changing, the scientists and epidemiologists at the Centers for Disease Control and Prevention, along with experts at Universities, must maintain a vigil and be prepared to create a "new" vaccine very quickly when a really different strain of Flu virus appears in their sights.
Try the links in the MadSci Library for more information on Virology.