The effectiveness of flu vaccination programs vary widely from season to season depending on the match of the vaccine to the virus it is designated to protect against, according to the Centers for Disease Control and Prevention. The effectiveness of the vaccine can also vary from person to person.
The two main factors in the effectiveness of a flu vaccine are the design of the vaccine, or how closely it matches the flu viruses that are most prevalent in a given year, and the age and health of the individual who is vaccinated, states the CDC. Other mitigating factors, such as type of vaccine used, also play a role. Every year, researchers review the effectiveness of previous vaccination programs, try to predict the type of flu viruses that are likely to be prevalent in a given season, and create the most effective vaccine possible against these likely widespread viruses. Some vaccines protect against three types of flu viruses, others against four.
Vaccines are designed both to prevent infection from these viruses and to lessen the effects of the disease if infection occurs, asserts the CDC. Flu vaccination programs are specifically significant for those most likely to be infected because of failing or not fully developed immune systems, such as the elderly, children and individuals suffering from chronic diseases.