What to know
CDC conducts studies each year to determine how well influenza (flu) vaccines protect against flu. Various factors influence how well flu vaccines work including repeat vaccinations.
Networks
Information about the networks CDC uses to study flu vaccine effectiveness is available at CDC's Influenza Vaccine Effectiveness Networks.
Background
How well flu vaccines work (or their ability to protect against a certain outcome) can vary from season to season. Protection can vary depending on who is being vaccinated. At least two factors play an important role in determining the likelihood that vaccination will protect a person from flu illness: 1) characteristics of the person being vaccinated (such as their age and health), and 2) how similar vaccines are to the influenza viruses spreading in the community. When flu vaccines are not very similar to some viruses spreading in the community, vaccination may provide little or no protection against illness caused by those viruses. However, the vaccines may still provide protection against other influenza viruses that circulate during the season. When there is high similarity between vaccines and circulating viruses, vaccination provides substantial benefits by preventing flu illness and complications.
Each flu season, researchers try to determine how well flu vaccines work to prevent flu illness and complications. Estimates of how well a flu vaccine works can vary based on study design, outcome(s) measured, population studied, and type of flu vaccine. Differences between studies must be considered when results are compared. More information is available at How Flu Vaccine Effectiveness and Efficacy are Measured.
Repeat Vaccinations
Scientists continue to study and learn more about how a person's flu vaccine history affect vaccine effectiveness. Studies have shown that previous flu vaccinations or infections can affect the immune response to flu vaccination in variable ways. A 2021 study conducted over six flu seasons (2012-2013 to 2017-2018) found that among people who get a flu vaccine each year (i.e., repeat vaccinees), getting the current season's flu vaccine generally was beneficial in reducing flu illness that results in a medical visit. There was no significant loss in protection among study participants who received the current and prior season's flu vaccine compared to those who were only vaccinated during the current season. Importantly, among people with prior season vaccination, current season vaccination provided significant added protection overall and against A(H1N1)pdm09- and influenza B-associated illnesses. These findings support the current ACIP recommendations for annual flu vaccination and demonstrate that study participants vaccinated during the prior flu season benefit overall from getting vaccinated during the current season.
Some studies have shown reduced flu vaccine effectiveness–particularly against A(H3N2) viruses–among people who were vaccinated in both the current and prior season compared with those who had only been vaccinated in the current season. However, this finding could be the result of study bias related to the "carryover" benefits of the prior season's vaccine. Of note, when the benefits of current flu vaccines are analyzed separately for people only vaccinated this season and people vaccinated both this season and the prior season, benefits of current season flu vaccination are observed for both groups. CDC is learning more about how repeat vaccination affects vaccine effectiveness, but current findings continue to support the Advisory Committee on Immunization Practices' recommendations for annual flu vaccination.