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How Effective Are Flu Shots?
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Vaccine shot
Vaccine shot A registered nurse measuring out a dose of vaccine for influenza A H1N1, a strain of influenza virus known for causing widespread outbreaks of acute upper or lower respiratory tract infection.

How Effective Are Flu Shots?

Written by
Kara Rogers
Kara Rogers is the senior editor of biomedical sciences at Encyclopædia Britannica, where she oversees a range of content from medicine and genetics to microorganisms. She joined Britannica in 2006 and...
Kara Rogers
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Britannica Editors
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
Britannica Editors
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The effectiveness of the flu vaccine changes from year to year but typically falls between 40 and 60 percent, meaning that in a vaccinated population 40 to 60 percent fewer people will become seriously ill from influenza infection.

The most significant factor driving flu vaccine effectiveness is its resemblance to the actual circulating influenza viruses. Flu viruses are constantly changing through a process known as antigenic drift. Public health experts from the World Health Organization (WHO) and various National Influenza Centers meet months in advance to analyze global surveillance data that capture changes in influenza viruses. This information is used to determine which strains are most likely to circulate in the upcoming season, thereby guiding vaccine production. Nonetheless, a new variant can emerge after the vaccine strains have been selected, potentially leading to a mismatch. Even in seasons with a mismatch, however, the vaccine still offers significant protection against severe illness, hospitalization, and death from influenza.

The effectiveness of flu vaccines also differs depending on factors such as age and overall health. In general, healthy young people typically respond best, while older adults and those with chronic conditions often need specialized high-dose or adjuvanted vaccines, which contain either more antigen than a standard-dose vaccine or an adjuvant (immune booster) to generate a stronger immune response. The flu vaccine is also most effective when received in the fall, before flu activity peaks. Because immunity fades over time and influenza viruses change rapidly, vaccination is recommended every year.

Flu vaccination also plays an important role beyond individual protection. When more people are vaccinated, the virus spreads less easily within communities, indirectly protecting those who are unable to receive the vaccine—an effect known as herd immunity. This broader protection is especially important for high-risk groups, particularly those with weakened immune systems, including infants and older people. Together, these factors help explain why annual flu vaccination remains a central strategy for reducing the overall impact of influenza.

Kara Rogers