Flu shots are generally effective with their efficacy typically ranging between 40% and 60% each year, depending on how well the vaccine matches the circulating flu strains. Even in years when effectiveness is lower, flu vaccines still provide critical protection by reducing the risk of severe illness, hospitalization, ICU admission, and death related to flu. For example, vaccination can lower flu-related death risk by about 31% and ICU admission by 26% among adults. Effectiveness tends to be higher against hospitalization, especially in children and adolescents, where it can range from 63% to 78% reduction in hospitalizations. Additionally, flu vaccines help reduce complications in people with chronic conditions like lung and heart diseases.
Effectiveness can vary by age group and virus subtype: it tends to be higher in children and adolescents and lower in older adults. The vaccination's protection also wanes over time, generally declining by about 8-9% per month after vaccination, which means timing the shot around September or October is recommended to optimize protection through the flu season.
Even with variability and some years showing lower vaccine match against certain strains (such as H3N2), receiving the flu vaccine remains the best public health tool to reduce flu illness severity, hospitalizations, and deaths every season.
