The flu shot's effectiveness varies each year and depends on factors such as the match between the vaccine strains and circulating viruses, age, and immune function. For the 2024-2025 flu season, vaccine effectiveness in the U.S. ranged approximately from 32% to 60% in outpatient settings and was higher in preventing hospitalization, especially among children and adolescents (up to 78%). Adult effectiveness ranged from 36% to 55%. Protection tends to be highest 14-29 days after vaccination and diminishes over time. While vaccine effectiveness is often modest to moderate, annual vaccination reduces hospitalizations, mortality, and duration of illness, and is particularly beneficial for higher-risk groups. Some recent studies have shown mixed results, including a preprint study indicating a negative effectiveness estimate, but these require cautious interpretation as not all findings are peer-reviewed or conclusive. Overall, the flu shot remains a key preventive tool despite variable effectiveness.
Key Points on Flu Shot Effectiveness
- Effectiveness generally ranges from about 30% to 60% against symptomatic flu in most seasons.
- Higher effectiveness is often observed in children and adolescents compared to older adults.
- Vaccination reduces risk of flu-related hospitalization significantly.
- Protection wanes over the months following vaccination.
- Annual vaccination is recommended due to viral changes and variability in effectiveness.
- Some recent studies have raised questions but are not definitive enough to change recommendations.
The flu shot is effective in reducing flu burden but not perfect, still it is the best preventive measure available each flu season.
