Vaccination cut the risk of testing positive for influenza by 40% during the severe 2024-2025 flu season, according to a study of over 1.1 million Californians.
Real-World Evidence from a High-Severity Season
A study published in JAMA Network Open analyzed data from over 1.1 million Californians tested for influenza between October 1, 2024 and May 31, 2025. The researchers found that vaccinated individuals were significantly less likely to test positive for influenza compared to unvaccinated individuals.
Among adults aged 65 and older who tested positive, vaccination was associated with a 29% lower odds of death from influenza-related causes within 30 days.
A Record-Breaking Flu Season
The 2024-2025 flu season was classified as high severity, with the highest hospitalization rate in 24 years. Nationwide estimates included:
- 610,000 to 1.3 million hospitalizations
- 27,000 to 130,000 deaths, including 289 pediatric deaths
This pediatric death toll surpassed the record from the 2009-2010 H1N1 pandemic.
The CDC continues to recommend annual vaccination for everyone aged 6 months and older.
Background on Vaccine Effectiveness
The study provides crucial real-world evidence of vaccine effectiveness during a severe season. Effectiveness varies annually, depending on how well the vaccine matches the circulating strains. The predominant strains in 2024-2025 were influenza A viruses.
California's comprehensive immunization records allowed researchers to adjust for key variables, strengthening the reliability of the findings.