Flu Shots Cut Infection Risk by 40% in High-Severity Season, California Study Finds

SACRAMENTO, Calif. — Influenza vaccination provided moderate protection during the severe 2024-25 flu season, with people who received the shot 40% less likely to test positive for the virus compared to those who did not, according to a large new study of more than 1.1 million Californians.

The analysis, published in JAMA Network Open, offers fresh evidence of the vaccine's benefits amid one of the most intense flu seasons in recent memory, marked by record pediatric deaths and elevated hospitalizations nationwide.

Researchers from the California Department of Public Health examined data from individuals aged 6 months and older who were tested for influenza between Oct. 1, 2024, and May 31, 2025. The case-control study found clear associations between vaccination and reduced odds of both infection and severe outcomes.

"These findings are consistent with protection against severe and fatal influenza among people vaccinated against influenza," the study authors wrote.

For adults aged 65 and older who tested positive for flu, vaccination was linked to 29% lower odds of dying from an influenza-related cause within 30 days of testing. This protection against mortality underscores the vaccine's role in safeguarding the most vulnerable populations during peak transmission periods.

The 2024-25 season was classified as high severity, with the highest rate of influenza-associated hospitalizations in 24 years. Nationwide estimates cited in the study pointed to between 610,000 and 1.3 million hospitalizations and 27,000 to 130,000 deaths. Tragically, the flu claimed the lives of 289 children, surpassing the previous record from the 2009-2010 H1N1 pandemic season.

Public health officials have long emphasized annual vaccination as the primary tool for mitigating flu's impact. The Centers for Disease Control and Prevention recommends everyone 6 months and older get vaccinated each season, noting that even when effectiveness is moderate, shots substantially reduce the risk of serious complications.

This latest California data aligns with broader national interim estimates from the 2024-2025 season, which showed vaccine effectiveness ranging from 32% to 60% against outpatient visits in different networks and 41% to 78% against hospitalizations in certain groups.

The study's scale provides robust real-world evidence. By leveraging comprehensive testing and vaccination records in California's large population, researchers could account for key variables while focusing on laboratory-confirmed cases, strengthening the reliability of the findings.

Experts note that flu vaccines work by prompting the immune system to produce antibodies against specific strains predicted to circulate. Because influenza viruses evolve rapidly, effectiveness varies annually depending on how well the vaccine matches circulating strains. During the 2024-25 season, predominant strains included influenza A viruses, against which protection was observed.

Beyond individual protection, vaccination contributes to community-level benefits by reducing overall transmission. Lower infection rates among vaccinated people mean fewer opportunities for the virus to spread to unvaccinated or high-risk individuals.

The findings come as health authorities prepare for the 2025-2026 season. Vaccine formulations are updated each year based on global surveillance data from the World Health Organization and national centers. Manufacturers typically produce hundreds of millions of doses, distributed through clinics, pharmacies and public health programs.

Despite consistent recommendations, vaccination coverage varies. Rates are typically higher among older adults and young children but lag in working-age adults. Barriers include access, misinformation, and perceptions that the flu is mild for healthy people — a view contradicted by hospitalization and death statistics.

The California study adds to a body of evidence accumulated over decades. Previous seasons have shown similar moderate effectiveness, with vaccines preventing millions of illnesses and tens of thousands of hospitalizations annually even in years with imperfect strain matches.

For older adults, who face the highest risk of severe outcomes, the 29% reduction in flu-related mortality is particularly meaningful. This group often has weakened immune responses, making any additional protection valuable. Enhanced vaccines, such as high-dose or adjuvanted formulations, are available specifically for those 65 and older.

Children also benefited significantly in the data. The record 289 pediatric deaths highlight the virus's danger to younger age groups, where vaccination can prevent not only infection but also complications like secondary bacterial pneumonia or exacerbation of chronic conditions.

Broader context from the CDC indicates the 2024-25 season strained healthcare systems. High hospitalization rates, especially among the elderly and those with underlying conditions, led to increased emergency department visits and intensive care admissions in many regions.

Antiviral medications like oseltamivir (Tamiflu) remain an important treatment tool when administered early, but prevention through vaccination is the cornerstone of public health strategy. Combined with hygiene practices, staying home when sick, and masking in high-risk settings, shots form part of a layered defense.

Looking forward, ongoing research explores improved vaccines, including universal candidates targeting conserved parts of the virus to provide broader, longer-lasting protection. Until those become available, current annual shots offer the best available shield.

The study authors emphasized the importance of continued surveillance and high vaccination uptake. With flu seasons varying in intensity, consistent public health messaging helps maintain awareness even as other respiratory viruses like COVID-19 and RSV compete for attention.

Health officials urge eligible individuals to get vaccinated as soon as doses become available in the fall, ideally before peak season. For the 2025-2026 campaign, updated recommendations are expected from the CDC's Advisory Committee on Immunization Practices.

This California research reinforces that while no vaccine is 100% effective, flu shots deliver meaningful protection against infection, severe disease and death. In a high-burden season, that 40% reduction in positivity translated to thousands of prevented cases and associated complications across the state.

As summer approaches and planning for the next respiratory virus season intensifies, the data serve as a timely reminder of vaccination's value. Public health campaigns will likely highlight these results to encourage uptake, particularly among groups with historically lower coverage rates.

The findings also highlight the power of large-scale, real-world data analysis in evaluating vaccine performance. California's comprehensive immunization information system enabled this detailed assessment, providing insights applicable beyond state borders.

In summary, the new evidence strengthens the case for annual influenza vaccination as a safe, effective measure that saves lives and reduces healthcare burden, even during particularly challenging seasons.