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The following is a summary of “Vaccine Effectiveness Against Influenza A(H1N1), A(H3N2), and B–Associated Hospitalizations—United States, September 1, 2023–May 31, 2024,” published in the April 2025 issue of Journal of Infectious Diseases by Lewis et al. 

Researchers conducted a retrospective study on influenza activity from December 2023 to May 2024, analyzing the circulation of Influenza A(H1N1), A(H3N2), and B viruses.  

They analyzed data from a multistate sentinel surveillance network of 24 medical centres across 20 states (September 1, 2023–May 31, 2024), enrolling adults aged ≥18 years hospitalized with acute respiratory illness (ARI). A test-negative design was used, where cases tested positive for influenza viruses by molecular or antigen tests, while controls tested negative for both influenza viruses and SARS-CoV-2. Vaccine effectiveness (VE) against influenza-associated hospitalization was determined using the formula (1 − adjusted odds ratio for vaccination) × 100%.  

The results showed that among 7690 individuals, 1170 had influenza (33% vaccinated), and 6520 served as controls. The VE was 40% (95% CI: 31%–48%), with age-specific estimates of 53% (18–49 years [34%–67%]), 47% (50–64 years [31%–60%]), and 31% (≥65 years [16%–43%]). Protection was comparable in immunocompetent (40% [30%–49%]) and immunocompromised individuals (32% [7%–50%]), VE was significant for influenza B (67% [35%–84%]) and A(H1N1) (36% [21%–48%]) but crossed the null for A(H3N2) (19% [-8%–39%]). The VE was higher within 14–60 days post-vaccination (54% [40%–65%]) than after >120 days (18% [-1%–33%]).  

Investigators concluded that influenza vaccination during 2023–2024 reduced the risk of hospitalizations among adults associated with H1N1 and influenza B, with lower effectiveness observed in those vaccinated more than 120 days before illness onset compared to those vaccinated 14–60 days prior. 

Source: academic.oup.com/jid/advance-article-abstract/doi/10.1093/infdis/jiaf185/8108408