How effective were 2024–2025 influenza vaccines at preventing hospitalization during H3N2-dominant seasons?

1. What the surveillance networks measured and what they found

Four U.S. vaccine‑effectiveness networks reported interim 2024–25 estimates showing that vaccination reduced influenza‑associated hospitalizations overall and that protection against A(H3N2) hospitalization was measurable but variable: the IVY inpatient network estimated 51% effectiveness against H3N2‑associated hospitalization, while other U.S. network estimates and combined analyses produced hospitalization VE estimates in the 41%–55% range for adults in some analyses and higher (63%–78%) among children in some datasets ^{[1] [2]}.

2. International corroboration and the lower bound of protection

Independent Southern Hemisphere data covering eight countries found vaccines cut influenza‑associated hospitalizations by about one half overall, but subtype‑specific estimates were lower for H3N2 — roughly 37% against H3N2 hospitalization in that evaluation — underscoring that subtype and regional strain differences shift point estimates ^[3]. Meta‑analyses and single‑country studies from prior seasons also show H3N2 estimates can be low or cross the null in some analyses, reinforcing that a mid‑season VE of 30%–50% against hospitalization is plausible and consistent with prior seasons ^{[6] [7]}.

3. Age, timing and subgroup variation matter

Effectiveness against hospitalization was not uniform: children in several U.S. datasets showed higher protection than adults in the same season (hospitalization VE in children often exceeded 60% in some reports), while estimates for older adults were more variable and sometimes lower, reflecting both immune response differences and small sample sizes in subgroup analyses ^{[2] [1]}. Time since vaccination also matters in other seasons’ analyses — protection can wane, with lower effectiveness observed when vaccination occurred >120 days before illness onset in some studies — a factor that complicates interpretation of hospitalization VE ^[7].

4. Antigenic drift and the headline risk: H3N2 subclades

Genetic evolution of H3N2 mattered: only about half of circulating H3N2 viruses in 2024–25 were well recognized by the vaccine strain according to antigenic characterization, prompting warnings that protection against mild disease could be reduced while protection against severe outcomes would likely persist ^{[4] [8]}. Public health agencies and international authorities repeatedly stated preliminary data still pointed to continued protection against hospital attendance even when clinical disease effectiveness was uncertain, a position echoed in WHO and regional public health commentary ^{[5] [9]}.

5. How to read these numbers and remaining uncertainties

Interim VE estimates against H3N2‑associated hospitalization in 2024–25 clustered in a moderate protection band (roughly 35%–55%), but the precise value depends on the study network, age group, timing and circulating subclade; some point estimates lacked statistical significance and surveillance gaps limit precision ^{[1] [2] [3]}. Independent researchers and public health bodies warned that antigenic drift — including emergent subclade K later in 2025 — could change effectiveness in future seasons, reinforcing that these findings are conditional and subject to revision as more data accrue ^{[8] [10]}.

Conclusion

The best available interim evidence shows 2024–2025 influenza vaccines provided meaningful, though incomplete, protection against hospitalization during H3N2‑dominant periods — typically reducing the risk of hospitalization by about a third to a half depending on population and setting — while genetic drift and subgroup variability left important uncertainties and kept results from being uniformly robust across all analyses ^{[1] [2] [3] [4]}.