- In the Southern Hemisphere, the adjusted flu vaccine effectiveness rates against outpatient visits and hospitalizations were 50.4% and 49.7%.
- Only 21.3% of flu outpatients and 15.9% of hospitalized flu patients were vaccinated.
- Health authorities in the Northern Hemisphere might expect similar levels of protection against influenza, experts said.
Influenza vaccines have cut flu-associated outpatient visits and hospitalizations by half during the Southern Hemisphere's 2025 flu season, according to an interim analysis of surveillance data from eight countries.
Over the Southern Hemisphere's flu season from April through September, the adjusted vaccine effectiveness (VE) rate against outpatient visits associated with influenza-like illness (ILI) was 50.4% (95% CI 33.2-63.2), while the adjusted VE rate against hospitalizations for severe acute respiratory infection (SARI) was 49.7% (95% CI 46.3-52.8), reported Savanah Russ, PhD, of the CDC's National Center for Immunization and Respiratory Diseases, and colleagues.
Only 21.3% of people with ILI were vaccinated against the flu, with an even lower percentage for people with SARI (15.9%), they noted in the Morbidity and Mortality Weekly Report.
"Health authorities in Northern Hemisphere locations might anticipate similar levels of protection against influenza illness, should the same influenza viruses circulate during the upcoming season," Russ and colleagues wrote. The Southern Hemisphere's 2025 flu vaccine composition is the same as the composition for the upcoming 2025-2026 Northern Hemisphere season.
The CDC's Advisory Committee on Immunization Practices recommends routine flu vaccination for everyone ages 6 months and older without contraindications. U.S. hospitalizations for flu reached record overall levels and peak-weekly marks during the 2024-2025 season, topping all rates over the prior 14 seasons.
Influenza A was the most common identified virus in both outpatient and hospitalized patients. That strain was present in 82.4% of people with ILI and 94.9% of those with SARI. Influenza A(H1N1)pdm09 was the most prevalent variant, at 39.9% and 58.7% of ILI and SARI cases, respectively.
Among patients with ILI, the adjusted VE rate against any influenza A subtype was 45.4%, and 53.3% against influenza A(H1N1)pdm09. Among people with SARI, the adjusted VE rate was 46.1% against any influenza A subtype and 41.6% against influenza A(H1N1)pdm09. Adjusted VE rates against influenza B were 62.3% in those with ILI and 77.6% in those with SARI.
The researchers used Southern Hemisphere national vaccination registries, medical records, and self-reports to determine vaccination status. Anyone who had a vaccine at least 14 days before flu symptoms was considered vaccinated. The analysis included 2,122 patients with ILI and 42,752 patients with SARI. Among those with SARI, 85.3% were young children, people with comorbidities, or older adults, considered priority vaccination groups.
The VE data were contributed by Argentina, Australia, Brazil, Chile, New Zealand, Paraguay, South Africa, and Uruguay.
Study limitations included the potential for preliminary data to differ from estimates at the end of the flu season. The small sample size may reduce the accuracy of the VE estimate, and 61% of potential patients were excluded from the analysis because of missing reverse-transcription PCR flu test results.
Disclosures
Russ reported no disclosures.
Co-authors reported relationships with AstraZeneca, CSL Seqirus, Evo Health, GSK, Moderna, Pfizer, Sanofi, Novavax, the New Zealand Ministry of Health, the Australian Department of Health and Aged Care, the Australian Technical Advisory Group on Immunisation, the Pan American Health Organization, the NIH, the EuroQol Research Foundation, the National Medical and Research Council, the National Influenza Surveillance Committee of Australia, the CDC, the Gates Foundation, and the WHO.
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