Each year influenza causes about 5 million hospitalizations and 650,000 deaths worldwide. Flu season runs roughly April to September in the temperate Southern Hemisphere and October to May in the North — so how the vaccine performs down south offers an early look at what the North can expect. This study, by CDC, the Pan American Health Organization and partners in eight countries, estimated how well the 2025 Southern Hemisphere flu vaccine worked.
How it was measured
Sentinel surveillance networks tracked two groups from March to September 2025:
| Outpatients with influenza-like illness (ILI) | Hospitalized patients with severe acute respiratory infection (SARI) | |
|---|---|---|
| Countries | Australia, South Africa | Argentina, Australia, Brazil, Chile, New Zealand, Paraguay, Uruguay |
| Sites | 163 general practices | 3,157 hospitals |
| Patients included | 2,122 | 42,752 |
| Tested positive for flu | 563 (26.5%) | 17,787 (41.6%) |
| Vaccinated | 21.3% | 15.9% |
All patients were tested by RT-PCR. Using a test-negative design, researchers compared vaccination rates between patients who tested positive for flu and those who tested negative, adjusting for sex, age, timing and country. A patient counted as vaccinated if they got a 2025 dose at least 14 days before falling ill. All countries used WHO-recommended, egg-based inactivated vaccines.
Positive influenza tests among outpatients and hospitalized patients, 2025. Credit: Centers for Disease Control and Prevention.
Which viruses circulated
Influenza A dominated: 82.4% of positive outpatients and 94.9% of positive hospital patients. Among hospitalized patients, A(H1N1)pdm09 led (58.7% of influenza A), followed by A(H3N2) (26.6%). Sequenced viruses were mostly H1N1 clade 5a.2a.1, H3N2 clade 2a.3a.1, and B/Victoria clade V1A.3a.2.
How well the vaccine worked
| Protection against | Outpatient visits | Hospitalization |
|---|---|---|
| Any influenza | 50.4% | 49.7% |
| Any influenza A | 45.4% | 46.1% |
| A(H1N1)pdm09 | 41.6% | |
| A(H3N2) | 37.2% | |
| Influenza B | 77.6% |
Among high-priority groups, protection against hospitalization was 51.3% for young children, 51.9% for people with chronic conditions and 37.7% for older adults. These figures resemble interim estimates from the 2024–25 Northern Hemisphere season — 40%–56% against outpatient illness and 34%–52% against hospitalization.
Room to improve
Only about one in five outpatients and one in six hospitalized patients had been vaccinated. Surveys of vaccine knowledge, attitudes and practices could help design better messaging and campaigns. For the coming season, providers can also recommend antiviral drugs, where available, for suspected or confirmed flu.
Limitations
- These are interim estimates and may change by season's end.
- The outpatient sample was small, limiting precision and subgroup estimates.
- 61% of patients were excluded for missing test results, which could bias results.
- Young children who needed two doses couldn't be distinguished from those who got one.
- Sequenced viruses weren't necessarily from study patients, and results may not apply to countries with different circulating viruses.
What it means for the North
The 2025–26 Northern Hemisphere vaccine has the same composition as the 2025 Southern Hemisphere vaccine, so similar protection is possible if the same viruses circulate. CDC recommends a seasonal flu vaccine for everyone 6 months and older before flu season starts.
Sources
Based on Russ S, Nogareda F, Regan AK, et al., "Interim Effectiveness Estimates of 2025 Southern Hemisphere Influenza Vaccines in Preventing Influenza-Associated Outpatient and Hospitalized Illness — Eight Southern Hemisphere Countries, March–September 2025," Morbidity and Mortality Weekly Report 74(36), Centers for Disease Control and Prevention; a work of the United States government in the public domain. Original report.
Licence: CC0 1.0 (public domain) · Adapted from www.cdc.gov
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