Flu Vaccine in South America, 2024: A 34.5% Cut in Hospitalization Despite Low Uptake

Flu season runs April–September in the Southern Hemisphere and October–May in the Northern. Across the Americas, flu is estimated to cause 716,000–829,000 hospitalizations and 41,007–71,710 deaths a year, and most countries run vaccination programs. Early vaccine effectiveness (VE) estimates from the south hint at what the north can expect if the same viruses circulate.

The Pan American Health Organization's REVELAC-i network measured how well the 2024 Southern Hemisphere vaccine kept high-risk people out of hospital in Argentina, Brazil, Chile, Paraguay and Uruguay, March 13 to July 19, 2024.

How

• Patients: people hospitalized with severe acute respiratory illness (SARI) — fever and cough starting within 10 days — at 2,535 hospitals (2,477 of them in Brazil), in the three groups targeted for vaccination: young children, people with chronic conditions, and older adults.
• Design: test-negative — patients testing positive for flu were compared with those testing negative for both flu and COVID-19. Vaccination (at least 14 days before symptoms) came from national electronic immunization records.
• Vaccines: WHO-recommended egg-based Southern Hemisphere formulations — trivalent in four countries, quadrivalent in Paraguay.

Who was studied

11,751 patients qualified — 77.4% from Brazil — of whom 58.3% were young children, 14.5% people with chronic conditions, and 27.2% older adults. 3,848 (32.7%) tested positive for flu, and 98.6% of their viruses were influenza A; of the A viruses subtyped, 68.3% were A(H3N2) and 31.7% A(H1N1)pdm09. Only 26 had influenza B (all B/Victoria).

Patients hospitalized with severe acute respiratory infection who tested positive for influenza, by week, REVELAC-i Network, five South American countries, March–July 2024. CDC / MMWR

Few vaccinated

Only 21.3% of patients were vaccinated — 29.3% of older adults, 19.4% of young children and 14.5% of people with chronic conditions — well below pre-COVID levels, in line with post-pandemic declines across the Americas linked to misinformation, hesitancy and disrupted immunization services. Among flu patients 18.3% were vaccinated, against 22.8% of flu-negative patients.

But the vaccine worked

The overall 34.5% lies within the historical ranges (34–53% against H3N2, 18–56% against H1N1). VE was lowest in Brazil, probably because more of its cases were young children. Too few B cases were available to estimate VE against influenza B.

Most H3N2 viruses belonged to clade 2a.3a.1, subclade J.2 (92.3%); most H1N1 were clade 5a.2a.1 (64.2%) or 5a.2a (35.8%).

For the north

If similar viruses dominate the 2024–25 Northern Hemisphere season, and the updated A/Thailand/8/2022 (H3N2)-like component protects similarly, similar protection can be expected. So:

• Everyone 6 months and older who is eligible should get a flu vaccine (CDC and WHO recommendation).
• Doctors should give antivirals to people with suspected or confirmed flu as early as possible.
• Hand washing and masks add protection.

Limits

• Too few samples to estimate VE against influenza B.
• 63% of patients were excluded because their flu test results were not ready in time.
• Brazil — about three times the population of the other four combined — made up most of the data.
• Children with one and two vaccine doses were not distinguished; two doses may protect better.
• Results may not apply to other groups or countries with different viruses and strategies.

Sources

Based on Zeno EE, Nogareda F, Regan A, et al., "Interim Effectiveness Estimates of 2024 Southern Hemisphere Influenza Vaccines in Preventing Influenza-Associated Hospitalization — REVELAC-i Network, Five South American Countries, March–July 2024," MMWR Morbidity and Mortality Weekly Report, volume 73, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The report's counts of excluded patients do not add up to its total, and its shares of flu patients by group exceed 100%, so neither is given here.