2024-25 Flu vaccine may be less effective than 2023-24 vaccine, but still expected to significantly reduce hospitalization risk

Influenza vaccine effectiveness is known to vary by season. Influenza epidemics typically occur during the cool weather months of April–September in the Southern Hemisphere and October–May in the Northern Hemisphere.1 To reduce influenza-associated morbidity and mortality, countries in South America recommend annual influenza vaccination for persons at high risk for severe influenza illness, including young children, persons with preexisting health conditions and older adults.1Southern Hemisphere influenza vaccine effectiveness is not necessarily predictive of Northern Hemisphere vaccine effectiveness. However, interim estimates of influenza vaccine effectiveness from Southern Hemisphere countries can provide early information about the likely protective effects of vaccination and help inform Northern Hemisphere countries about expected vaccine effectiveness in advance of their flu season.1Using data from a network of five South American countries (Argentina, Brazil, Chile, Paraguay, Uruguay), investigators estimated interim vaccine effectiveness against influenza-associated severe acute respiratory illness hospitalization. All countries used World Health Organization (WHO)–recommended egg-based Southern Hemisphere trivalent formulations containing antigens from A/Victoria/4897/2022 (H1N1)pdm09–like virus, A/Thailand/8/2022 (H3N2)–like virus, and B/Austria/1359417/2021 (B/Victoria lineage)–like virus, which are also the antigens recommended for flu vaccines in North American countries in 2024/25. One country (Paraguay) used quadrivalent vaccines that also contained the B/Yamagata lineage–like virus.1During March 13 to July 19, 2024, the five countries identified 11,751 influenza-associated severe acute respiratory illness cases. Overall, only 21.3% of patients were vaccinated against influenza. Vaccination coverage varied by target group: 29.3% of older adults, 19.4% of young children, and 14.5% of persons with comorbidities.1 The adjusted vaccine effectiveness against any influenza-associated hospitalization was 34.5% overall, including 58.7% among persons with comorbidities, 39.0% among young children, and 31.2% among older adults. The adjusted vaccine effectiveness against the predominating subtypes was 36.5% for A(H3N2) and 37.1% for A(H1N1)pdm09.1 Last flu season, the flu vaccine in those five countries had an overall effectiveness of 51.9% against influenza-associated severe acute respiratory illness hospitalization.2The investigators comment that the documented influenza vaccination coverage levels (21.3%) were below pre–COVID-19 norms. This finding is consistent with postpandemic declines in vaccination coverage across the Americas, which may be associated with vaccine misinformation, vaccine hesitancy and disruptions in routine immunization services, prevalent during the COVID-19 pandemic.1These interim vaccine effectiveness estimates suggest that although the proportion of hospitalized patients who were vaccinated was only modest, vaccination with the Southern Hemisphere influenza vaccine significantly lowered the risk for hospitalization. If similar influenza viruses continue to predominate during the Northern Hemisphere influenza season, similar levels of protection might be anticipated.1 The investigators state that these findings support the CDC and World Health Organization recommendation to vaccinate all eligible persons six months of age or older against influenza. They recommend that Northern Hemisphere countries should anticipate the need for robust influenza vaccination campaigns to achieve optimal protection against influenza-associated complications. Other protective measures, such as handwashing and mask use, and early antiviral treatment, can complement vaccination for stronger protection against influenza illness and its complications.1Canadian guidelines A summary of the National Advisory Committee on Immunization (Canada) statement on seasonal influenza vaccine for 2024–25, guidance on flu vaccines for older adults and a discussion of the transition from quadrivalent to trivalent influenza vaccines is available here.3References:Zeno EE, Nogareda F, Regan A, et al; REVELAC-i Network. 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 Morb Mortal Wkly Rep 2024;73:861-8. (accessed October 15, 2024).Fowlkes AL, Nogareda F, Regan A, et al; REVELAC–i Network, Interim effectiveness estimates of 2023 Southern Hemisphere influenza vaccines in preventing influenza-associated hospitalizations — REVELAC–i Network, March–July 2023. MMWR Morb Mortal Wkly Rep 2023;72:1010-15. (accessed October 16, 2024).Murdoch L. NACI releases statement on seasonal influenza vaccine for 2024–25. National Advisory Committee on Immunization also issues guidance on flu vaccines for older adults, as well as the transition from quadrivalent to trivalent influenza vaccines. Canadian Healthcare Network August 1, 2024. (accessed October 16, 2024).

2024-25 Flu vaccine may be less effective than 2023-24 vaccine, but still expected to significantly reduce hospitalization risk

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