The 2024 influenza season has already occurred in the Southern Hemisphere. The H3N2 influenza virus has circulated and mutated so that the vaccine was not as effective as in previous years. The CDC found this year’s vaccine had a 34.5% effectiveness against hospitalization, compared to 51.9% effectiveness of last year’s vaccine. The United States has not changed its vaccine recommendations.
It has decided to use trivalent vaccines with the same antigens as those in Southern Hemisphere vaccines.If a similar influenza A(H3N2) strain circulates in the U.S., we can expect a similar efficacy during the upcoming flu season. Reference 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 Morb Mortal Wkly Rep 2024;73:861–868. DOI: http://dx.doi.org/10.15585/mmwr.mm7339a1
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