The effectiveness of the seasonal flu vaccine against hospitalization among high-risk groups during the 2024 seasonal flu season in five Southern Hemisphere countries was low (35%), down from 50% in 2023 but not outside the expected range, which may portend similar efficacy during the upcoming Northern Hemisphere flu season if similar A(H3N2) viruses predominate, Centers for Disease Control and Prevention (CDC) researchers report.
The interim estimates, published late last week in Morbidity and Mortality Weekly Report (MMWR), come from Argentina, Brazil, Chile, Paraguay, and Uruguay, where an average of only 21.3% of patients hospitalized for flu from March to July 2024 (the Southern Hemisphere flu season) had received the flu vaccine.
“While Southern Hemisphere data cannot predict exactly what will happen in the Northern Hemisphere, what we can say is that if we see similar influenza viruses (strains or clades) circulating this season in the Northern Hemisphere, we can expect similar vaccine effectiveness against influenza-related hospitalizations,” lead author Erica Zeno, PhD, epidemic intelligence service officer with the CDC’s Influenza Division, told CIDRAP News in an email.
This flu season, the United States is using trivalent (three-strain) vaccines with the same components as those in Southern Hemisphere vaccines, Zeno said.
On August 29, the CDC forecasted that the upcoming flu season “will be similar to or lower than that of the 2023-2024 season rate of 8.9 weekly laboratory-confirmed hospitalizations per 100,000, with moderate confidence,” it wrote. “However, past seasons have varied widely in the number of illnesses, hospitalizations, and deaths, depending on the subtypes circulating, population immunity to different subtypes, and vaccine effectiveness against circulating subtypes.”
Campaign targeted high-risk groups
The researchers used a test-negative, case-control design to analyze data from a multinational surveillance network to generate estimates of interim vaccine effectiveness (VE) against hospitalization with flu-related severe acute respiratory illness (SARI).
The study population was made up of 11,751 SARI patients from three Pan American Health Organization (PAHO) vaccination target groups: young children (58.3%), older children and people with underlying medical conditions (14.5%), and older adults (27.2%). Case-patients had SARI and tested positive for flu, while control patients had SARI and tested negative for flu and COVID-19.
Data were pooled from 30 hospitals in Argentina, 2,477 in Brazil, 13 in Chile, 5 in Paraguay, and 10 in Uruguay beginning 2 weeks after each country’s flu vaccination campaign. Vaccination status was confirmed using national electronic vaccination records.
All countries used World Health Organization (WHO)–recommended egg-based Southern Hemisphere formulations. Argentina, Brazil, Chile, and Uruguay used trivalent (three-strain) vaccines 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, while Paraguay used quadrivalent (four-strain) vaccines that also contained the B/Yamagata lineage–like virus.
Vaccination rate lower than prepandemic norms
The five countries reported 11,751 flu-related SARI cases and an average low seasonal flu vaccination rate. “The documented influenza vaccination coverage levels (21.3%) were below pre–COVID-19 norms,” the researchers wrote. “This finding is consistent with postpandemic declines in vaccination coverage across the Americas associated with vaccine misinformation, hesitancy, and disruptions in routine immunization services, prevalent during the COVID-19 pandemic.”
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Publish date : 2024-10-07 05:34:00
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