CDC: Influenza vaccine efficacy in South America is 35%, similar season could be expected in the US

The effectiveness of seasonal influenza vaccines against hospitalizations in high-risk groups during the 2024 seasonal influenza season in five Southern Hemisphere countries is low (35%); 50% Although it is expected in 2023, it is not outside the range of predictions, and if similar A(H3N2) viruses are dominant, it is possible that a similar effect can be expected in the future Northern Hemisphere influenza season. reported researchers at the Centers for Disease Control and Prevention (CDC).

of Intermediate estimatepublished late last week Morbidity and Mortality Weekly Report (MMWR) were from Argentina, Brazil, Chile, Paraguay and Uruguay, and on average only 21.3% of patients hospitalized with influenza from March to July 2024 (southern hemisphere influenza season) had received an influenza vaccination. It was.

“While data from the Southern Hemisphere cannot accurately predict what will happen in the Northern Hemisphere, what we can say is that if we see similar influenza viruses (strains or clades) circulating in the Northern Hemisphere this season, then influenza “We expect similar vaccine efficacy against influenza and associated hospitalizations,” lead author Dr. Erica Zeno, an epidemic information service officer in the CDC's Division of Influenza, told CIDRAP News in an email.

This flu season, the same trivalent (three-strain) vaccine is being used in the United States. component Zeno said it's the same as what's in vaccines in the southern hemisphere.

On August 29th, CDC The report states that “we have moderate confidence that the upcoming influenza season will be equal to or worse than the 2023-2024 season, which had 8.9 laboratory-confirmed hospitalizations per week per 100,000 people.” I predict it will be low.” “However, the numbers of illnesses, hospitalizations, and deaths in past seasons have varied widely depending on the subtypes that are prevalent, the immunity of the population to the different subtypes, and the effectiveness of vaccines against the subtypes that are prevalent. .”

Campaign targeting high-risk groups

Researchers analyzed data from a multinational surveillance network using a test-negative case-control design to estimate preliminary efficacy (VE) of vaccines against hospitalization for influenza-associated severe acute respiratory illness (SARI). generated a value.

The study population consisted 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%). ), elderly (27.2%). The case patient was infected with SARI and tested positive for influenza, whereas the control patient was infected with SARI and tested negative for influenza and the new coronavirus.

Data were collected from 30 hospitals in Argentina, 2,477 hospitals in Brazil, 13 hospitals in Chile, 5 hospitals in Paraguay, and 10 hospitals in Uruguay, starting two weeks after each country's influenza vaccination campaign. Vaccination status was confirmed using national electronic vaccination records.

All countries used the egg-based Southern Hemisphere formulation recommended by the World Health Organization (WHO). Argentina, Brazil, Chile, and Uruguay have launched trivalent (three-strain) vaccines containing antigens from A/Victoria/4897/2022 (H1N1)pdm09-like virus, A/Thailand/8/2022 (H3N2)-like virus, and B. I used it. /Austria/1359417/2021 (B/Victoria lineage)-like virus was used, while Paraguay used a quadrivalent (four-strain) vaccine that also included B/Yamagata lineage-like virus.

Vaccination rates lower than pre-pandemic norms

Five countries reported 11,751 influenza-related SARI cases and had low average seasonal influenza vaccination coverage. “Documented influenza vaccination coverage (21.3%) was below pre-COVID-19 norms,” ​​the researchers wrote. “This finding is related to vaccine misinformation, hesitancy, and interruptions in routine immunization services that were prevalent during the COVID-19 pandemic, and post-pandemic vaccination rates have declined across the Americas. It is consistent with the fact that