HealthDay News — Influenza vaccine effectiveness was 75% against life-threatening influenza in children during the 2019 to 2020 season, when vaccine-mismatched viruses predominated, according to a study published online in Clinical Infectious Diseases.

Samantha M. Olson, MPH, from the US Centers for Disease Control and Prevention in Atlanta, and colleagues enrolled children aged younger than 18 years admitted to the intensive care unit with acute respiratory infection across 17 hospitals and tested respiratory specimens for influenza viruses. A test-negative design was used to estimate vaccine effectiveness comparing the odds of vaccination in test-positive cases versus test-negative controls. A total of 159 critically ill influenza case patients (51% A/H1N1pdm09 and 25% B-Victoria viruses) and 132 controls were enrolled.

The researchers found that 52% of the 56 sequenced A/H1N1pdm09 viruses were vaccine-mismatched (A/H1N1pdm09/5A+156K) and 41% were vaccine-matched (A/H1N1pdm09/5A+187A,189E). Most (30 of 31) sequenced B-lineage viruses were vaccine-mismatched. Effectiveness was 75% and 57% against life-threatening and non-life-threatening influenza, respectively. Effectiveness was 78% and 47% against matched and mismatched A/H1N1pdm09 viruses, respectively, and 75% against mismatched B-Victoria viruses.

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“These data are particularly impressive in the context of circulation of 2 vaccine-mismatched viruses,” the authors write. “Our real-world findings suggest that accelerating efforts to bring influenza vaccines to all children could lead to appreciable reductions in critical illness and deaths from influenza worldwide.”

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