Studies have shown egg-adaptive mutations in influenza vaccine strains that might have impaired protection against circulating A(H3N2) influenza viruses during the 2016–2017 and 2017–2018 seasons. We used the test-negative design and multivariable models to assess vaccine effectiveness against influenza-associated hospitalization and emergency department visits among children (<18 years old) during the 2016–2017 and 2017–2018 seasons. Effectiveness was 71% (95% confidence interval, 59%–79%), 46% (35%–55%), and 45% (33%–55%) against A(H1N1)pdm09, A(H3N2), and B viruses respectively, across both seasons. During high-severity seasons with concerns for vaccine mismatch, vaccination offered substantial protection against severe influenza outcomes requiring hospitalization or emergency department visits among children.
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