Effect of Vaccination on Preventing Influenza-Associated Hospitalizations Among Children During a Severe Season Associated with B/Victoria Viruses, 2019-2020

The 2019-2020 influenza season was characterized by early onset with B/Victoria followed by A(H1N1)pdm09 viruses. Emergence of new B/Victoria viruses raised concerns about possible vaccine mismatch. We estimated vaccine effectiveness (VE) against influenza-associated hospitalizations and emergency department (ED) visits among U.S. children.

We assessed VE among children 6 months–17 years with acute respiratory illness and ≥10 days of symptoms enrolled at 7 pediatric medical centers in the New Vaccine Surveillance Network. Combined mid-turbinate/throat swabs were tested for influenza virus using molecular assays. Vaccination history was collected from parental report, state immunization information systems, and/or provider records. We estimated VE from a test-negative design using logistic regression to compare odds of vaccination among children testing positive versus negative for influenza.

Among 2029 inpatients, 335 (17%) were influenza positive: 37% with influenza B/Victoria alone and 44% with influenza A(H1N1)pdm09 alone.VE was 62% (95% confidence interval [CI], 52%–71%) for influenza-related hospitalization, 54% (95% CI, 33%–69%) for B/Victoria viruses and 64% (95% CI, 49%–75%) for A(H1N1)pdm09. Among 2102 ED patients, 671 (32%) were influenza positive: 47% with influenza B/Victoria alone and 42% with influenza A(H1N1)pdm09 alone. VE was 56% (95% CI, 46%–65%) for an influenza-related ED visit, 55% (95% CI, 40%–66%) for B/Victoria viruses and 53% (95% CI, 37%–65%) for A(H1N1)pdm09.

Influenza vaccination provided significant protection against laboratory-confirmed influenza-associated hospitalizations and ED visits associated with the two predominantly circulating influenza viruses among children, including against the emerging B/Victoria virus subclade.