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

27 Jan 2021
Campbell AP, Ogokeh C, Weinberg GA, Boom JA, Englund JA, Williams JV, Halasa NB, Selvarangan R, Staat MA, Klein EJ, McNeal M, Michaels MG, Sahni LC, Stewart LS, Szilagyi PG, Harrison CJ, Lively JY, Rha B, Patel M


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.