Sustained Within-season Vaccine Effectiveness Against Influenza-associated Hospitalization in Children: Evidence From the New Vaccine Surveillance Network, 2015–2016 Through 2019–2020
Leila C. Sahni, Eric A. Naioti, Samantha M. Olson, Angela P. Campbell, Marian G. Michaels, John V. Williams, Mary Allen Staat, Elizabeth P. Schlaudecker, Monica McNeal, Natasha Halasa, Laura S Stewart, James D. Chappell, Janet A. Englund, Eileen J. Klein, Peter G. Szilagyi, Geoffrey A. Weinberg, Christopher J. Harrison, Rangaraj Selvarangan, Jennifer E. Schuster, Parvin H. Azimi, Monica N Singer, Vasanthi Avadhanula, Pedro A. Piedra, Flor M. Muñoz, Manish M. Patel, Julie A. Boom
Abstract
BACKGROUND: Adult studies have demonstrated within-season declines in influenza vaccine effectiveness (VE); data in children are limited. METHODS: We conducted a prospective, test-negative study of children 6 months through 17 years hospitalized with acute respiratory illness at 7 pediatric medical centers during the 2015-2016 through 2019-2020 influenza seasons. Case-patients were children with an influenza-positive molecular test matched by illness onset to influenza-negative control-patients. We estimated VE [100% × (1 - odds ratio)] by comparing the odds of receipt of ≥1 dose of influenza vaccine ≥14 days before illness onset among influenza-positive children to influenza-negative children. Changes in VE over time between vaccination date and illness onset date were estimated using multivariable logistic regression. RESULTS: Of 8430 children, 4653 (55%) received ≥1 dose of influenza vaccine. On average, 48% were vaccinated through October and 85% through December each season. Influenza vaccine receipt was lower in case-patients than control-patients (39% vs 57%, P < .001); overall VE against hospitalization was 53% (95% confidence interval [CI]: 46, 60%). Pooling data across 5 seasons, the odds of influenza-associated hospitalization increased 4.2% (-3.2%, 12.2%) per month since vaccination, with an average VE decrease of 1.9% per month (n = 4000, P = .275). Odds of hospitalization increased 2.9% (95% CI: -5.4%, 11.8%) and 9.6% (95% CI: -7.0%, 29.1%) per month in children ≤8 years (n = 3084) and 9-17 years (n = 916), respectively. These findings were not statistically significant. CONCLUSIONS: We observed minimal, not statistically significant within-season declines in VE. Vaccination following current Advisory Committee on Immunization Practices (ACIP) guidelines for timing of vaccine receipt remains the best strategy for preventing influenza-associated hospitalizations in children.