Impact of a quadrivalent inactivated influenza vaccine on influenza-associated complications and health care use in children aged 6 to 35 months: Analysis of data from a phase III trial in the Northern and Southern Hemispheres.

Background: A multi-season phase III trial conducted in the Northern and Southern Hemispheres demonstrated the efficacy of a quadrivalent split-virion inactivated influenza vaccine (IIV4) in children 6-35 months of age.

Methods: Data collected during the phase III trial were analysed to examine the vaccine efficacy (VE) of IIV4 in preventing laboratory-confirmed influenza in age subgroups and to determine the relative risk for IIV4 vs. placebo for severe outcomes, healthcare use, and parental absenteeism from work associated with laboratory-confirmed influenza.

Results: VE (95% confidence interval [CI]) to prevent laboratory-confirmed influenza due to any A or B strain was 54.76% (40.24-66.03%) for participants aged 6-23 months and 46.91% (23.57-63.53%) for participants aged 24-35 months. VE (95% CI) to prevent laboratory-confirmed influenza due to vaccine-similar strains was 74.51% (53.55-86.91%) for participants aged 6-23 months and 59.78% (19.11-81.25%) for participants aged 24-35 months. Compared to placebo, IIV4 reduced the risk (95% CI) by 31.28% (8.96-89.34%) for acute otitis media, 21.76% (6.46-58.51%) for acute lower respiratory infection, 40.80% (29.62-55.59%) for healthcare medical visits, 29.71% (11.66-67.23%) for parent absenteeism from work, and 39.20% (26.89-56.24%) for antibiotic use.

Conclusion: In children aged 6-35 months, vaccination with IIV4 reduces severe outcomes of influenza as well as the associated burden for their parents and the healthcare system. In addition, vaccination with IIV4 is effective at preventing against influenza in children aged 6-23 and 24-35 months.

Trial Registration: EudraCT no. 2013-001231-51.