Efficacy, immunogenicity, and safety of a quadrivalent inactivated influenza vaccine in children aged 6-35 months: A multi-season randomised placebo-controlled trial in the Northern and Southern Hemispheres.

Background: A quadrivalent split-virion inactivated influenza vaccine (VaxigripTetra™, Sanofi Pasteur; IIV4) containing two A strains (H1N1 and H3N2) and B strains from both lineages (Victoria and Yamagata) was approved in Europe in 2016 for individuals aged ≥ 3 years. This study examined the efficacy and safety of IIV4 in children aged 6-35 months.

Methods: This was a phase III randomised controlled trial conducted in Latin America, Asia, Africa, and Europe during the Northern Hemisphere 2014/2015 and 2015/2016 and Southern Hemisphere 2014 and 2015 influenza seasons. Healthy children aged 6-35 months not previously vaccinated against influenza were randomised to receive two full doses 28 days apart of IIV4, placebo, the licensed trivalent split-virion inactivated vaccine (IIV3), an investigational IIV3 containing a B strain from the alternate lineage. The primary objective was to demonstrate efficacy against influenza illness caused by any strain or vaccine-similar strains.

Results: The study enrolled 5806 participants. Efficacy, assessed in 4980 participants completing the study according to protocol, was demonstrated for IIV4. Vaccine efficacy was 50.98% (97% CI, 37.36-61.86%) against influenza caused by any A or B type and 68.40% (97% CI, 47.07-81.92%) against influenza caused by vaccine-like strains. Safety profiles were similar for IIV4, placebo, and the IIV3s, although injection-site reactions were slightly more frequent for IIV4 than placebo.

Conclusions: IIV4 was safe and effective for protecting children aged 6-35 months against influenza illness caused by vaccine-similar or any circulating strains.

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