BACKGROUND: We report results of Years 2 and 3 of consecutive cluster-randomized controlled trials of trivalent inactivated influenza vaccine (IIV3) in Senegal.
METHODS: We cluster-randomized (1:1) 20 villages to annual vaccination with IIV3 or inactivated poliovirus vaccine (IPV) of age-eligible residents (6 months - 10 years). The primary outcome was total vaccine effectiveness against laboratory-confirmed influenza illness (LCI) among age-eligible children (modified-intention-to-treat population [mITT]). Secondary outcomes were indirect (herd protection) and population (overall community) vaccine effectiveness.
RESULTS: We vaccinated 74% of 12,408 age-eligible children in Year 2 (June 2010-April 11) and 74% of 11,988 age-eligible children in Year 3 (April 2011-December 2011) with study vaccines. Annual cumulative incidence of LCI was 4.7 (Year 2) and 4.2 (Year 3) per 100 mITT child vaccinees of IPV villages. In Year 2, IIV3 matched circulating influenza strains. The total effectiveness was 52.8% (95% CI: 32.3%-67.0%), and the population effectiveness was 36.0% (95% CI: 10.2%-54.4%) against LCI caused by any influenza strain. The indirect effectiveness against LCI by A/H3N2 was 56.4% (95% CI: 39.0%-68.9%). In Year 3, 74% of influenza detections were vaccine-mismatched to circulating B/Yamagata and 24% were vaccine-matched to circulating A/H3N2. The Year 3 total effectiveness against LCI was -14.5% (95% CI: -81.2%-27.6%). Vaccine effectiveness varied by type/subtype of influenza in both years.
CONCLUSION: IIV3 was variably effective against influenza illness in Senegalese children, with total and indirect vaccine effectiveness present during the year when all circulating strains matched the IIV3 formulation.