Estimates of Inactivated Influenza Vaccine Effectiveness Among Children in Senegal: Results From 2 Consecutive Cluster-Randomized Controlled Trials in 2010 and 2011

Abstract

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.

Effectiveness of Influenza Vaccines in the HIVE Household Cohort Over 8 Years: Is There Evidence of Indirect Protection?

“…Our results are consistent with cluster randomized trials from India [ 12 ], which demonstrated substantial total VE, even in the absence of indirect protection. Similar patterns of total and indirect VE were also found in Senegal [ 13 ].…”

“…In previous studies indirect effects have varied by age-group [ 11 , 13 ]. We explored the potential for effect modification using age-stratified models of indirect VE.…”

“…A modeling study informed by another randomized trial in Hong Kong estimated that, for influenza B, the indirect protection for household contacts could reach 20% under certain scenarios of household transmission and vaccination coverage [ 11 ]. Additional cluster-randomized studies in India [ 12 ] and Senegal [ 13 , 14 ] have estimated indirect and total VE, with mixed results.…”

Evaluation of the Clinical Effectiveness of Oseltamivir for Influenza Treatment in Children

“…In addition to anti-influenza drugs, the usefulness of influenza vaccines cannot be overemphasized. Influenza vaccination in susceptible children has been shown to be an effective measure for preventing influenza virus infection, but a “Universal” vaccine with a broad spectrum of protection needs to be developed due to rapid viral mutations ( Rolfes et al, 2019 ; Niang et al, 2021 ).…”

Using Social Contact Data to Improve the Overall Effect Estimate of a Cluster-Randomized Influenza Vaccination Program in Senegal

“…A map of the 20 villages analysed is included in Figure 1. This study, ClinicalTrials.gov NCT00893906, is closed, and the primary results for the trial have been published separately for Year 1 (Diallo et al., 2019) and Year 2 (Niang et al., 2020).…”