Effectiveness of seasonal influenza vaccinations against laboratory-confirmed influenza-associated infections among Singapore military personnel in 2010-2013

Abstract

Background: Limited information is available about seasonal influenza vaccine effectiveness (VE) in tropical communities.

Objectives: Virus subtype-specific VE was determined for all military service personnel in the recruit camp and three other non-recruit camp in Singapore's Armed Forces from 1 June 2009 to 30 June 2012.

Methods: Consenting servicemen underwent nasal washes, which were tested with RT-PCR and subtyped. The test positive case and test negative control design was used to estimate the VE. To estimate the overall effect of the programme on new recruits, we used an ecological time series approach.

Results: A total of 7016 consultations were collected. The crude estimates for the VE of the triavalent vaccine against both influenza A(H1N1)pdm09 and influenza B were 84% (95% CI 78-88%, 79-86%, respectively). Vaccine efficacy against influenza A(H3N2) was markedly lower (VE 33%, 95% CI -4% to 57%). An estimated 70% (RR = 0.30; 95% CI 0.11-0.84), 39% (RR = 0.61;0.25-1.43) and 75% (RR = 0.25; 95% CI 0.11-0.50) reduction in the risk of influenza A(H1N1)pdm09, influenza A(H3N2) and influenza B infections, respectively, in the recruit camp during the post-vaccination period compared with during the pre-vaccination period was observed.

Conclusions: Overall, the blanket influenza vaccine programme in Singapore's Armed Forces has had a moderate to high degree of protection against influenza A(H1N1)pdm09 and influenza B, but not against influenza A(H3N2). Blanket influenza vaccination is recommended for all military personnel.

Keywords: influenza A(H1N1)pdm09; influenza A(H3N2); influenza B; influenza vaccination; vaccine effectiveness.

Figure 1

Distribution of Weekly National Influenza Cases versus Singapore Armed Forces (SAF) Influenza Cases by Subtype (A, B, C) and Weekly National Upper Respiratory Tract Infection (URTI) Cases versus SAF Febrile Respiratory Illness (FRI) Cases (D). On panels A–C, national influenza cases are represented by lines, while SAF influenza cases are represented by bars (blue for unvaccinated, orange for those vaccinated ≤14 days prior to consultation and white for vaccinated >14 days prior). On panel D, weekly national URTI cases are represented by the black line, while the weekly FRI cases in military camps are represented by red bars. The scales for y-axes used are different for B and D. The time is measured in calendar months on each panel, and the longer tick marks at each x-axis represent the start and end of each vaccination period. An additional time axis is presented at the foot, where period 1 refers to the pre-vaccination period; period 2 refers to the period new recruits were given monovalent vaccination; period 3 refers to the period new recruits were given trivalent vaccination; and period 4 refers to the period all SAF servicemen received trivalent vaccination.

Figure 2

Comparison of Recruit Camp Influenza Cases (A, B, C) and Febrile Respiratory Illness (FRI) Cases (D) with Predictions from the Ecological Time Series Model. Lines represent observations (red) and predictions (blue). Shaded regions are 95% confidence intervals (dark blue) and prediction intervals (light blue). The y-axis scales differ for B and D.