Effectiveness of Seasonal Influenza Vaccinations Against Laboratory-Confirmed Influenza-Associated Infections Among Singapore Military Personnel in 2010–2013

Hin Peow Ho; Xiahong Zhao; Junxiong Pang; Mark I.-C. Chen; Vernon J. M. Lee; Li Wei Ang; Raymond V. Tzer Pin Lin; Christine Q. Gao; Li Yang Hsu; Alex R. Cook

Disclosures

Influenza Resp Viruses. 2014;8(5):557-566. 

In This Article

Abstract and Introduction

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.

Introduction

Singapore is a city state that lies off the southern tip of the Malay Peninsula. Its location, <200 km from the equator, bestows a near constant year-round temperature and humidity and only moderately elevated rainfall during the monsoons. As a result, the timing of influenza outbreaks in Singapore is much less predictable than in temperate countries, with year-round potential for influenza transmission.

All male residents of Singapore undertake 2 years of national service on reaching adulthood and most serve in the Singapore Armed Forces (SAF). Servicemen initially undertake training at a dedicated camp ('the recruit camp' henceforth) off the main island of Singapore, Pulau Ujong. On completing training, they are posted to other camps, mostly on Pulau Ujong itself. Their service takes place in a semi-closed environment, and servicemen usually return to their families during the weekend, which may promote the transmission of infections between servicemen and the public. While on camp, soldiers live, work and socialise in close proximity, with potential for large outbreaks of respiratory infections such as influenza, akin to those observed in other closed or semi-closed populations, such as boarding schools or prisons.[1]

The SAF implemented routine vaccination of new recruits with the H1N1-2009 vaccine when it became available at the end of 2009. Previous research found that this policy reduced H1N1-2009 disease burden among servicemen and hence recommended routine influenza vaccination of new recruits with the trivalent vaccine.[2] Other settings have seen mixed findings on the effectiveness of non-pandemic or seasonal influenza vaccines. A study of seasonal influenza vaccine effectiveness (VE) among US military basic trainees in the 2005–2006 season showed that the VE against laboratory-confirmed influenza was 92%.[3] Outside militaries, a 2011 systemic review showed pooled effectiveness of 59% for seasonal influenza by trivalent inactivated vaccine in the protection of adults aged 18–65 years.[4] Other post-pandemic studies revealed an overall VE of 56% in the UK,[5] 60% in the USA,[6] 55% in Spain,[7] and 30% against severe cases in high-risk individuals in France for H1N1-2009.[8] Although these estimates indicate only moderate effectiveness on an individual basis, the protection afforded by herd immunity in a closed or semi-closed population, such as the SAF, may potentially result in greater effectiveness for a vaccine programme in reducing morbidity and absenteeism.

There is a paucity of studies that assess the effectiveness of blanket seasonal influenza vaccination over time in the same community. As a result, the expansion of the SAF vaccination programme to include all servicemen using the trivalent vaccine from 2011 onwards provides a unique opportunity to assess the effect of mass influenza vaccination to approximately half of an entire birth year cohort in a country. We estimated effectiveness of the 2011–2012 and 2012–2013 trivalent influenza vaccines in the Singapore military population using the standard test positive and test negative methodology, and the impact of the programme on reducing both influenza-related morbidity and overall febrile respiratory illness using a time series analysis approach.

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