The efficacy of influenza vaccination in healthcare workers in a tropical setting: a prospective investigator blinded observational study.
BACKGROUND
Influenza vaccine has been shown to be highly effective in temperate regions with well-defined seasonal influenza. Healthcare workers (HCWs) are advised to receive regular influenza vaccination to protect themselves and their patients. However, there are limited data on the efficacy of influenza vaccine in HCWs in the tropics.


METHODS
In this observational, investigator blinded cohort study, bi-monthly questionnaires recording influenza-like illness (ILI) episodes and medical leave were administered to 541 HCWs at the Singapore National University Hospital and KK Women's and Children's Hospital from 2004 to 2005. ILI was defined according to a standard symptom score.


RESULTS
Baseline characteristics were comparable in both the vaccinated and non-vaccinated groups. Overall, the relative risk of self-reported ILI in vaccinated HCWs was 1.13 [95% confidence interval (CI), 0.98-1.13; P=0.107]; medical leave taken was lower in the vaccinated group [mean 0.26+/-0.6 days per visit, compared with 0.30+/-0.5 days in the non-vaccinated group (P=0.40)]. Because of the reported Northern Hemisphere 2003/04 vaccine mismatch, we stratified the cohort and determined that the group which received a matched vaccine had a relative risk of ILI of 0.49 (95% CI, 0.37-0.66; P<0.001), achieving a vaccine efficacy of 51%. Mean medical leave decreased significantly in HCWs who received the matched vaccine, compared with those who did not receive vaccination (0.13+/-0.3 vs 0.30+/-0.5; P<0.001) and with HCWs vaccinated with mismatched strains (0.13+/-0.3 vs 0.39+/-0.9; P=0.01).


CONCLUSIONS
A well-matched influenza vaccine is effective in preventing ILI and reducing sickness absence in healthcare workers in tropical settings. Efforts need to be made to increase influenza vaccination rates and to improve the currently available vaccines.
Kheok
Si Wei
SW
Yong Loo Lin School of Medicine, National University of Singapore, Singapore.
Chong
Chia Yin
CY
McCarthy
Grace
G
Lim
Wai Yee
WY
Goh
Khean Teik
KT
Razak
Lubna
L
Tee
Nancy W S
NW
Tambyah
Paul Ananth
PA
D000328:Adult; D001291:Attitude of Health Personnel; D015331:Cohort Studies; D016001:Confidence Intervals; D005260:Female; D006282:Health Personnel / Q000706:statistics & numerical data*; D006801:Humans; D007252:Influenza Vaccines; D007251:Influenza, Human / Q000453:epidemiology / Q000517:prevention & control*; D008297:Male; D016273:Occupational Exposure; D016272:Occupational Health; D011446:Prospective Studies; D012306:Risk; D012846:Singapore / Q000453:epidemiology; D016037:Single-Blind Method; D011795:Surveys and Questionnaires; D014329:Tropical Climate
D007252:Influenza Vaccines