June 2008, Vol. 37 No. 6 465 Influenza Vaccine Efficacy in Healthcare Workers—Si Wei Kheok et al The Efficacy of Influenza Vaccination in Healthcare Workers in a Tropical Setting: A Prospective Investigator Blinded Observational Study Si Wei Kheok, 1 Chia Yin Chong, 2 MBBS, M Med (Paeds), FRCPCH, Grace McCarthy, 1,3 MD, Wai Yee Lim, 2 RN, Khean Teik Goh, 4 MBBS, M Med (Fam Med), Lubna Razak, 4 MPH, Nancy WS Tee, 2 MBBS, FRCPA, Paul Ananth Tambyah, 1,4 MBBS, FAMS Introduction Influenza causes significant morbidity, mortality and economic impact in Singapore. According to a recent report, there are about 4.2 million cases of influenza-like illness (ILI) annually in Singapore, leading to 3.5 million doctor visits and 2.1 million days of documented medical leave. 1 The authors of that report indicated that up to 15% of patients with ILI were virologically confirmed with influenza virus infection. 1 The influenza vaccine has been shown to be effective in temperate regions with well-defined seasonal influenza. However, there are limited data on the efficacy of trivalent influenza vaccine in healthcare workers (HCWs) in the tropics, where influenza is believed to be a perennial disease with occasional peaks but no regular clearly defined “season”. 2 To our knowledge, the only published study to date reported a vaccine efficacy of 52.6% in reducing ILI, and 76% in reducing medical leave due to ILI in Malaysian dental students and staff. 3 In the Malaysian study, HCWs were only followed up for 4 months after vaccination. Waning immunity in the months after initial vaccination may further decrease vaccine effectiveness in a setting with 1 Yong Loo Lin School of Medicine, National University of Singapore, Singapore 2 Kandang Kerbau Women’s and Children’s Hospital, Singapore 3 Duke University Health System, Durham NC, USA 4 National University Hospital, Singapore Address for Correspondence: A/Prof Paul Ananth Tambyah, Division of Infectious Diseases, Yong Loo Lin School of Medicine, National University of Singapore, 5 Lower Kent Ridge Rd, Singapore 119074. Email: mdcpat@nus.edu.sg Abstract Introduction: 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. Materials and 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. Ann Acad Med Singapore 2008;37:465-9 Key words: Efficacy, Healthcare workers, Influenza vaccination, Vaccine mismatch Original Article