Effectiveness of the 2013 and 2014 Southern Hemisphere Influenza Vaccines Against Laboratory-confirmed Influenza in Young Children Using a Test-negative Design, Bangkok, Thailand

Background: The Thai Advisory Committee on Immunization Practices recommends annual influenza vaccination for children 6 months through 2 years of age, although older children may be vaccinated on request. We evaluated the effectiveness of the 2013 and 2014 inactivated influenza vaccines to reduce medically attended, laboratory-confirmed influenza illness among Thai children aged 7–60 months. Methods: From September 2013–May 2015, children with influenza-like illness were screened with a rapid influenza diagnostic test. Enrolled children had nasal and throat swabs tested for influenza viruses using polymerase chain reaction. Cases and controls were subjects testing positive and negative, respectively, for influenza viruses by polymerase chain reaction. Vaccination status was ascertained from vaccination cards. Vaccine effectiveness (VE) was calculated as 100% × (1 − odds ratio of vaccination among cases vs. controls). Results: Of the 1377 children enrolled, cases (n = 490) and controls (n = 887) were similar in demographic characteristics. Cases were less likely to receive influenza vaccine than controls in 2013 (6% vs. 14%; P = 0.02), but not in 2014 (6% vs. 7%; P = 0.57). Among cases, 126 (26%) were positive for influenza A(H1N1)pdm09 virus, 239 (49%) for influenza A(H3N2) and 124 (25%) for influenza B. One specimen was positive for both influenza A(H3N2) and B viruses. VE for full vaccination against all viruses was 64% (95% confidence interval: 21% to 84%) in 2013 and 26% (95% confidence interval: −47% to 63%) in 2014. Conclusions: Influenza vaccination was low among Thai children in this study, and VE varied by year, highlighting the need for annual monitoring of VE to better understand vaccine program effectiveness.