Seasonal influenza vaccine effectiveness among children, 2010–2012

BACKGROUND The annual differences in the seasonal influenza vaccine and the circulating strains make it necessary to assess influenza vaccine effectiveness (VE) yearly. We assessed the effectiveness of the trivalent inactivated influenza vaccine for the 2010-2011 and 2011-2012 influenza seasons among children in Guangzhou, China. METHODS We conducted a 1:2 matched case-control study based on date of birth (±7 days), gender, and area of residence. The influenza cases from surveillance sites in Guangzhou were laboratory-confirmed during the 2010-2012 seasons. The controls were randomly selected from children aged 6-59 months in the Children's Expanded Programmed Immunization Administrative Computerized System. The influenza vaccination information for both cases and controls was retrieved from this system. RESULTS We analyzed the vaccination information for 1255 influenza cases and 2510 matched controls in 2 influenza seasons in Guangzhou, China. We found that the VE for vaccination during the 2010-2011 and 2011-2012 seasons of virus circulation was 73·2% (95% confidence interval (CI), 52·2-85·0%) and 52·9% (95% CI, 42·1-61·7%), respectively. The VE decreased from 68·9% (95% CI, 57·5-77·2%) in the period between January and March to 48·4% (95% CI, 33·8-59·7%) in the period between April and June. CONCLUSIONS This post-licensing study of VE found moderate protection against influenza for vaccinated children aged 6-59 months. Although the influenza vaccine strains for the 2010-2011 and the 2011-2012 seasons were the same, our study indicated that annual vaccination is recommended even for those who received the vaccine during the previous season.