Effectiveness of the influenza a(H1N1)PDM09 vaccine in adults recommended for annual influenza vaccination: A case-control study

Background: Because of variability in published A(H1N1)pdm09 influenza vaccine effectiveness estimates, we aimed to assess the effectiveness of MF59-adjuvanted A(H1N1)pdm09 vaccine in a matched case-control study. Objectives: We aimed to assess the effectiveness of MF59- adjuvanted A(H1N1)pdm09 influenza vaccine in a matched case-control study. Methods: This study was conducted during the pandemic influenza season 2009-2010 in adults with underlying comorbidities and healthy subjects of 60 years and older recommended for annual influenza vaccination in the Netherlands. Sixteen laboratory- confirmed and eligible A(H1N1)pdm09 influenza cases registered by four Community Health Services of the Netherlands were included. The control population was retrieved from a general practice database and consisted of subjects who had not been registered with influenza code R80 (according to the International Classification of Primary Care) during the A(H1N1)pdm09 influenza season. The controls were matched to cases on sex, age and comorbidities. The primary outcome was laboratory-confirmed A(H1N1)pdm09 influenza. Odds ratios (OR) and their 95% confidence intervals (95% CI) were calculated. Vaccine effectiveness (VE) was expressed as VE = (1 - OR)∗100%. Results: The A(H1N1)pdm09 vaccination rates in cases and controls were 6% and 76% respectively. After matching cases with controls in a ratio of 1:10, vaccine effectiveness was 98% (95% CI 84-100%). Conclusions: Even though we cannot entirely rule out that selection bias has played a role in our study, the present results indicate that the MF59-adjuvanted A(H1N1)pdm09 influenza vaccine has been effective in preventing laboratory-confirmed A(H1N1)pdm09 influenza in adults with underlying comorbidities and healthy subjects of 60 years and older during the A(H1N1)pdm09 influenza pandemic