Effectiveness of the AS03-Adjuvanted Vaccine against Pandemic Influenza Virus A/(H1N1)2009-A Comparison of Two Methods; Germany, 2009/10

During the autumn wave of the pandemic influenza virus A/(H1N1)2009 (pIV) the German population was offered an AS03-adjuvanted vaccine. The authors compared results of two methods calculating the effectiveness of the vaccine (VE). The test-negative case-control method used data from virologic surveillance including influenza-positive and negative patients. An innovative case-series methodology explored data from all nationally reported laboratory-confirmed influenza cases. The proportion of reported cases occurring in vaccinees during an assumed unprotected phase after vaccination was compared with that occurring in vaccinees during their assumed protected phase. The test-negative case-control method included 1,749 pIV cases and 2,087 influenza test-negative individuals of whom 6 (0.3%) and 36 (1.7%), respectively, were vaccinated. The case series method included data from 73,280 cases. VE in the two methods was 79% (95% confidence interval (CI) = 35-93%; P = 0.007) and 87% (95% CI = 78-92%; P, 0.001) for individuals less than 14 years of age and 70% (95% CI = 245%-94%, P = 0.13) and 74% (95% CI = 64-82%; P, 0.001) for individuals above the age of 14. Both methods yielded similar VE in both age groups; and VE for the younger age group seemed to be higher.