ANTIBODY EFFICACY AS A KEEN INDEX TO EVALUATE INFLUENZA VACCINE EFFECTIVENESS

The efficacy of the influenza vaccine is often underestimated, due to the dilution of the outcome by noninfluenzal illnesses. We thus explor ed the methodology, to evaluate the effect of the inactivated influenz a vaccine under the following strict conditions: an assessment of the effectiveness on clinical illness among healthy adults in a small-scal e mixed epidemic during the 1991-1992 season, The vaccine antigens inc luded were A/Yamagata/32/89 (HIN1), A/Beijing/352/89 (H3N2), and B/Ban gkok/163/90. Two indices were analyzed. ''vaccine efficacy,'' a compar ison between the vaccinees and the nonvaccinees; and ''antibody effica cy'', a comparison between those with and those without a protective l evel of pre-epidemic hemagglutination-inhibition (HAI) antibody. The o dds ratio (OR) and its 95% confidence interval (95% CI) was calculated by the logistic regression model. A decrease in the age-adjusted OR o f vaccination was not statistically significant: 0.54 (95% CI. 0.19-1. 53) corresponding to vaccine efficacy (1 - OR) of 46% (-53% to 81%). A mong the vaccinees, a significantly decreased OR in those with a highe r titer. to A/Beijing was observed 0.14 (0.02-0.92) adjusted for the m utual effects of pre-epidemic antibodies to different vaccine antigens . The adjusted ORs thus calculated for A/Yamagata and B/Bangkok were n ot found to be statistically significant. The antibody efficacy (1 - O R) was estimated to be 86% (8% to 98%) against illnesses related to A/ Beijing-like viruses. The product of antibody efficacy (86%) and the p roportion of those who achieved a protective level of antibody after v accination (73% for A/Beijing strain) was 63%, which is theoretically equivalent to the vaccine efficacy. Thus, the antibody efficacy is con sidered to be an important index, while the vaccine efficacy against c linical illnesses is easily, disturbed by extraneous factors in the fi eld trials. (C) 1997 Elsevier Science Ltd.