FIELD INVESTIGATION OF INFLUENZA VACCINE EFFECTIVENESS ON MORBIDITY

Our objective was to evaluate influenza vaccine effectiveness during a n influenza epidemic by means of a matched case-control study The stud y was performed by 35 general practitioners who collected specimens fo r influenza virus testing from 610 patients who consulted for infectio us syndrome: 168 (28%) were influenza-positive. Two designs were used for selecting controls to take into account the high incidence-rate of influenza-like illness and the various possible protective effects of the vaccine. A first disease-free control matched for age and sex was selected during the same week as the case. A second control matched f or age and sex was selected at the end of the epidemic period, irrespe ctive of disease history during the epidemic period Upper and lower bo unds of vaccine effectiveness can be derived from these case-control d esigns. After adjustment for chronic conditions and exposure to an ind ex case analysis of the matched-pairs whose case was influenza-positiv e showed with the first group of controls, an influenza vaccine effect iveness of 68% (95% CI, 10% to 88%) and, in the second group, 53% (95% CI, -19% to 82%), Among the pairs whose case was negative for influen za, vaccine effectiveness was, respectively, 31% (95% CI, -17% to 59%) and 12% (95% CI, -47% to 47%). Vaccine effectiveness was highest for the H3N2 subtype whose vaccine strain was identical to that of the wil d-type strain. The results suggest that influenza vaccine is effective in the field in preventing influenza morbidity. (C) 1998 Elsevier Sci ence Ltd. All rights reserved.