Background: Influenza may rapidly disseminate within populations living in confined settings, causing considerable morbidity and disrupting daily activities. The French military health-care system set up since 1994 a prevention strategy based on triennial anti-influenza vaccination. The aim of this study was to evaluate the effectiveness of this strategy during the 2003-2004 influenza season.
Methods: We conducted a matched case-control study from 10/01/2003 through 3/31/2004. Cases were laboratory-confirmed influenza cases. The controls were not to have presented influenza during all the period of study. Controls were matched to cases by sex, army unit and age. Subgroups were categorized into four groups by vaccination regimen [0-1 year], [1-2 years], [2-3 years], [3 years and more or unvaccinated].
Results: One hundred and eighteen cases and 435 controls were included. The proportion of correctly vaccinated subjects (</=3 years) was not statistically significant between cases and controls (P=0.22) but the proportion of</=1 year' vaccinated subjects was statistically significant between cases and controls (P=0.01). The effectiveness of the influenza vaccine was 50% (95%CI: 20-70%).
Conclusion: The results of this study, obtained in an influenza season during which the circulating virus A/Fujian/411/2002 (H3N2) was not perfectly matched with the A(H3N2) component of the influenza vaccine (the most unfavourable case), are compatible with those reported in the literature but do not call into question triennial vaccination. The epidemiologic influenza indicators of the military units show that the impact of influenza did not deteriorate the operational activity of the forces during the study period.