Tuberculin skin reactivity in Italian military recruits tested in 1996-1997.

In 1996-1997 data was collected and a Mantoux tuberculin test performed in 2882 Italian military recruits aged 18-23 years in order to establish the prevalence of tuberculin reactivity. In addition, the annual risk of infection, defined as the probability that a non-infected individual would be infected during the following year, was calculated. Of the 2882 recruits, 513 had received a BCG vaccination, the remaining 2369 had not. The overall prevalence of subjects with a tuberculin skin reaction size >5 mm (the cut-off point for positivity corresponding to the antimode in the reaction size frequency curve) was 6.1% (144/2369). The prevalence of skin reactivity increased with age but remained similar when related to area of residence, duration of father's school education and family size. The same general trend was observed if the standard pre-established cut-off point of 10 mm was used. In this case the overall prevalence of a positive skin reaction was 4% (95/2369). The annual risk of infection was 0.3% for a prevalence of tuberculin skin reactivity of 6.1% (cut-off point 5 mm) and 0.19% for a prevalence of 4% (cut-off point 10 mm). Analysis of the population sample vaccinated with BCG showed a lack of correlation between the positive reaction after vaccination reported retrospectively by the subject and the current skin reaction observed by the physician in this study (K = 0.254). Moreover, a significant difference was observed between the skin reaction in subjects vaccinated with BCG in 1993-1994 (average size 12.5 mm) and that of subjects vaccinated in 1995-1996 (average size 10.1 mm, P<0.01), probably as a consequence of mycobacteria circulating in the general population which act as a natural booster in people already vaccinated with BCG. A booster effect of tuberculin in Mantoux assays also cannot be excluded.