Revaccination with BCG: its effects on skin tests in Kuwaiti senior school children.

Following a policy of BCG vaccination adopted in Kuwait more than 20 yrs ago, children receive their first vaccination just before starting school. Those who have a response of less than 10 mm induration to 2 tu of RT23 PPD, when they are 13 yrs old, are revaccinated. The effects of this revaccination on skin test positivity in a group of 18 yr old senior school children have been investigated. In a random study group 23% were found to have received BCG a second time. Revaccination resulted in a significant increase in positivity to tuberculin, and to the other 6 reagents tested, that was much more than would have been expected due to the passage of time alone in low responders. Scars of the second BCG vaccination were larger than those after the first vaccination, and showed a sex difference, with scars being significantly larger in boys than in girls. Boys also tended to show the largest responses to skin tests, with the notable exception of tuberculin to which girls showed the largest response. In most cases responses to skin tests were larger after revaccination than after a single vaccination. Based on this study, it is impossible to be sure that revaccination improved protective immunity, but the increase in tuberculin responsiveness, and recognition of environmental mycobacterial species may be indirect evidence supporting this conclusion.