Interventions for Tuberculosis Control and Elimination 2002

If environmental mycobacteria do indeed provide protection against
M. tuberculosis, and infection with them occurs before the administration
of BCG, then the effect of the latter will be at least partially masked. 813
This may explain the larger protection conferred by BCG given earlier in
life than if given later as demonstrated in Chingleput. 765
Furthermore, the risk of tuberculosis would be expected to be greater
in initially tuberculin negative persons than in individuals with small tuberculin
skin test reaction sizes (more likely attributable to infection with environmental
than tubercle bacilli).
In Puerto Rico, protection from BCG was lower in rural areas, where
non-specific sensitivity was higher than in urban areas, where protection
from BCG was higher. 830 However, in Chingleput, the rate of tuberculosis
among persons with a reaction size of more than nine millimeters to a
sensitin produced from M. avium complex (PPD-B) was identical to that
among those with zero to nine millimeters reaction sizes. 765
In the United Kingdom, the risk of tuberculosis was higher among initially
tuberculin skin test negative adolescents than among those reacting
to 100 tuberculin units only, but the risk decreased over time (figure 72). 776
The protection afforded against tuberculosis by a tuberculin skin test reaction
that can be elicited only by this large dose of tuberculin is remarkably
similar (but smaller) to that imparted by BCG vaccination (figure 73).
In the Karonga, Malawi, trial the risk of tuberculosis during followup
was lowest among those with an initial tuberculin skin test reaction size
of six to 10 mm (figure 74). 831 After adjustment for age and sex, the risk
was also lower among those with reactions of one to five millimeters than
among non-reactors. 832
That different species of mycobacteria seem to act on the immune system
has also been demonstrated by observations from Sweden. After the
cessation of mass BCG vaccination, there was a large increase in peripheral
lymphadenitis due to environmental mycobacteria (figure 75) ( 703,833,834
and Romanus V, personal written communication, Feb 18, 2000). Similarly,
in the Czech Republic, the incidence of lymphadenitis among children due
to M. avium following cessation of BCG vaccination was 3.6, compared to
0.2 per 100,000 person-years among children vaccinated on the insistence
of their parents, 835 suggesting a protection of 95% (95% confidence interval
88% to 98%) from BCG against lymphadenits due to M. avium.
While not all findings are consistent with the hypothesis that environmental
mycobacteria may mask the protection that BCG can confer in
their absence, 832 it may explain to a considerable extent certain variations
in observed efficacy.
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