Interventions for Tuberculosis Control and Elimination 2002
Interventions for Tuberculosis Control and Elimination 2002
Interventions for Tuberculosis Control and Elimination 2002
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
infected persons may thus mask any protective effect of BCG vaccination,<br />
as vaccination is not expected to provide protection against those who are<br />
already infected. 803<br />
The argument fails to account <strong>for</strong> the fact that BCG provided no protection<br />
at all in some trials. Depending on the proportion of individuals<br />
who had escaped infection with environmental mycobacteria at the point<br />
of BCG vaccination, masking of protection by BCG vaccination would be<br />
expected to be incomplete.<br />
Differences in risk attributable to exogenous re-infection tuberculosis<br />
BCG vaccination is expected to provide protection against tuberculosis<br />
resulting from infection acquired subsequent to vaccination. It is not<br />
expected to provide greater protection than a naturally acquired primary<br />
infection. Protection conferred by a primary infection against disease from<br />
re-infection is incomplete. 804-811 Thus, the protective efficacy of BCG might<br />
be increasingly masked as the contributory fraction of cases attributable to<br />
re-infection increases. 812,813 Thus, following this argument, the protection<br />
af<strong>for</strong>ded by BCG is expected to be lower where the risk of infection with<br />
M. tuberculosis (<strong>and</strong> thus re-infection) is high.<br />
This is not borne out by observations. The annual risk of infection<br />
in the United Kingdom decreased considerably over time, 814 yet the level<br />
of protection af<strong>for</strong>ded by BCG remained high <strong>and</strong> virtually unchanged. 761<br />
Differences in genetic make-up of vaccinees<br />
Because differences in protection from BCG among males <strong>and</strong> females were<br />
observed in at least one study, 766 other genetic factors may also play a role<br />
in the differential protection conferred by BCG. Nevertheless, the finding<br />
that BCG gave virtually no protection to children in Chingleput, 765 but high<br />
protection in children from the Indian sub-continent living in the United<br />
Kingdom 758,761 would tend to disfavor this hypothesis.<br />
Differences in nutritional status of vaccinees<br />
As nutritional status affects the functioning of the cellular immune system,<br />
it might be expected that poor nutritional status would adversely affect the<br />
protective efficacy of BCG vaccination. However, BCG provided very high<br />
protection against tuberculosis death among poorly nourished North American<br />
Indian children, even somewhat higher than among well-nourished British<br />
adolescents, 776 a finding that would tend to contradict this hypothesis.<br />
118
Change language