Impact of BCG vaccination on tuberculin surveys to estimate the
annual risk of tuberculosis infection in south India
P.G. Gopi, R. Subramani, T. Nataraj & P.R. Narayanan
Tuberculosis Research Centre, (ICMR), Chennai, India
Received October 14, 2005
Background & objectives: Annual risk of tuberculosis infection (ARTI) computed from prevalence
of infection measures the extent of tuberculosis transmission in the community and it is used to
monitor the tuberculosis control programme. This is usually derived from tuberculin surveys among
children not vaccinated with BCG. This study explores whether the estimated ARTI among BCG
vaccinated children is comparable to that of unvaccinated children.
Methods: Three tuberculin surveys were undertaken among children aged <10 yr as part of assessing
the impact of DOTS implemented in Tiruvallur district, south India. The prevalence of infection
was estimated using the anti-mode method among vaccinated and unvaccinated children. The ARTI
was computed separately and compared in all the three surveys.
Results: The prevalence of infection among unvaccinated and vaccinated children in the first survey
were 7.8 per cent (95% CI: 7.1-8.6) and 7.9 per cent (95% CI: 7.1-8.8) respectively (ARTI was
estimated to be 1.6 per cent in both groups) and the difference was not statistically significant. The
corresponding figures for children test read in the second and third surveys were 6.9 per cent
(95% CI: 6.2-7.6) and 6.8 per cent (6.0-7.5) and; 6.0 per cent (5.2-6.7) and 6.0 per cent (5.5-6.5)
respectively. The computed ARTI was respectively 1.4 and 1.2 per cent among unvaccinated children
in the second and third surveys; and 1.4 and 1.2 per cent among vaccinated children in the second
and third rounds.
Interpretation & conclusion: There was no difference in the infection with Mycobacterium tuberculosis among vaccinated and unvaccinated children. BCG vaccinated children may thus be included for estimation of infection to assess the extent of transmission in the community as well as for monitoring purpose.
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