To obtain precise information for computing the indices of tuberculosis situation in a community, with passage of time, reliance has been placed on tuberculosis infection rates obtained by carrying out tuberculin surveys. In most developing countries, covered extensively by BCG vaccination without prior tuberculin testing, the tuberculin test has problems of interpretation for demarcating the infected persons from the uninfected. To overcome the problem, therefore, the test results are analysed among persons who do not show a BCG scar and are, thus, considered as normal population. In this paper, an attempt is made to show that BCG vaccination not always lead to the formation of a scar, and also that the scar resulting from BCG vaccination may fade away with time and the person, thus, may be wrongly included in the unvaccinated group. It has also been found that there is greater fading of scars in the younger age groups: in children 0-2 years of age, upto 52% of the scars faded away within 21 months of vaccination. This proportion steadily decreased to about 8% in the 10-14 years age group.
The implication of the finding is that the demarcation line between uninfected and infected persons may require to be shifted from survey to survey, based on the distributions among the 'no scar' population. Moreover, in a totally vaccinated community, the differences of reactions may provide the answer to the problem of identifying the newly infected persons.