The incidence of infection with mycobacterium tuberculosis is an index of the risk of infection to which a community is exposed. An accurate estimation of incidence rate is of considerable importance in understanding the epidemiology of tuberculosis in organising control measures. A new method of estimating incidence of infection is discussed. The material from 3 studies of National TB Institute has been utilized. Study I: is a part of a survey of a random sample of 134 villages. No previous tuberculin testing or BCG vaccination had been carried out in the area, but each person was examined for BCG scar in order to exclude persons vaccinated probably from other areas. After a complete census, a Mantoux test with 1 TU of PPD RT 23 with Tween 80 given on two occasions (Round I and II). Those with reaction of 13mm or less at Round I were offered a test with 20 TU with Tween 80 within a week of 1 TU test. The interval between the rounds was about 18 months. From the analysis of the data from the first 50 villages for which complete information for both rounds was available, it was seen that there was a general increase in the size of reactions elicited in the second round. Study 2: tuberculin testing was carried out with 1 TU and 20 TU among selected ‘control’ groups which provided the data regarding the “enhancing of tuberculin allergy” seen in repeat tuberculin tests. Study 3: in the course of the longitudinal “survey reader assessments” were carried out periodically to judge the standards of the tuberculin test readers. Inter & intra-reader comparisons were made. The findings have been used to estimate the magnitude of reader variation. The data was also used to study variations in the technique of testing and reading.

It was estimated that on an average inter & intra-reader variations between the rounds were unlikely to exceed 6mm or more in more than 5% of the observations. The reading errors have an equal chance of being positive or negative except at extreme ends of the distribution where zero readings of Round I can only show an increase, and the very large reactions had a greater chance of showing only a decrease at a subsequent round. The study mainly concerns with the problems of estimating the incidence of tuberculous infection in a community. Calculations based on age-specific prevalence rates or on rates of tuberculin conversion or both subject to gross error, leading to unreliable epidemiological conclusions. For estimating the newly infected, a new approach has been suggested based on the drawing of a curve for the distribution of differences in reaction size from one round of tuberculin testing to another. It is assumed that if new infection causes a distinct rise in the degree of tuberculin sensitivity which is greater than the combined rise due to enhancement and reader variation, the distribution of differences between the rounds should indicate the newly infected. It is shown that the newly infected probably constitute a homogeneous group with an increase in mean reaction size of about 24mm and standard deviation of 4mm. Accordingly, 98% of the newly infected show an increase in reaction size of 16mm or more.

Enhancing effect of tuberculin allergy as a result of repeat tests with 1 TU RT 23 on groups tested with I TU, 20 TU and placebo was studied by random allocation among population not vaccinated with BCG in 8 villages. In all, 2357 persons were tested with 1 TU and 759 with normal saline at first round. Based on testing at three rounds the study population could be divided into eight different groups and were labelled with alphabets 'a' to 'h' having been tested once, twice or thrice. The groups 'a', 'c', 'e' & ’g' were tested at 2 months, round two with 1 TU RT 23 and remaining half were not tested. However, all available persons in the 8 groups were retested at the third round, 18 months after the initial test. Thus, eight groups cannot be treated as independent samples but representative of the whole population.

The study did not show enhancing effect due to previous tuberculin test with 1 TU alone among groups tested once, twice or thrice after an interval of 18 months. Part of population was tested with 20 TU at round one; boosting effect was seen at 2 months when test was repeated. However, it was not seen after 18 months but when exactly the boosting effect disappeared was not known. Thus, there was no increase in reaction even among those who were tested with a higher dose of 20 TU earlier after 18 months. The groups provided the largest number for comparison between tested and the control groups. It is inferred from the study that boosting with high dose or repeat tests with the same dose does not persist after 18 months. Hence, for classifying positive tuberculin reactors, no correction is required to the same individuals/population after an interval of 18 months or more, as no boosting effect after 18 months has been observed, on the basis of this analysis.

Under the auspices of the Indian Council of Medical Research, a third assessment of the mass BCG campaign in India was carried out from 1955-58. It is a continuation of the work started by the WHO, of evaluating the level of allergy among groups vaccinated in the campaign. WHO team used 5 TU RT 14, 20, 21, while the present assessment team used 5 TU RT 22. A total of 18,367 school children distributed over 262 schools in 169 different localities were retested for post-vaccination tuberculin sensitivity. The groups were vaccinated in mass BCG campaign with 91 different batches of vaccine produced in Madras. The interval between vaccination and retest varied from 1½ to 42 months.

The mean size of reactions varied from 8.3 to 16.6 with overall mean of 12.5 mm. Less than 10% of the mean values were under 10 mm and less than 10% over 15 mm. Analysis also showed that BCG vaccination was responsible for an increase of 6-7 mm in the mean size of reaction over the pre-vaccination level of the non-infected. One third of the groups had their sensitivity increased upto 6 mm and two third by 7-11 mm. Comparing with the highest attainable degree of tuberculin sensitivity in the infected 1/3rd of the vaccinated group fell short of it by 5-9 mm, whereas 2/3rd were within 4 mm of this level. There is no appreciable difference in the post-vaccination allergy according to the state and prevalence of non- specific tuberculin sensitivity. However, there is an increase in allergising capacity of the BCG vaccine after introduction of modifications in the production procedures in 1955 and again in 1956 in the BCG Laboratory at Madras. Waning of allergy upto 20 months after vaccination and boosting thereafter probably due to super infection was also observed. Findings show that a large proportion of the vaccinated groups retested have achieved attainable allergy with the vaccine used. In view of the above, there is an urgent need to produce Freeze-Dried vaccine than the present liquid vaccine for achieving high levels of allergy.

Freeze-Dried vaccine holds out promise for use in the mass BCG campaign. A continued and expanded research is needed into the protective value of BCG vaccination with the level of allergy which the mass campaign can produce under the epidemiological circumstances existing in India and other technically developed countries.