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.

The utility of repeated estimates of prevalence rates of infection in children as a tool for surveillance in tuberculosis is now well recognized. Two prevalence surveys at an interval of 5 years were conducted by National Tuberculosis Institute, Bangalore, with the main objective of studying changes in prevalence rate of infection among children in the age group of 0-9 years. A total population of 42,343 residing in 90 randomly selected villages of Doddaballapur taluk, Bangalore, were registered; of them, 12,535 were children in the age group of 0-9 years. Children were further classified into two sub groups 0-4 and 5-9 years, with or without BCG scars. The unvaccinated children in these two age groups formed the study population.

The population in the study area during the 2nd repeat survey was similar to that of first survey with regard to age, sex distribution, except that a growth rate of 1.1% per year was registered. The BCG scar rate, among children in the age group 0-4, 5-9 years, was 8% & 39% respectively at survey I. All the unvaccinated children below 10 years were given tuberculin test with 1 TU PPD RT 23 and reactions were read 72 to 96 hours after tuberculin testing. In the first survey, level of demarcation to classify the infected children was 10 mm and above, while in II survey it was 12 mm and above. It was observed that the prevalence rate of infection from I survey to II survey was not altered (2.58% & 2.46%) in the 0-4 years of age, while there was an increase in the rate from 8.93% to 12.3% in 5-9 years of age in the II survey. The increase in the infection rate could be attributed to the rising trend of infection, over reading by tuberculin-readers', skills of both tuberculin tester and reader, boosting of tuberculin reaction or scarless BCG vaccination. In conclusion, the study of changes in the prevalence rate of infection in the younger age group is simple, cheap, less time consuming. The data can be used for calculating annual risk of infection as well trend of transmission of infection.

It has been reported that a substantial proportion of the new cases arise from the previously infected population. Hence, it appears that exogenous reinfection and/or endogenous reactivation play a major role in the development of post-primary disease. Though the risk of disease associated with exogenous reinfection and endogenous reactivation has not been computed in Indian conditions, the data collected during a longitudinal study by National TB Institute, Bangalore was analysed to estimate the above mentioned risk rates.

The risk of disease associated with exogenous reinfection was 6.55% per year compared with 0.21% yearly due to endogenous reactivation. To test the validity of the computed risk rates these were applied to the interval between the 3rd and 4th surveys. It was then estimated that 64 new cases should have been diagnosed in that survey interval as against 57 cases actually diagnosed. It was also estimated that 1.9% of the total population would be having recent infection, 1.3% would be previously infected with recent reinfection and 32.7% with previous infection but no recent infection leaving 64.1% who are not infected at all (uninfected). Among the new cases diagnosed, 28% would have progressive primary disease, 41% cases arise due to exogenous reinfection and 31% due to endogenous reactivation. In other words, the 1.9% population with recent infection contributes 28% of the total new cases, the 1.3% reinfected population contributes 41% and the 32.7% previously infected population contributes the remaining 31% of the total new cases.

The question of carrying out a repeat epidemiological survey in India has been engaging the attention of many for quite some time. The first nationwide tuberculosis prevalence survey was conducted in India during 1955-58. It served as an eye opener and produced data which were profitably used by the planners to decide about the form and state of national control programme. Doing a repeat survey will be useful only if it would be capable of yielding epidemiological information on the future course of action. At the time of formulation of the District Tuberculosis Programme (DTP), it was perhaps presumed that programme would work with optimum efficiency as in the operational studies and as such the real performance was not envisaged. Secondly, due to low prevalence rates of tuberculosis as shown in all the surveys could reflect a small rate of change or no change at all, thus these longitudinal surveys with inadequate samples, did not have enough discriminatory power to observe a statistically valid change with time.

It is now globally realised that instead of looking at mortality rates or small changes in the prevalence rates of cases, it is the Annual Risk of Infection (ARI) which holds the key to epidemiological trend in a community. However, through a model recently constructed at the National Tuberculosis Institute, it is possible to extrapolate the findings of well planned small surveys in certain areas. It gives an idea what to expect over a period of 50 years - a slow decline. Therefore, when the present efficiency of Case-finding programme is about 33%, treatment efficiency also of the same order or even worse and with persistent rise in the population, it is futile to talk of epidemiological assessment through repeat surveys. Instead, we should concentrate on raising the efficiency of the DTP as near to the level which could be called the critical level of efficiency. Till then nation wide surveillance through the calculation of ARI is the only choice.

It has been postulated that BCG scar disappears in a good number of children and some of the vaccinated children will get included in the non- vaccinated group and cause difficulty in interpreting the results of tuberculin test. It was decided to analyse information on BCG scar status in the younger population of a rural community in 3 taluks of Bangalore district with an objective to find out whether disappearance of BCG scar is dependent on the age of the child, size of post-vaccination induration at initial survey and tuberculin sensitivity status of children in whom BCG scar has disappeared, in comparison with children in whom the BCG scar has not disappeared. In all, 1095 children aged 0 to 14 years were found with BCG scar in 119 randomly selected villages during an epidemiological survey done in 1961 at the time of intake. Following two groups of children were studied for disappearance of the scar. Of them, a) 796 children who had BCG scar at the first survey, and whose BCG scar status was available at 4th survey, b) 299 who showed no BCG scar at first survey but were found with BCG scar at 2nd survey and whose BCG scar status was available at 4th survey.

Of the BCG scars recorded at intake, 26.4% and 32.5% disappeared subsequently during three and a half and five year periods respectively. The waning of BCG scars was independent of age of the child and tuberculin sensitivity status at intake. Tuberculin sensitivity status in children in whom scar had disappeared was the same as that found in children in whom scar had persisted at intake and after five years. The misclassification of children, in whom scars have disappeared, as unvaccinated leads to a difficulty in interpreting the results of tuberculin test done for the purpose of computation of the Annual Risk of Infection. Further, the extent of misclassification increases in proportion with the increase in BCG coverage of the population. This finding justifies the practice of identifying the demarcation level on the basis of the distribution of tuberculin induration sizes for classifying the infected persons in a population in each survey.