In view of the variable and low levels of post-vaccination allergy elicited in the Indian Mass BCG Campaign vaccinated groups as observed by WHO and Indian BCG Assessment Teams, some studies were carried out to investigate some of the factors considered having influenced the levels of post-vaccination allergy. The reasons were potency and storage of vaccine, techniques, interval between vaccination and retesting and tuberculins used. Potency of the vaccine: Madras vaccine was compared with Danish vaccine. The retest done at 3 months with 1 TU RT 23 showed 11.8 mm induration with Danish and 11.9 mm with the Madras vaccine. Test done after one year with 20 TU induration was 18.3 mm for both the vaccines. It is reasonable to assume that the vaccine produced at Madras is as potent as Danish vaccine. Storage of vaccine: A comparison of post-vaccination allergy in respect of storage of vaccine by the ICMR Assessment Team and by Mass Campaign Teams was carried out in five groups of villages in the neighbourhood places where five BCG teams were working in Tamil Nadu. The non-reactors were vaccinated randomly by vaccines stored by either team and with the placebo III group as control. The retest done after 3 months showed that in all the five groups combined the mean size of indurations vaccinated with vaccine stored by Assessment Team were more in the vaccine stored by Mass Campaign Team. The loss of potency was 0.7 mm per week in vaccine stored by Assessment Team and 1.5 mm per week stored by Mass Campaign Team. Techniques: The above study was extended to five more groups vaccinated and tested by the Mass Campaign Team. Assessment Team also vaccinated separate groups with the same vaccine and tested on the same day. Mean size induration for groups combined was higher for assessment team but not significant. So, the techniques do not seem to make much difference. Interval between vaccination & retesting: Analysis of data collected on post-vaccination allergy in the groups vaccinated by the Mass Campaign Teams retesting according to intervals showed that the mean size induration decreased with the increase in interval between vaccination and retesting from 1-20 months. Thereafter, it rose again. These results indicate a tendency for the allergy after vaccination to wane with passage of time. The rise is presumably due to super infection or difference in batches of vaccine. In the third year (not included in this paper) greater proportion of people had bigger reactions. Besides, tuberculin used for retesting is also found to give differences in mean size induration. Tuberculin 5 TU RT 22 gave larger reactions than 1 TU RT 23 with tween, the differences were above 3 mm.

It can be concluded that at present Madras vaccine is satisfactorily potent, the post-vaccination allergy is influenced by storage of vaccine, by interval between vaccination and retesting and type of tuberculin used for eliciting the allergy. Technique of testing, reading and vaccination may not influence the results. Under Indian climatic conditions the liquid vaccine should not be used for more than 2 weeks. There is a need for Freeze-Dried vaccine.