A study was carried out in Bangalore city corporation area with the following objectives; when BCG vaccination is administered simultaneously with primary smallpox vaccination to infants; (i) whether any immunological interferences take place as indicated by the development of vaccination lesion and post-vaccination allergy due to BCG vaccination and the development of the local lesion (take rate) of smallpox vaccination; (ii) whether the incidence of complications are higher among those simultaneously vaccinated and, (iii) whether the population will accept a procedure involving two vaccinations. BCG technicians and the smallpox vaccinators registered all the eligible children after house to house visit and randomly allocated to three groups. A total of 789 children aged below one year were admitted to the study. While 315 were vaccinated simultaneously with BCG and smallpox vaccines (BCG on the right arm and smallpox on the left), 255 were vaccinated with smallpox vaccine only and 219 with BCG vaccine. All 789 children were followed up on the 5th, 21st, 90th and 93rd day of vaccination. The 5th and 21st day followups were to study the development and healing of smallpox vaccination lesions, whereas the 21st, 90th and 93rd day followups were for BCG vaccination lesions. The 90th and 93rd followups were for tuberculin testing and reading.

It was found that there was no evidence of immunological interference between the two vaccines when administered simultaneously i.e., the development of lesion of smallpox vaccination among the simultaneously vaccinated group was similar to the development of the smallpox vaccination lesion among the only smallpox vaccinated group and, the post-vaccination allergy due to BCG among the simultaneously vaccinated group was similar to the post-vaccination allergy among the only BCG vaccinated group. The complications due to vaccinations were very few and similar among the simultaneously vaccinated as compared to the other respective groups. The acceptability of simultaneous vaccination was higher than BCG alone. The above study has demonstrated that BCG and smallpox vaccinations can be administered simultaneously.