Method In this study, the inoculum of live BCG organisms in vaccinated infants at birth was evaluated and the correlation between amount of CFU and PPD reaction were examined at 3 months of age. For this purpose, 854 newborn infants in health centers affiliated to Shaheed Beheshti University of Medical Sciences were studied at the age of 3 months. Tuberculin skin tests were performed in the pasteur lnstitute and other health centers. After 72 hours, PPD induration diameter was measured. The comparison of the results with chi - square test showed there is a significant correlation between the two variables (size of PPD reaction and live organisms of BCG vaccination).

Results After evaluating live organisms in BCG vaccination, it was concluded that the amount of live organisms in many vaccines is more than 3 milliom while others are less. The following have been obtained as a result of assessment of PPD reaction in vaccinated infants: those vaccinated by live orgrnisms, more than 3 million 93.4% showed PPD positive reaction in vaccinaged infants but less than 3 million 69.2% showed PPD positive reaction.

Conclusion Thus, the number of live organisms in vaccination can play an effective role in the size of tuberculin skin reaction.