Six years' experience with the discontinuation of BCG vaccination. 3. Transmission of tubercle bacilli in an unvaccinated population.

Setting: Mass BCG vaccination of newborns has been discontinued since 1986 in one third of the Czech Republic (30,000 newborns annually) except for babies with a high risk of tuberculosis.

Objective: The transmission and risk of tuberculosis infection were evaluated in non-BCG-vaccinated children representing a population living in satisfactory socio-economic conditions.

Design: In 1986-92, of 184,648 children tuberculin tested at 2-year intervals, 283 were infected.

Results: The risk of infection was low (0.04%). Infections very rarely occurred in children below the age of 1 year and the highest rate was in children 2-3 years old. The sources of infection were found in the families of only one half of the youngest children and in 10% of the older ones. In the remaining children occasional contacts with sources outside the family must be taken into account, but the mode of contact and transmission of infection were not identified. The only factor contributing to tuberculosis transmission was the high population density.

Conclusion: The risk of infection was low and identified sources of tuberculosis were rare in a stable population of children without evident risk factors. Children were infected by occasional contact with tuberculous cases outside of their families. The unfavourable factors associated with living in overcrowded industrialised settlements contributed to the risk of infection of non-BCG-vaccinated children.