Objectives To establish source case and risk factors for infection with Mycobacterium tuberculosis in young children during an incident of extensive transmission of mycobacterium tuberculosis in a Junior School.

Methods Following the diagnosis of pulmonary tuberculosis in a 9-year old boy (the index case) with a right upper lobe consolidation and Mycobacterium tuberculosis grown from a smear-negative sputum, we detected a high rate of infection in his family and class contacts. This led to screening of all pupils (n = 200) and staff (n = 108) of a UK junior school for M. tuberculosis infection using a gamma interferon release assay and chest x-rays.

Results Altogether, 85 (42%) pupils of the junior school had a reactive gamma interferon release assay indicating infection with M. tuberculosis. The infection rate in the class of the index case was significantly higher (79%) than the infection rate among the other pupils at the school (35%) (p<0.01). Children without previous BCG vaccination had a significantly higher risk of being infected (OR 6.34, 95% CI 1.79 to 22.52, p<0.01). None of the adult contacts screened had pulmonary tuberculosis. Genetic finger printing revealed that the strain of M. tuberculosis matched a strain of a family contact of the index case (and the most likely source for this school outbreak) encountered six years previously.

Conclusions Smear-negative sputum and lack of cavitating disease on a chest x-ray do not exclude significant risk of transmission of infection from a patient with pulmonary tuberculosis. BCG immunisation was associated with a lower risk of infection.