High rate of transmission among contacts in large London outbreak of isoniazid mono-resistant tuberculosis

Background For a continuing London outbreak of isoniazid mono-resistant tuberculosis (TB), we aimed to determine transmission rates and risk factors for contacts of early cases, in order to inform future guidance on contact tracing Methods Paper-based proformas were completed by TB nurses, and then analysed using Epilnfo/SAS statistical software Results Forty community contacts (11%) became cases, 45(13%) were recommended chemoprophylaxis and 270(76%) were discharged clear of infection The highest transmission rate was among contacts exposed to two or more cases (29% became cases) and close contacts of sputum smear-positive cases (22%) Other risk factors were being male and exposure to drug-using cases or cases with prison links The number needed to be screened (NNS) to detect one case was lowest [5 (95% CI 4-8)] for contacts of sputum smear-positive pulmonary cases, although the NNS was still only 20 (95% Cl 8-72) for casual contacts of smear-positive cases Conclusions Transmission of disease to contacts was high (11%) compared with other documented outbreaks (0 7-2%) The results support recommended guidelines for contact tracing but also provide grounds to recommend, for outbreak cases, screening of casual contacts of smear-positive cases and contacts exposed to more than one case, drug users or prisoners