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

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Abstract
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. Paper-based proformas were completed by TB nurses, and then analysed using EpiInfo/SAS statistical software. 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% CI:8–72) for casual contacts of smear-positive cases. 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.