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 EpiInfo/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% CI: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.
Neely
F
F
Health Protection Agency (HPA), London region, 7th Floor, Holborn Gate, 330 High Holborn, London WC1V 7PP, UK.
Maguire
H
H
Le Brun
F
F
Davies
A
A
Gelb
D
D
Yates
S
S
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D016358:Contact Tracing; D004196:Disease Outbreaks; D055030:Drug Users; D005260:Female; D006801:Humans; D007538:Isoniazid; D008131:London / Q000453:epidemiology; D008297:Male; D008403:Mass Screening; D008875:Middle Aged; D015999:Multivariate Analysis; D009169:Mycobacterium tuberculosis / Q000302:isolation & purification; D011329:Prisoners; D012307:Risk Factors; D012737:Sex Factors; D018088:Tuberculosis, Multidrug-Resistant / Q000453:epidemiology / Q000382:microbiology / Q000635:transmission*; D055815:Young Adult
D007538:Isoniazid