An outbreak of pulmonary tuberculosis and a follow-up investigation of latent tuberculosis in a high school in an eastern city in China, 2016-2019

Background In October 2016, a senior high school student was diagnosed with sputum-smear positive [SS(+)] pulmonary tuberculosis (TB). We conducted an investigation of an outbreak in the school, including among students and teachers diagnosed with latent TB, who we followed until July 2019. Methods We defined latent TB infection (LTBI) as a tuberculin skin test (TST) induration of 15mm or larger; probable TB as a chest radiograph indicative of TB plus productive cough/hemoptysis for at least 2 weeks, or TST induration of 15mm or larger; and confirmed TB as two or more positive sputum smears or one positive sputum smear plus a chest radiograph indicative of TB or culture positive with M. tuberculosis. We conducted mycobacterial interspersed repetitive unit-variable number tandem repeat (MIRU-VNTR) typing based on 24 loci in the isolates. Results Between October 2016 and July 2019, we identified 52 cases, including nine probable, six confirmed, and 37 LTBI cases. The index case-student had attended school continuously despite having TB symptoms for almost three months before being diagnosed with TB. We obtained three isolates from classmates of the index case in 2016; all had identical MIRU-VNTR alleles with the index case. The LTBI rate was lower among students (7.41%, 30/405) than among teachers (26.92%, 7/26) (rate ratio [RR] = 0.28, 95% confidential interval [CI]: 0.13-0.57). Among the 17 students who had latent TB and refused prophylaxis in October 2016, 23.53% (4/17) became probable/confirmed cases by July 2019. None of the six teachers who also refused prophylaxis became probable or confirmed cases. Of the 176 students who were TST(-) in October 2016, 1.70% (3/176) became probable/confirmed cases, and among the 20 teachers who were TST(-), 1 became a probable case. Conclusions Delayed diagnosis of TB in the index patient may have contributed to the start of this outbreak; lack of post-exposure chemoprophylaxis facilitated spread of the outbreak. Post-exposure prophylaxis is strongly recommended for all TST-positive students; TST-negative students exposed to an SS(+) case should be followed up regularly so that prophylaxis can be started if LTBI is detected.