Objective  To obtain the latent infection and distribution characteristics of Mycobacterium tuberculosis in freshmen in senior high schools and boarding junior high schools in Ningbo by analyzing the tuberculin skin test results of some students, and provide evidence for the improvement of tuberculosis (TB) prevention and control strategies in schools.

  Methods  TB symptom screening and purified protein derivative (PPD) test were conducted in 16,795 freshmen. The students with strong positive results of PPD were examined by chest X-ray, sputum smear and sputum culture. χ² test and Fisher's exact test were used to compare the differences in rates among different groups. Multivariate logistic regression analysis was used to identify the influencing factors of strong PPD positive reaction and tuberculosis infection risk.

  Results  The positive rate of PPD tests in the freshmen in senior high schools and boarding junior high schools in Ningbo was 17.23%, the strong positive rate was 2.69% and the infection rate was 7.83%. Three active TB cases were found in this screening, and the incidence of TB was 17.86/100 000. The results of multivariate logistic regression analysis showed that gender, school type and age were the influencing factors for the risk of strong PPD positive action and infection in the freshmen. The risks for strong PPD positive reaction (OR=1.445, 95%CI: 1.197–1.745） and infection (OR=1.413, 95%CI: 1.262–1.582) in girls were higher than those in boys. The risks for strong PPD positive reaction (OR=1.535, 95%CI: 1.23–1.914) and infection (OR=1.525, 95%CI: 1.330–1.748) in freshmen in vocational middle schools were higher than those in freshmen in non-vocational middle schools. Compared with freshmen aged less than 13 years, those aged 14–15 years (OR=1.315, 95%CI: 1.037–1.666) and 16–17 years (OR=1.436, 95%CI: 1.115–1.851) were at a higher risk for strong PPD positive reaction, and the students aged 16–17 years (OR=1.195, 95%CI: 1.029–1.386) and ≥18 years (OR=2.009, 95%CI: 1.108–3.642) were at higher risk for infection.

  Conclusion  PPD screening in freshmen can help identify infection sources as early as possible for the control of TB transmission, and find the key population in the prevention and control of TB and take targeted prevention measures in schools. Given the importance of PPD screening in TB prevention and control in schools, the PPD screening in freshmen is promoted to cover all the middle school students in Ningbo, especially the girl students, vocational middle school students, and the students in minor ethnic groups.