Table 1. General characteristics of the study population.

		Total
		N	%
Total		2,982	100.0
Age (mean ±SD)		15.1±1.3
Region	Seoul	889	29.8
	Gyeong-gi	1,126	37.8
	Incheon	474	15.9
	Other	493	16.5
BCG scar present	Yes	1,818	61.0
	No	1,164	39.0
Family history of TB	Yes	20	0.7
	No	2,962	99.3
Relationship with index case	Classmates*	1,499	50.3
	Exchanging classmate†	907	30.4
	Club mates‡	82	2.7
	Others	494	16.6
Smoking	Non-smoker	2,348	78.7
	Current-smoker	157	5.3
	Ex-smoker	123	4.1
	No response	30	1.0
	Not follow up	324	10.9
BMI§	mean ± SD	21.0±3.4
	median(IQR)	20.3(18.8, 22.6)
	<18.5	633	21.2
	18.5–25.0	1,997	67.0
	>25.0	352	11.8

^*Classmates: The same classmates with index case.

^†Exchanging classmate: who contact with the index case in other classroom over 1 h per week.

^‡Club mates: contact with the index case during extracurricular activities.

^§BMI: body mass index.