Table 5. Risk factors associated with symptomatic, asymptomatic and uninfectious groups^*.

	Middle school in Shandong				Primary school in Guizhou
	Group A (n = 29)	Group B (n = 71)	Group C (n = 89)	P Value	Group A (n = 27)	Group B (n = 43)	Group C (n = 50)	P Value
Median age (year, range)	12(11–14)	13(11–14)	13(11–15)	0.156	12(7–15)	11(6–14)	12(7–14)	0.832
Male/female ratio	2.6(21/8)	0.8(32/39)	0.8(39/50)	0.021	0.5(9/18)	0.6(16/27)	0.8(22/28)	0.639
BMI	17.5(14.0–21,1)	18.0(12.0–24.0)	17.2(12.6–21.8)	0.135	18.1(12.3–23.9)	18.1(11.0–25.2)	17.5(10.0–25.0)	0.631
Dormitory (yes/no)	29/0	70/1	84/5	0.259	17/10	29/14	31/19	0.852
Class distribution				<0.001				0.968
Class 1	13	40	18		8	15	16
Class 2	2	7	31		13	17	21
Class 3	12	16	16		6	11	13
Class 4	2	8	24

^*Symptomatic cases are denoted Group A, asymptomatic are denoted Group B and uninfected are denoted Group C.