Table 1.

Participant demographics

		Seropositive group (n=225)	Seronegative group (n=2851)
Mean age, years		19·0 (1·8)	19·1 (1·9)
Sex
	Female	22 (10%)	229 (8%)
	Male	203 (90%)	2622 (92%)
Race
	Non-Hispanic White	56 (25%)	1698 (60%)
	Non-Hispanic Black	50 (22%)	349 (12%)
	Non-Hispanic Other	7 (3%)	183 (6%)
	Hispanic	112 (50%)	621 (22%)
Non-US residence
	No	216 (96%)	2749 (96%)
	Yes	4 (2%)	19 (1%)
	N/A*	5 (2%)	83 (3%)
Non-US birth
	No	189 (84%)	2626 (92%)
	Yes	32 (14%)	192 (7%)
	N/A	4 (2%)	33 (1%)

Data are n (%) or mean (SD). A total of 173 participants were excluded because they were lost to follow-up, did not have a valid baseline IgG, or became PCR positive during the quarantine period. The table includes all 3076 participants who entered training and were followed prospectively, including the 640 participants who were later excluded from further analysis (figure 1). N/A=not applicable.

^*If a participant answered unknown or left a question blank, then the value is grouped into N/A.