Table 2. Secondary Transmission and Exposure Characteristics, Fall 2020/Spring 2021 (Univariable Analysis).

Exposure	No. of students, faculty, and staffa					SAR, %	Fisher exact P value
	Index cases	Contacts	Contacts tested	No in-school transmission	In-school transmission
Contact role
Staff	151	278	233	224	9	3.9	.08
Student	216	1492	1093	1073	20	1.8
Contact grade level and staff
Prekindergarten, kindergarten, elementaryb	119	793	628	617	11	1.8	.18
Middle schoolc	40	205	136	133	3	2.2
High schoold	36	328	255	251	4	1.6
Staff	151	278	233	224	9	3.9
Classroom exposure
No	114	480	368	356	12	3.3	.14
Yes	187	1291	959	942	17	1.8
Lunch exposure
No	258	1728	1291	1266	25	1.9	.007
Yes	18	43	36	32	4	11.1
Sports exposure
No	254	1532	1152	1128	24	2.1	.58
Yes	19	239	175	170	5	2.9
Distance during exposure, ft
<3	20	82	81	77	4	4.9	.13
<6 (Not specified)	210	1248	927	910	17	1.8
3-6	48	431	311	303	8	2.6
Masking during exposure
Both case and contact masked	250	1687	1256	1235	21	1.7	<.001
Case masked and contact unmasked	3	3	2	1	1	50.0
Neither masked	28	67	60	53	7	11.7
Semester
Fall 2020	95	541	384	377	7	1.8	.68
Spring 2021	171	1211	938	916	22	2.3
Index case role
Staff	94	495	360	347	13	3.6	.04
Student	173	1276	967	951	16	1.7
Index case grade level and staff
Prekindergarten, kindergarten, elementaryb	81	621	504	498	6	1.2	.12
Middle schoolc	39	224	145	142	3	2.1
High schoold	51	407	295	288	7	2.4
Staff	94	495	360	347	13	3.6
Means of identifying index case
In-school close contact	5	13	11	9	2	18.2	.04
Out-of-school contact	83	588	413	408	5	1.2
School-based asymptomatic testing program	57	384	325	318	7	2.2
Tested because of symptoms	120	766	569	554	15	2.6
Tested before or after travel	2	20	9	9	0	0
Index case symptoms
No	112	741	555	547	8	1.4	.13
Yes	155	1030	772	751	21	2.7
SVI overall quartilee
<0.25 (Lowest)	91	797	680	673	7	1.0	<.001
0.25-0.50	84	543	357	352	5	1.4
0.51-0.75 (Highest)	92	431	290	273	17	5.9
SVI SES quartilee
<0.25 (Lowest)	162	1304	1003	993	10	1.0	<.001
0.25-0.50	95	394	257	246	11	4.3
0.51-0.75 (Highest)	10	73	67	59	8	11.9

Abbreviations: SAR, secondary attack rate (calculated as [probable and possible transmissions / contacts tested] × 100); SES, socioeconomic status; SVI, Social Vulnerability Index.

^a Unknown or missing values were removed if the number of unknown or missing values comprised <5% of the total number of contacts tested for each exposure category.

^b Grades 1 through 5.

^c Grades 6 through 8.

^d Grades 9 through 12.

^e Overall quartiles as found in the school districts included in the study.