Table 1. Characteristics of 1884 classes and 1604 index cases for which a school contact with COVID-19 cases was suspected, before, and after the intervention.

	n (%)	Before interventionn=490	After interventionn=1394
Classes (n=1884)
Type of school
Infant-toddler centre	350 (18.5)	107 (21.8)	243 (17.4)
Primary school	540 (28.7)	125 (25.5)	415 (29.8)
Middle school	496 (26.3)	128 (26.1)	368 (26.4)
High school	478 (25.4)	129 (26.3)	349 (25.0)
Other educational services	20 (1.1)	1 (0.2)	19 (1.4)
Calendar period
September/October	248 (13.1)
November	263 (13.9)
December	316 (16.8)
January	265 (14.1)
February	523 (27.8)
March/April	269 (14.3)
Class size
<21	862 (45.7)	191 (39.0)	671 (48.1)
≥21	1011 (53.7)	293 (59.8)	718 (51.5)
Missing	11 (0.6)	6 (1.2)	5 (0.4)
Secondary transmission
No	1157 (61.4)	307 (62.6)	850 (61.0)
Yes	727 (38.6)	183 (37.4)	544 (39.0)
Number of secondary cases*	1 (1-3)	2 (1-3)	1 (1-3)
Mean attack rate	0.1 (0.04–0.12)	0.1 (0.04–0.12)	0.1 (0.04–0.12)
Part of a school cluster
No	1 367 (72.6)	368 (75.1)	999 (71.7)
Yes	517 (27.4)	122 (24.9)	395 (28.3)
Tracing delay*	3 (2-5)	7 (5-10)	3 (2-4)
Testing delay*	4 (2-8)	5 (3-8)	4 (2-7)
Index cases (n=1604)		n=429	n=1,175
Type of index case
Student	1213 (75.6)	321 (74.8)	892 (75·9)
Teacher	391 (24.4)	108 (25.2)	283 (24.1)
Index case symptomatic
No	298 (18.6)	63 (14.7)	235 (20)
Yes	1306 (81.4)	366 (85.3)	940 (80)
Potential source of infection
Unknown	814 (50.7)	283 (66·0)	531 (45.2)
Known	790 (49.3)	146 (34·0)	644 (54.8)
Type of source
Household outbreak	614 (77.7)	97 (66.4)	517 (80.3)
Social contact	26 (3.3)	7 (4.8)	19 (2.9)
Sport contact	18 (2.3)	7 (4.8)	11 (1.7)
Unidentifiable contact	132 (16.7)	35 (24.0)	97 (15.1)

^*Median (IQR), calculated only in classes with secondary transmission.