Table 1.

Risk of SARS-CoV-2 transmission in educational settings, by the characteristic of the index case, Germany, 2020

Characteristic of index case	Index casesa	Incident SARS-CoV-2 cases among 14 591 exposed high-risk contacts, during 14 days of follow-up (quarantine) N = 441 cohorts						Index casesb	Incident SARS-CoV-2 clusters among high-risk contacts, during 14 days of follow-up (quarantine) N = 784 cohorts
		Secondary cases (clusters)	No. of contacts	% Contacts with PCR	SAR (%) [95% CI]	Crude IRR [95% CI]	P-value		Clusters (secondary cases)	Mean cluster size	Cluster risk [95% CI]	Crude RR [95% CI]	P-value
Role
Student/child	346	99 (53)	10 716	87.47	0.92 [0.64–1.29]	1.00	ref	591	76 (145)	1.9	0.13 [0.10–0.16]	1.00	ref
Teacher	75	91 (25)	2858	92.93	3.18 [1.95–4.89]	3.17 [1.79–5.59]	<0.001	157	48 (169)	3.5	0.31 [0.23–0.38]	2.38 [1.73–3.26]	<0.001
unknown	20	6 (3)	1017	95.97	0.59 [0.09–1.93]	0.77 [0.23–2.63]	0.68	36	6 (15)	2.5	0.17 [0.06–0.33]	1.30 [0.61–2.77]	0.50
Symptom status
Symptomatic	300	166 (64)	10 566	88.11	1.57 [1.12–2.14]	1.00	ref	550	100 (262)	2.6	0.18 [0.15–0.22]	1.00	ref
Asymptomatic	127	21 (13)	3523	91.26	0.60 [0.25–1.19]	0.47 [0.25–0.89]	0.02	203	24 (55)	2.3	0.12 [0.08–0.17]	0.65 [0.43–0.99]	0.04
Unknown	14	9 (4)	502	95.62	1.79 [0.43–4.84]	1.34 [0.37–4.85]	0.65	31	6 (12)	2.0	0.19 [0.07–0.37]	1.06 [0.51–2.23]	0.87
Type of institutionc
Day-care (0–6 years)	99	110 (32)	4392	90.64	2.50 [1.60–3.72]	3.23 [1.76–5.91]	<0.001	205	61 (203)	3.3	0.30 [0.24–0.37]	2.78 [1.88–4.10]	<0.001
Primary schools	88	27 (15)	2389	85.64	1.13 [0.57–1.99]	1.62 [0.80–3.31]	0.18	157	21 (40)	1.9	0.13 [0.08–0.20]	1.25 [0.75–2.09]	0.40
Secondary schools	173	41 (25)	5970	90.08	0.69 [0.39–1.12]	1.00	ref	299	32 (50)	1.6	0.11 [0.07–0.15]	1.00	ref
Vocational schools	52	5 (4)	1181	84.17	0.42 [0.10–1.14]	0.65 [0.21–1.99]	0.45	70	8 (14)	1.8	0.11 [0.05–0.21]	1.07 [0.51–2.22]	0.18
Other/unknown	29	13 (5)	659	91.96	1.97 [0.44–5.46]	2.99 [1.12–7.98]	0.03	53	8 (22)	2.8	0.15 [0.07–0.28]	1.41 [0.69–2.89]	0.35
Age
0–5 years	42	31 (11)	1828	90.43	1.70 [0.79–3.18]	1.29 [0.55–3.03]	0.55	77	18 (45)	2.5	0.23 [0.14–0.34]	2.00 [1.14–3.53]	0.02
6–10 years	89	15 (12)	2410	82.53	0.62 [0.29–1.17]	0.72 [0.32–1.63]	0.43	155	18 (31)	1.7	0.12 [0.07–0.18]	1.00 [0.56–1.79]	0.99
11–15 years	113	35 (20)	3358	89.93	1.04 [0.55–1.80]	1.00	ref	189	22 (38)	1.7	0.12 [0.07–0.17]	1.00	ref
16–20 years	90	17 (9)	2884	88.04	0.59 [0.21–1.30]	0.56 [0.25–1.24]	0.15	157	16 (37)	2.3	0.10 [0.06–0.16]	0.88 [0.48–1.61]	0.67
21–34 years	42	30 (13)	1321	88.19	2.27 [1.02–4.33]	1.97 [0.84–4.64]	0.12	80	23 (50)	2.2	0.29 [0.19–0.40]	2.47 [1.46–4.17]	0.001
35 years and older	45	62 (13)	1773	93.80	3.50 [1.80–6.07]	2.80 [1.27–6.20]	0.01	90	27 (113)	4.2	0.30 [0.21–0.41]	2.58 [1.56–4.27]	<0.001
unknown	20	6 (3)	1017	95.97	0.59 [0.09–1.93]	0.65 [0.18–2.35]	0.51	36	6 (15)	2.5	0.17 [0.06–0.33]	1.43 [0.62–3.28]	0.40
Type of groups
Stable groups	313	80 (52)	8650	88.15	0.92 [0.66–1.26]	1.00	ref	536	75 (123)	1.6	0.14 [0.11–0.17]	1.00	ref
Dynamic groups	111	91 (25)	5430	90.90	1.68 [1.04–2.56]	1.45 [0.84–2.50]	0.18	199	44 (168)	3.8	0.22 [0.17–0.29]	1.58 [1.13–2.21]	0.007
Unknown	17	25 (4)	511	86.89	4.89 [1.16–12.91]	3.21 [1.07–9.59]	0.04	49	11 (38)	3.5	0.22 [0.12–0.37]	1.60 [0.92–2.81].	0.10
Sex
Female	194	66 (33)	5808	88.86	1.14 [0.70–1.73]	1.00	ref	332	43 (89)	2.1	0.13 [0.10–0.17]	1.00	ref
Male	241	128 (47)	8555	89.32	1.50 [0.99–2.16]	0.78 [0.46–1.31]	0.35	441	86 (238)	2.8	0.20 [0.16–0.24]	0.66 [0.47–0.93]	0.02
Unknown	6	2 (1)	228	89.04	0.88 [0.00–7.01]	0.64 [0.07–5.67]	0.69	11	1 (2)	2.0	0.09 [0–0.41]	0.47 [0.07–3.05]	0.43
Month of infection
August	33	6 (5)	909	97.36	0.66 [0.19–1.65]	1.00	ref	33	5 (6)	1.2	0.15 [0.05–0.32]	1.00	ref
September	78	9 (9)	2548	97.84	0.35 [0.16–0.67]	0.45 [0.12–1.78]	0.26	78	9 (9)	1.0	0.12 [0.05–0.21]	0.76 [0.28–2.10]	0.60
October	95	84 (23)	3986	93.15	2.11 [1.23–3.35]	1.92 [0.58–6.42]	0.29	149	24 (85)	3.5	0.16 [0.11–0.23]	1.06 [0.44–2.58]	0.89
November	151	74 (30)	4492	85.28	1.65 [1.03–2.50]	1.77 [0.55–5.75]	0.34	349	56 (150)	2.7	0.16 [0.12–0.20]	1.06 [0.46–2.46]	0.89
December	84	23 (14)	2656	78.43	0.87 [0.38–1.69]	1.11 [0.32–3.91]	0.87	175	36 (79)	2.2	0.21 [0.15–0.27]	1.36 [0.58–3.20]	0.49
Total	441	196 (81)	14 591	89.09	1.34 [0.99–1.78]	n.a.	n.a.	784	130 (329)	2.5	0.17 [0.14–0.19]	n.a.	n.a.

Left side of the table displays secondary attack rates (SARs), defined as the proportion of SARS-CoV-2-PCR positive secondary cases in a given cohort of close contact persons around that index case. Incidence rate ratios (IRRs) and corresponding confidence intervals (CIs) and P-values from negative binomial regression in 14 591 high-risk contacts clustered in cohorts around 441 index cases compare the mean count of secondary cases per index-cohort by the characteristic of index case, where an IRR of 1.0 corresponds to H₀ = ‘there is no difference in the SAR between groups’. The analyses take account of the clustering of secondary cases within index cases. The right side of the table displays the cluster risk, i.e. the risk of causing at least one secondary infection among high-risk contacts, and associated risk ratios (RRs) from binomial regression for the comparison between groups, where an RR of 1.0 corresponds to H0 = ‘there is no difference in cluster risk between comparison groups’.

^aSubgroup of index cases with complete information on a number of contact persons and a number of PCR tests.

^bComplete study population, i.e. presents additional data on secondary cases around index cases with incomplete information on negative PCR tests in close contact persons, thus not allowing to calculate SARs.

^cPresented data include teachers.