Attack rates of SARS-CoV-2 infection within 14 days after index test date (AR14), grouped by personal characteristics of the index case
| Characteristics of index case | Overall AR14 (%) (95% CI) | AR14 to students (%) (95% CI) | AR14 to staff (%) (95% CI) |
| All index cases, all school levels | 0.33 (0.32 to 0.33) | 0.33 (0.32 to 0.34) | 0.31 (0.29 to 0.34) |
| School size (all index cases): | |||
| <100 | 0.60 (0.49 to 0.74) | 0.69 (0.56 to 0.87) | 0.35 (0.21 to 0.58) |
| 100–299 | 0.36 (0.34 to 0.39) | 0.38 (0.35 to 0.40) | 0.30 (0.25 to 0.35) |
| ≥300 | 0.32 (0.31 to 0.32) | 0.32 (0.31 to 0.33) | 0.32 (0.29 to 0.35) |
| Students: | 0.32 (0.31 to 0.33) | 0.33 (0.32 to 0.34) | 0.28 (0.25 to 0.30) |
| Primary school age | 0.31 (0.30 to 0.32) | 0.31 (0.30 to 0.32) | 0.30 (0.27 to 0.34) |
| Lower secondary school age | 0.34 (0.32 to 0.36) | 0.36 (0.34 to 0.37) | 0.23 (0.20 to 0.27) |
| School staff: | 0.34 (0.32 to 0.36) | 0.32 (0.30 to 0.34) | 0.45 (0.40 to 0.52) |
| Primary school | 0.37 (0.35 to 0.40) | 0.35 (0.32 to 0.38) | 0.50 (0.43 to 0.59) |
| Combined schools* | 0.25 (0.22 to 0.29) | 0.23 (0.20 to 0.27) | 0.36 (0.27 to 0.49) |
| Lower secondary school | 0.34 (0.30 to 0.39) | 0.34 (0.29 to 0.38) | 0.40 (0.29 to 0.54) |
| Teacher | 0.37 (0.35 to 0.39) | 0.35 (0.33 to 0.37) | 0.49 (0.43 to 0.57) |
| Teaching assistants | 0.25 (0.21 to 0.29) | 0.24 (0.19 to 0.29) | 0.31 (0.21 to 0.47) |
| Child care workers | 0.23 (0.18 to 0.29) | 0.21 (0.16 to 0.27) | 0.35 (0.21 to 0.57) |
95% confidence intervals (CI) were calculated with the Wilson method for binomial proportions, multiplied by 100 to obtain percentages. AR14 was calculated as a percentage, with the number of subsequent individuals with covid-19 (infected non-index students and staff members within 14 days of the index case) in the numerator and the total number of non-index students and staff members in all schools with an index case in the denominator, multiplied by 100.
*Primary and lower secondary schools combined.