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. 2022 Aug 5;1(1):e000026. doi: 10.1136/bmjmed-2021-000026

Table 2.

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.