Attack rates of SARS-CoV-2 infection within 14 days after index test date (AR14) in study population, by birth year
| Characteristics of index case | AR14 to students (%) (95% CI) | Numerator/denominator |
| School grade (birth year): | ||
| 1 (2014) | 0.65 (0.62 to 0.72) | 283/43 351 |
| 2 (2013) | 0.78 (0.70 to 0.87) | 349/44 685 |
| 3 (2012) | 0.65 (0.59 to 0.73) | 313/47 827 |
| 4 (2011) | 0.69 (0.62 to 0.76) | 358/51 955 |
| 5 (2010) | 0.66 (0.59 to 0.73) | 337/51 435 |
| 6 (2009) | 0.68 (0.62 to 0.75) | 397/58 043 |
| 7 (2008) | 0.73 (0.66 to 0.80) | 413/56 964 |
| 8 (2007) | 0.56 (0.52 to 0.61) | 568/101 337 |
| 9 (2006) | 0.57 (0.53 to 0.62) | 615/107 805 |
| 10 (2005) | 0.67 (0.62 to 0.72) | 701/104 650 |
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 within 14 days of the index case) in the numerator and the total number of non-index students in the denominator, stratified by birth year of the index case, multiplied by 100. Only students born in the same year and attending the same school as the index case were included in the equation.