Table 2. Association between class or index case characteristics (potential confounders) and number of secondary cases.
| Number of classes with secondary transmission | Number of secondary cases | IRR* (95% CI) | |
|---|---|---|---|
| Classes (n=1884) | n=727 | n=1706 | |
| Type of school | |||
| Infant-toddler centre | 131 (18.2) | 349 | ref |
| Primary school | 217 (29.8) | 553 | 1.03 (0.80–1.31) |
| Middle school | 172 (23.7) | 386 | 0.78 (0.60–1.01) |
| High school | 202 (27.8) | 409 | 0.86 (0.66–1.11) |
| Other educational services | 5 (0.7) | 9 | 0.45 (0.17–1.18) |
| Class size | |||
| <21 | 316 (43.5) | 751 | ref |
| ≥21 | 411 (56.5) | 955 | 1.08 (0.91–1.29) |
| Index cases (n=1604) | n=640 | ||
| Type of index case | |||
| Student | 477 (74.5) | 1047 | ref |
| Teacher | 163 (25.5) | 658 | 0.75 (0.61–0.92) |
| Screening | 0 | 1 | na |
*
Relative risks are computed with negative binomial models with the count of secondary cases as a dependent variable.