School clusters of COVID-19, South Korea, February-December 2020
| Variables | Kindergartens | Primary schools | Secondary schools | Total |
| N (%) | N (%) | N (%) | N (%) | |
| Total schools (n) | 8705 | 6120 | 5590 | 20 415 |
| Schools with 0 case | 8444 (97.0) | 3747 (61.2) | 2683 (48.0) | 14 874 (72.9) |
| Schools with ≥1 case | 211 (2.4) | 1430 (23.4) | 1811 (32.4) | 3452 (16.9) |
| Schools with ≥2 cases | 50 (0.6) | 943 (15.4) | 1096 (19.6) | 2089 (10.2) |
| School clusters (n)* | 21 | 90 | 196 | 307 |
| Student index case | 7 (33.3) | 18 (20.0) | 39 (19.9) | 64 (20.8) |
| Staff index case | 4 (19.0) | 11 (12.2) | 17 (8.7) | 32 (10.4) |
| Unidentified index case | 10 (47.6) | 61 (67.8) | 140 (71.4) | 211 (68.7) |
| Median SIR (range)† | 1.54 (0.43–8.33) | 0.25 (0.11–3.25) | 0.41 (0.08–8.15) | 0.41 (0.08–8.33) |
*Documented potential transmission within schools identified as of December 2020.
†Secondary infection rate in students in corresponding schools.
SIR, secondary infection rate.