Characteristics of well family members: univariable analysis
| Family members | Total (n (%)) (n=121) | Families without transmission (n (%)) (n=81) | Families with transmission (n (%)) (n=40) | P value | Unadjusted OR (95% CI) |
| Family size (median (IQR)) | 4 (3–5) | 3 (3–5) | 4 (3–6) | – | – |
| ≤3 | 56 (46.3) | 41 (50.6) | 15 (37.5) | – | Ref |
| >3 | 65 (53.7) | 40 (49.4) | 25 (62.5) | 0.18 | 1.71 (0.79 to 3.71) |
| Close contact with primary cases at home (within 1 m or 3 feet) (No of times)* | – | – | – | – | – |
| 0 | 41 (33.9) | 36 (44.4) | 5 (12.5) | – | Ref |
| 1–3 | 61 (50.4) | 38 (46.9) | 23 (57.5) | 0.005 | 4.55 (1.57 to 13.20) |
| ≥4 | 19 (15.7) | 7 (8.7) | 12 (30.0) | <0.001 | 12.34 (3.30 to 46.23) |
| No of family members wearing mask at home before primary case’s illness onset date (median (IQR))† | 0 (0–1) | 0 (0–2) | 0 (0–0) | – | – |
| None | 90 (74.4) | 54 (66.7) | 36 (90.0) | – | Ref |
| One or more | 31 (25.6) | 27 (33.3) | 4 (10.0) | 0.009 | 0.22 (0.07 to 0.69) |
| No of family members wearing mask at home after primary case’s illness onset date (median (IQR))‡ | 1 (0–3) | 2 (0–3) | 0 (0–3) | – | – |
| None | 47 (38.8) | 26 (32.1) | 21 (52.5) | – | Ref |
| Some | 38 (31.4) | 24 (29.6) | 14 (35.0) | 0.47 | 0.72 (0.30 to 1.73) |
| All | 36 (29.8) | 31 (38.3) | 5 (12.5) | 0.004 | 0.20 (0.07 to 0.60) |
*Family members stay with the primary case at a short distance (within 1 m or 3 feet) for more than 10 min at a time. For example, they have dinner with the primary case around a table or watch TV sitting near.
†Before the primary case developed the illness, the primary case or his/her family contacts wear masks all the time at home.
‡When the primary case developed the illness, the primary case’s family contacts wear masks all the time living with the primary case at home.