Characteristics of primary cases of COVID-19: univariable analysis
| Primary cases | Total (n (%)) (n=124) | Families without transmission (n (%)) (n=83) | Families with transmission (n (%)) (n=41) | P value | Unadjusted OR (95% CI) |
| Age (years) (median (IQR)) | 45.0 (35.7–60.0) | 42.0 (34.0–57.5) | 52.0 (39.3–61.0) | – | – |
| <18 | 0 | 0 | 0 | – | – |
| 18–59 | 92 (74.2) | 63 (75.9) | 29 (70.7) | – | Ref |
| ≥60 | 32 (25.8) | 20 (24.1) | 12 (29.3) | 0.54 | 1.30 (0.56 to 3.02) |
| Sex | – | – | – | – | – |
| Men | 61 (49.2) | 40 (48.2) | 21 (51.2) | – | Ref |
| Women | 63 (50.8) | 43 (51.8) | 20 (48.8) | 0.75 | 0.89 (0.42 to 1.87) |
| Education level | – | – | – | – | – |
| High school or lower | 26 (21.0) | 18 (21.7) | 8 (19.5) | – | Ref |
| Bachelor degree | 69 (55.6) | 47 (56.6) | 22 (53.7) | 0.53 | 0.75 (0.30 to 1.86) |
| Graduate degree | 29 (23.4) | 18 (21.7) | 11 (26.8) | 0.65 | 0.77 (0.25 to 2.38) |
| Clinical severity | – | – | – | – | – |
| Mild | 96 (77.4) | 63 (75.9) | 33 (80.4) | – | Ref |
| Severe | 20 (16.1) | 16 (19.3) | 4 (9.8) | 0.22 | 0.48 (0.15 to 1.54) |
| Critical | 8 (6.5) | 4 (4.8) | 4 (9.8) | 0.38 | 1.91 (0.45 to 8.13) |
| Fever (≥37.3℃) | – | – | – | – | – |
| No | 18 (14.5) | 9 (10.8) | 9 (22.0) | – | Ref |
| Yes | 106 (85.5) | 74 (89.2) | 32 (78.0) | 0.11 | 0.43 (0.16 to 1.19) |
| Cough* | – | – | – | – | – |
| No | 66 (53.2) | 45 (54.2) | 21 (51.2) | – | Ref |
| Yes | 58 (46.8) | 38 (45.8) | 20 (48.8) | 0.75 | 1.13 (0.53 to 2.39) |
| Diarrhoea† | – | – | – | – | – |
| No | 109 (87.9) | 76 (91.6) | 33 (80.5) | – | Ref |
| Yes | 15 (12.1) | 7 (8.4) | 8 (19.5) | 0.08 | 2.63 (0.88 to 7.85) |
| Comorbidity | – | – | – | – | – |
| No | 103 (83.1) | 72 (86.7) | 31 (75.6) | – | Ref |
| Yes | 21 (16.9) | 11 (13.3) | 10 (24.4) | 0.13 | 2.11 (0.81 to 5.48) |
| Time interval from illness onset to first hospital visit (days) (median (IQR))‡ | 3.0 (1.0–7.0) | 3.0 (1.0–7.0) | 4.0 (2.0–7.0) | – | – |
| ≤2 | 47 (37.9) | 35 (42.2) | 12 (29.3) | – | Ref |
| >2 | 77 (62.1) | 48 (57.8) | 29 (70.7) | 0.17 | 1.76 (0.79 to 3.93) |
| Time interval from illness onset to medical isolation (days) (median (IQR)) | 5.0 (2.0–7.0) | 5.0 (2.0–7.0) | 5.0 (3.0–9.0) | – | – |
| ≤2 | 32 (25.8) | 26 (31.3) | 6 (14.6) | – | Ref |
| >2 | 92 (74.2) | 57 (68.7) | 35 (85.4) | 0.05 | 2.66 (1.00 to 7.12) |
| Time interval from illness onset to laboratory confirmation (days) (median (IQR)) | 7.0 (4.7–10.2) | 7.0 (4.4–9.9) | 8.0 (5.6–12.9) | – | – |
| ≤3 | 16 (12.9) | 13 (15.7) | 3 (7.3) | – | Ref |
| >3 | 108 (87.1) | 70 (84.3) | 38 (92.7) | 0.20 | 2.35 (0.63 to 8.77) |
| Knowledge score on COVID-19 before illness onset (14 in total) (median (IQR))§ | 5 (0–9) | 5 (0–9) | 5 (0–10) | – | – |
| ≥10 | 31 (25.0) | 18 (21.7) | 13 (31.7) | – | Ref |
| 3–9 | 45 (36.3) | 32 (38.6) | 13 (31.7) | 0.24 | 0.56 (0.22 to 1.47) |
| ≤2 | 48 (38.7) | 33 (39.7) | 15 (36.6) | 0.33 | 0.63 (0.25 to 1.61) |
| Self-awareness of being infected with SARS-CoV-2 when developed illness | – | – | – | – | – |
| Likely | 45 (36.3) | 35 (42.2) | 10 (24.4) | – | Ref |
| Unlikely | 79 (63.7) | 48 (57.8) | 31 (75.6) | 0.06 | 2.26 (0.98 to 5.21) |
| Knowledge of their own infectiousness after illness onset | – | – | – | – | – |
| Likely | 84 (67.7) | 62 (74.7) | 22 (53.7) | – | Ref |
| Unlikely | 40 (32.3) | 21 (25.3) | 19 (46.3) | 0.02 | 2.55 (1.16 to 5.61) |
| Wear mask at home after illness onset¶ | – | – | – | – | – |
| Never | 41 (33.1) | 24 (28.9) | 17 (41.5) | – | Ref |
| Sometimes | 37 (29.8) | 21 (25.3) | 16 (39.0) | 0.76 | 1.15 (0.46 to 2.87) |
| All the time | 46 (37.1) | 38 (45.8) | 8 (19.5) | 0.02 | 0.30 (0.11 to 0.82) |
| Self-isolated after illness onset | – | – | – | – | – |
| Yes | 79 (63.7) | 58 (69.9) | 21 (51.2) | – | Ref |
| No | 45 (36.3) | 25 (30.1) | 20 (48.8) | 0.05 | 2.17 (1.00 to 4.70) |
| Eat separately at home after illness onset | – | – | – | – | – |
| Yes | 70 (56.5) | 54 (65.1) | 16 (39.0) | – | Ref |
| No | 54 (43.5) | 29 (34.9) | 25 (61.0) | 0.008 | 2.86 (1.32 to 6.19) |
| Eat with separate tableware | – | – | – | – | – |
| Yes | 81 (65.3) | 58 (69.9) | 23 (56.1) | – | Ref |
| No | 43 (34.7) | 25 (30.1) | 18 (43.9) | 0.14 | 1.78 (0.82 to 3.88) |
| Score on hand hygiene (8 in total) (with 11 missing values) (median (IQR)) | 8 (7–8) | 8 (7–8) | 7 (6–8) | – | – |
| ≥6 | 103 (91.2) | 68 (93.2) | 35 (87.5) | – | Ref |
| 4–5 | 7 (6.2) | 4 (5.5) | 3 (7.5) | 0.63 | 1.46 (0.31 to 6.88) |
| ≤3 | 3 (2.6) | 1 (1.3) | 2 (5.0) | 0.28 | 3.88 (0.34 to 44.29) |
*Primary case ever had the symptom of cough when living with others at home.
†Primary case ever had the symptom of diarrhoea (change of character of stool) when living with others at home.
‡Date on which cases self-reported the appearance of either fever (≥37.3℃) or any respiratory symptom during epidemiological investigation. Date of hospital visit was the earliest date that cases sought medical service for COVID-19 related illness.
§A composite variable involving the primary case’s knowledge on the infectivity of SARS-CoV-2, contagious population, transmission route, susceptible population, incubation period, common symptoms and preventive measures.
¶Refers to the primary case or family members wearing a face mask at home, regardless of whether it was a N95 mask, disposable surgical mask or a common mask, including cloth mask. Wearing masks all the time means the primary case wears a mask all the time except when having dinner or sleeping at home.
**A composite variable involving the primary case’s hand washing practice, including using running water, washing frequency, using sanitiser and under what conditions.