Table 1

Characteristics of primary cases of COVID-19: univariable analysis

Primary casesTotal (n (%))
(n=124)
Families without transmission (n (%)) (n=83)Families with transmission (n (%)) (n=41)P valueUnadjusted 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)
 <18000
 18–5992 (74.2)63 (75.9)29 (70.7)Ref
 ≥6032 (25.8)20 (24.1)12 (29.3)0.541.30 (0.56 to 3.02)
Sex
 Men61 (49.2)40 (48.2)21 (51.2)Ref
 Women63 (50.8)43 (51.8)20 (48.8)0.750.89 (0.42 to 1.87)
Education level
 High school or lower26 (21.0)18 (21.7)8 (19.5)Ref
 Bachelor degree69 (55.6)47 (56.6)22 (53.7)0.530.75 (0.30 to 1.86)
 Graduate degree29 (23.4)18 (21.7)11 (26.8)0.650.77 (0.25 to 2.38)
Clinical severity
 Mild96 (77.4)63 (75.9)33 (80.4)Ref
 Severe20 (16.1)16 (19.3)4 (9.8)0.220.48 (0.15 to 1.54)
 Critical8 (6.5)4 (4.8)4 (9.8)0.381.91 (0.45 to 8.13)
Fever (≥37.3℃)
 No18 (14.5)9 (10.8)9 (22.0)Ref
 Yes106 (85.5)74 (89.2)32 (78.0)0.110.43 (0.16 to 1.19)
Cough*
 No66 (53.2)45 (54.2)21 (51.2)Ref
 Yes58 (46.8)38 (45.8)20 (48.8)0.751.13 (0.53 to 2.39)
Diarrhoea†
 No109 (87.9)76 (91.6)33 (80.5)Ref
 Yes15 (12.1)7 (8.4)8 (19.5)0.082.63 (0.88 to 7.85)
Comorbidity
 No103 (83.1)72 (86.7)31 (75.6)Ref
 Yes21 (16.9)11 (13.3)10 (24.4)0.132.11 (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)
 ≤247 (37.9)35 (42.2)12 (29.3)Ref
 >277 (62.1)48 (57.8)29 (70.7)0.171.76 (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)
 ≤232 (25.8)26 (31.3)6 (14.6)Ref
 >292 (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)
 ≤316 (12.9)13 (15.7)3 (7.3)Ref
 >3108 (87.1)70 (84.3)38 (92.7)0.202.35 (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)
 ≥1031 (25.0)18 (21.7)13 (31.7)Ref
 3–945 (36.3)32 (38.6)13 (31.7)0.240.56 (0.22 to 1.47)
 ≤248 (38.7)33 (39.7)15 (36.6)0.330.63 (0.25 to 1.61)
Self-awareness of being infected with SARS-CoV-2 when developed illness
 Likely45 (36.3)35 (42.2)10 (24.4)Ref
 Unlikely79 (63.7)48 (57.8)31 (75.6)0.062.26 (0.98 to 5.21)
Knowledge of their own infectiousness after illness onset
 Likely84 (67.7)62 (74.7)22 (53.7)Ref
 Unlikely40 (32.3)21 (25.3)19 (46.3)0.022.55 (1.16 to 5.61)
Wear mask at home after illness onset¶
 Never41 (33.1)24 (28.9)17 (41.5)Ref
 Sometimes37 (29.8)21 (25.3)16 (39.0)0.761.15 (0.46 to 2.87)
 All the time46 (37.1)38 (45.8)8 (19.5)0.020.30 (0.11 to 0.82)
Self-isolated after illness onset
 Yes79 (63.7)58 (69.9)21 (51.2)Ref
 No45 (36.3)25 (30.1)20 (48.8)0.052.17 (1.00 to 4.70)
Eat separately at home after illness onset
 Yes70 (56.5)54 (65.1)16 (39.0)Ref
 No54 (43.5)29 (34.9)25 (61.0)0.0082.86 (1.32 to 6.19)
Eat with separate tableware
 Yes81 (65.3)58 (69.9)23 (56.1)Ref
 No43 (34.7)25 (30.1)18 (43.9)0.141.78 (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)
 ≥6103 (91.2)68 (93.2)35 (87.5)Ref
 4–57 (6.2)4 (5.5)3 (7.5)0.631.46 (0.31 to 6.88)
 ≤33 (2.6)1 (1.3)2 (5.0)0.283.88 (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.