Secondary attack rate (SAR) and risk for infection among close contacts traced during the Omicron epidemic in Shanghai from 1 March to 1 June 2022.
| Characteristics* | Secondary cases/total close contacts (N/N) | SAR (95% CI) | Crude OR (95% CI) | Adjusted OR (95% CI)* |
| Exposure settings | ||||
| Farmers’ market | 2883/11 709 | 24.62 (23.79 to 25.50) | 3.47 (2.79 to 4.32) | 3.62 (2.87 to 4.55) |
| Household | 32 821/141 162 | 23.25 (23.04 to 23.44) | 2.73 (2.21 to 3.36) | 2.68 (2.15 to 3.35) |
| Hotels or restaurants | 106/1419 | 7.47 (6.04 to 8.88) | 1.99 (1.46 to 2.72) | 1.97 (1.42 to 2.74) |
| Workplaces | 3742/27 072 | 13.82 (13.40 to 14.24) | 1.83 (1.47 to 2.26) | 1.84 (1.47 to 2.31) |
| Healthcare settings | 644/4655 | 13.83 (12.76 to 14.88) | 1.75 (1.37 to 2.24) | 1.78 (1.37 to 2.32) |
| Transportation | 17/1264 | 1.34 (0.77 to 2.07) | 1 (Reference) | 1 (Reference) |
| Other settings | 17 312/93 611 | 18.49 (18.26 to 18.75) | 2.81 (2.28 to 3.46) | 2.73 (2.19 to 3.40) |
| Contact frequency | ||||
| Daily | 25 335/116 636 | 21.72 (21.50 to 21.94) | 1.00 (0.96 to 1.04) | 1.00 (0.96 to 1.04) |
| Several times | 17 580/91 745 | 19.16 (18.93 to 19.42) | 1.08 (1.04 to 1.13) | 1.05 (1.00 to 1.09) |
| First time | 11 399/61 882 | 18.42 (18.11 to 18.72) | 1 (Reference) | 1 (Reference) |
| Sex of close contacts | ||||
| Male | 46 133/261 711 | 17.63 (17.52 to 17.74) | 1 (Reference) | 1 (Reference) |
| Female | 36 294/185 933 | 19.52 (19.36 to 19.67) | 1.11 (1.09 to 1.12) | 1.11 (1.09 to 1.13) |
| Occupation of close contacts | ||||
| Medical workers | 350/3308 | 10.58 (9.53 to 11.75) | 1 (Reference) | 1 (Reference) |
| Taxi and bus drivers | 223/1502 | 14.85 (12.79 to 16.80) | 1.29 (1.07 to 1.55) | 1.25 (1.03 to 1.51) |
| Students | 1143/8644 | 13.22 (12.44 to 13.91) | 1.57 (1.37 to 1.79) | 1.49 (1.29 to 1.72) |
| Public service staff | 1830/8578 | 21.33 (20.43 to 22.26) | 1.59 (1.39 to 1.82) | 1.50 (1.30 to 1.72) |
| Building workers | 6083/35 890 | 16.95 (16.49 to 17.30) | 1.60 (1.41 to 1.81) | 1.52 (1.33 to 1.74) |
| Retired | 2555/8589 | 29.75 (28.59 to 30.88) | 2.05 (1.80 to 2.33) | 1.60 (1.40 to 1.84) |
| Food market workers | 1036/4793 | 21.61 (20.40 to 23.01) | 1.69 (1.46 to 1.96) | 1.60 (1.37 to 1.87) |
| Delivery workers | 322/1548 | 20.80 (18.54 to 22.97) | 1.62 (1.33 to 1.96) | 1.60 (1.30 to 1.95) |
| Preschoolers | 127/585 | 21.71 (17.84to 25.67) | 1.64 (1.29 to 2.07) | 1.61 (1.26 to 2.05) |
| Unemployed | 1194/4587 | 26.03 (24.74 to 27.73) | 1.89 (1.65 to 2.17) | 1.77 (1.53 to 2.04) |
| Others | 7861/41 988 | 18.72 (18.35 to 19.17) | 1.58 (1.40 to 1.78) | 1.51 (1.33 to 1.71) |
| Comorbidities | ||||
| No | 58 761/341 713 | 17.20 (17.12 to 17.27) | 1 (Reference) | 1 (Reference) |
| Yes | 23 706/109 057 | 21.74 (21.50 to 21.96) | 1.19 (1.17 to 1.21) | 1.10 (1.09 to 1.12) |
| Vaccination status of contacts | ||||
| No vaccination | 21 259/126 588 | 16.79 (16.59 to 16.97) | 1 (Reference) | 1 (Reference) |
| Partial vaccination | 1784/8638 | 20.65 (19.79 to 21.66) | 1.19 (1.14 to 1.25) | 1.32 (1.26 to 1.38) |
| Full vaccination | 24 781/124 037 | 19.98 (19.76 to 20.18) | 1.19 (1.17 to 1.21) | 1.30 (1.28 to 1.32) |
| Booster vaccination | 34 243/188 881 | 18.13 (18.00 to 18.27) | 1.13 (1.11 to 1.14) | 1.20 (1.18 to 1.22) |
*Adjusted for age group, sex, occupation, comorbidities and vaccination status of index cases and close contacts; and for clinical severity, self-reported symptoms and time from positive test to isolation of index cases.
†'Unknown’ for each characteristic was excluded from the analysis.