Table 2. Estimating the association of potential risk factors with the risk of acquiring and transmitting the SARS-CoV-2 Delta variant.
| Characteristic | Univariate analysis | Multivariate analysis | |||
| OR (95% CI) | P value | OR (95% CI) | P value | ||
| Note: “−” in the column of “OR (95% CI)” indicated that the corresponding variable was not incorporated into the analysis. All children and adolescents (under 18 years) were unvaccinated, as they were not covered by the COVID-19 vaccination program in the Chinese mainland during the Delta outbreak. Vaccinated adults denote fully vaccinated adults, while partially vaccinated adults were deemed unvaccinated adults in the analysis. Abbreviation: CI=confidence interval; OR=odds ratio; COVID-19=coronavirus disease 2019; SARS-CoV-2=severe acute respiratory syndrome coronavirus 2. * P<0.05, ** P<0.01. *** P<0.001. † Contacts who were aged at or over 65 years or whose infectors were aged at or over 65 years were excluded from this analysis due to the small sample size. § For a contact who had interacted with more than one SARS-CoV-2 infection, we chose the first infected individual they had been exposed to as the potential infector. If they had exposed to multiple infectors at the same time, we randomly chose one from among these infectors as the potential infector. | |||||
| Age (years) and vaccination status of contacts† | |||||
| Unvaccinated children (0–9 years) | 3.42 (1.49, 7.89) | 0.004** | 2.44 (1.03, 5.80) | 0.044* | |
| Unvaccinated adolescents (10–17 years) | 1.56 (0.66, 3.69) | 0.310 | 1.20 (0.46, 3.10) | 0.707 | |
| Unvaccinated adults (18–64 years) | Reference | Reference | |||
| Vaccinated adults (18–64 years) | 0.49 (0.25, 0.95) | 0.035* | 0.46 (0.23, 0.93) | 0.030* | |
| Age (years) and vaccination status of infectors§ | |||||
| Unvaccinated children (0–9 years) | 1.79 (0.45, 7.19) | 0.410 | 0.82 (0.19, 3.46) | 0.784 | |
| Unvaccinated adolescents (10–17 years) | 0.32 (0.07, 1.51) | 0.149 | 0.63 (0.16, 2.52) | 0.513 | |
| Unvaccinated adults (18–64 years) | Reference | Reference | |||
| Vaccinated adults (18–64 years) | 0.53 (0.17, 1.61) | 0.260 | 0.51 (0.19, 1.40) | 0.192 | |
| Type of contact | |||||
| Household contact | 7.54 (3.67, 15.48) | <0.001*** | 6.79 (3.20, 14.44) | <0.001*** | |
| Health care | 0.44 (0.10, 1.93) | 0.274 | 0.22 (0.04, 1.11) | 0.067 | |
| Social contact | Reference | Reference | |||
| Workplace contact | 0.64 (0.15, 2.75) | 0.550 | 0.73 (0.17, 3.20) | 0.678 | |
| Other | 0.07 (0.02, 0.20) | <0.001*** | 0.05 (0.02, 0.15) | <0.001*** | |
| Sex of contacts | |||||
| Female | Reference | Reference | |||
| Male | 1.03 (0.62, 1.73) | 0.906 | 0.72 (0.40, 1.28) | 0.261 | |
| Sex of infectors | |||||
| Female | Reference | Reference | |||
| Male | 1.04 (0.39, 2.79) | 0.942 | 0.67 (0.27, 1.67) | 0.391 | |
| Clinical severity of primary cases | |||||
| Asymptomatic infection | Reference | − | − | ||
| Symptomatic infection | 1.05 (0.23, 4.75) | 0.951 | − | − | |