Table 2.
Univariate and multivariable analysis of factors associated with SARS CoV-2 infection in study participants from 12 December 2021 to end of the study (31 January 2023)
| Rate per 1000 participant days | Univariate Cox Regression | Multivariable Cox Regression | |||
|---|---|---|---|---|---|
| Rate (95% CI) | Hazard ratio, 95% CI | P value | Hazard ratio 95% CI | P value | |
| Early SARS-CoV-2 infection1 | |||||
| No (non-EI) | 1.75 (1.41-2.18) | Reference | Reference | ||
| Yes (EI) | 0.86 (0.74-0.99) | 0.49 (0.38-0.64) | <.0001 | 0.56 (0.43-0.74) | <.001 |
| Sex at birth | |||||
| Male | 0.94 (0.78-1.13) | Reference | |||
| Female | 1.09 (0.93-1.29) | 1.16 (0.90-1.48) | 0.25 | - | |
| Age group | |||||
| 18-49 years | 1.40 (1.17-1.67) | Reference | |||
| 50-64 years | 0.91 (0.73-1.13) | 0.66 (0.50-0.87) | 0.0001 | NE6 | 0.022 |
| ≥65 years | 0.72 (0.55-0.93) | 0.52 (0.38-0.71) | |||
| Underlying illness | |||||
| Any2 | 0.94 (0.76-1.15) | 0.87 (0.68-1.13) | 0.31 | - | |
| Diabetes mellitus | 0.84 (0.61-1.15) | 0.79 (0.56-1.12) | 0.18 | ||
| Cardiac disease | 0.80 (0.54-1.18) | 0.76 (0.50-1.15) | 0.20 | ||
| Immunocompromised | 1.27 (0.79-2.04) | 1.24 (0.76-2.03) | 0.38 | ||
| Severity of 2020 illness | |||||
| Hospitalized | 0.82 (0.68-1.00) | Reference | - | ||
| Not hospitalized | 1.21 (1.03-1.41) | 1.46 (1.14-1.87) | 0.003 | ||
| Children (≤17yrs) in household | |||||
| No | 0.85 (0.73-1.00) | Reference | Reference | ||
| Yes | 1.53 (1.25-1.86) | 1.77 (1.37-2.27) | <.0001 | 1.60 (1.19-2.15) | 0.002 |
| Ethnic Background | |||||
| Caucasian | 1.29 (1.08-1.53) | Reference | Reference | ||
| Asian (East/SE/South) | 0.88 (0.69-1.12) | 0.68 (0.51-0.92) | 0.006 | 0.57 (0.41-0.79) | <.001 |
| Black | 0.63 (0.40-0.99) | 0.50 (0.31-0.80) | 0.57 (0.34-0.95) | 0.03 | |
| Other | 0.94 (0.70-1.25) | 0.73 (0.52-1.03) | 0.70 (0.49-1.00) | 0.05 | |
| Education | |||||
| Secondary school or less | 0.64 (0.47-0.87) | Reference | Reference | ||
| College/diploma/certificate | 0.94 (0.75-1.18) | 1.46 (1.00-2.14) | 0.0002 | 1.32 (0.09-1.95) | 0.16 |
| Undergraduate university degree | 1.31 (1.07-1.61) | 2.03 (1.40-2.94) | 1.59 (1.08-2.32) | 0.02 | |
| Graduate or professional degree | 1.42 (1.08-1.85) | 2.19 (1.46-3.29) | 1.52 (0.99-2.32) | 0.06 | |
| Neighbourhood income quintile | |||||
| 1-lowest | 0.71 (0.54-0.92) | Reference | Reference | ||
| 2 | 0.92 (0.71-1.20) | 1.30 (0.89-1.88) | 0.001 | 1.27 (0.88,1.83) | 0.20 |
| 3 | 1.15 (0.87-1.51) | 1.61 (1.10-2.36) | 1.46 (0.99-2.14) | 0.06 | |
| 4 | 1.57 (1.17-2.11) | 2.19 (1.47-3.25) | 1.95 (1.31-2.91) | 0.001 | |
| 5 – highest | 1.26 (0.97-1.65) | 1.77 (1.22-2.58) | 1.43 (0.96-2.12) | 0.08 | |
| Work status, Omicron3 | |||||
| Other | 0.88 (0.73-1.06) | Reference | Reference | ||
| Essential worker | 1.17 (1.00-1.38) | 1.33 (1.04-1.70) | 0.02 | 1.29 (0.98-1.68) | 0.07 |
| Hand-to-face habits4 | |||||
| None | 0.74 (0.56-0.97) | Reference | |||
| ≤5 times per day | 1.12 (0.96-1.31) | 1.51 (1.10-2.06) | 0.02 | - | |
| ≥6 times per day | 1.25 (0.95-1.64) | 1.68 (1.14-2.47) | |||
| COVID-19 vaccine doses before Omicron5 | |||||
| Not vaccinated | 1.28 (0.73-2.25) | Reference | - | ||
| One dose | 0.53 (0.13-2.13) | 0.42 (0.09-1.89) | 0.46 | ||
| Two doses | 0.99 (0.86-1.13) | 0.78 (0.43-1.39) | |||
| Three doses | 1.18 (0.89-1.57) | 0.93 (0.49-1.75) | |||
| Number of vaccine doses during Omicron5 | |||||
| None | 1.67 (1.40-1.99) | Reference | NE7 | ||
| One dose | 1.03 (0.85-1.24) | 0.62 (0.48-0.81) | <.0001 | ||
| Two doses | 0.38 (0.25-0.60) | 0.23 (0.15-0.38) | |||
| Three doses | 0.21 (0.08-0.55) | 0.13 (0.05-0.34) | |||
| Time since most recent vaccine dose | NE7 | ||||
| Not vaccinated | 1.32 (0.73-2.38) | NA | |||
| ≥120 days | 1.12 (0.97-1.30) | Reference | |||
| 75 - 119 days | 0.97 (0.69-1.37) | 0.95 (0.64-1.41) | 0.78 | ||
| 14-74 days | 0.75 (0.56-1.00) | 0.71 (0.50-1.01) | 0.06 | ||
Abbreviations: SE southeast, FT Full-time, PT part time, CI Confidence Interval, NE not estimable
1Early infection is defined as an RT-PCR confirmed SARS-CoV-2 infection between January 24 and 1 September 30, 2020
2Any underlying illness predisposing to complications of viral respiratory illness [24]. Diabetes mellitus and underlying cardiac disease were the only two conditions present in at least 10% of participants. Immunocompromised includes both immunocompromising illness and therapy [10]
3Work status was categorized as essential if participants reported working full- or part- time in-person outside of the home. Other comprised those working only at home or who were students, retired or unemployed
4Hand-to-face habits were defined as rubbing eyes, biting nails or cuticles or habitually putting fingers into the mouth or nose
5The start of the Omicron wave in the population area was December 12, 2021 (see text); vaccine dates are lagged by 14 days
6In multivariable analysis, age was modelled with a restricted cubic spline; the hazard ratio per decade of age was 0.82 (95% CI0.66–0.90)
7Vaccine doses were modelled as time varying in the multivariable analysis such that direct estimates are not available