Table 3.
Incidence of Omicron infection by time period1 in those with and without RT-PCR confirmed SARS-CoV-2 infection prior to 30 September 2020 (early infection)
| Early Infection (EI) |
No Early Infection (non-EI) |
EI vs. non -EI IRR (95% CI) |
EI vs. Non-EI Hazard Ratio2 (95% CI) | |
|---|---|---|---|---|
| Prior to Omicron | ||||
| Number of COVID-19 infections | 5 | 9 | ||
| Participant-days | 369,651 | 101273.7 | ||
| COVID-19 rate/1000 participant days | 0.014 | 0.089 | 0.15 (0.05–0.45) | |
| Omicron BA.1/BA.2 | ||||
| Number of COVID-19 infections | 91 | 40 | ||
| Participant-days | 107,331 | 30,142 | ||
| COVID-19 rate/1000 participant days | 0.85 | 1.33 | 0.64 (0.44–0.93) | 0.66 (0.45–0.97) |
| Omicron BA.4/BA.5 | ||||
| Number of COVID-19 infections | 68 | 38 | ||
| Participant-days | 96,844 | 26,221 | ||
| COVID-19 rate/1000 participant days | 0.70 | 1.45 | 0.48 (0.33–0.72) | 0.44 (0.28–0.68) |
| Mixed BA.5/BQ1.1/XBB | ||||
| Number of COVID-19 infections | 27 | 10 | ||
| Participant-days | 44,859 | 11,817 | ||
| COVID-19 rate/1000 participant days | 0.60 | 0.85 | 0.71 (0.34–1.47) | 0.71 (0.32–1.56) |
1Time periods are defined as follow: prior to Omicron, 25 January 2020–11 December 2021; Omicron BA1/BA.2, 12 December 2021–3 Jun 2022; Omicron BA4/BA5, 4 June 1–13 November 2022; and mixed BA.5/BQ1.1/XBB, 14 November 2022–31 January 2023 (end of study)
2Hazard ratios from the primary analysis: multivariable Cox regression adjusted for age, COVID-19 vaccination, severity of 2020 illness, neighborhood income quantile, ethnicity, education, work status during omicron and presence of children in the household