Table 2. Hazard ratio estimates for hospitalisation with the Omicron compared with the Delta variant of SARS-CoV-2, using stratified Cox regression, overall and by subgroup analysis, Norway, 6 December 2021–9 January 2022 (n = 91,005).
| Hospitalisation | |||||
|---|---|---|---|---|---|
| Delta cases | Omicron cases | Omicron vs Delta, adjusted hazard ratio (95%CI) | |||
| n | % a | n | % a | ||
| Overall/main analysis | 552 | 1.1 | 91 | 0.2 | 0.27 (0.20–0.36) |
| Subgroup analysis by | |||||
| Sex | |||||
| Female | 228 | 0.9 | 53 | 0.3 | 0.45 (0.30–0.67) |
| Male | 324 | 1.3 | 38 | 0.2 | 0.17 (0.11–0.27) |
| Age group (years) | |||||
| 0–29 | 36 | 0.1 | 11 | 0.1 | 0.24 (0.09–0.60) |
| 30–44 | 95 | 0.7 | 14 | 0.1 | 0.23 (0.11–0.47) |
| 45–54 | 80 | 1.2 | 18 | 0.4 | 0.40 (0.19–0.85) |
| 55–64 | 116 | 3.2 | 17 | 0.6 | 0.22 (0.10–0.45) |
| 65–74 | 96 | 6.2 | 11 | 1.3 | 0.20 (0.07–0.51) |
| ≥ 75 | 129 | 19 | 20 | 4.4 | 0.41 (0.17–0.98) |
| Country of birth | |||||
| Norway | 323 | 0.8 | 52 | 0.2 | 0.27 (0.18–0.40) |
| Overseas | 203 | 1.7 | 34 | 0.3 | 0.23 (0.14–0.38) |
| Unknown | 26 | 4.4 | 5 | 1.4 | 2.33 (0.13–41.6) |
| Risk for severe COVID-19b | |||||
| No underlying comorbidities | 297 | 0.7 | 36 | 0.1 | 0.23 (0.15–0.36) |
| Medium-risk comorbidity | 145 | 2.8 | 31 | 0.9 | 0.35 (0.21–0.59) |
| High-risk comorbidity | 110 | 13 | 24 | 5.1 | 0.16 (0.06–0.42) |
| Vaccination status at date of positive test | |||||
| Not vaccinated | 335 | 1.5 | 15 | 0.2 | 0.13 (0.07–0.23) |
| One dose < 21 days before positive test | 8 | 2.3 | 1 | 0.6 | NA |
| Partially completed primary vaccination series ≥ 21 days before positive testc | 9 | 0.2 | 3 | 0.1 | NA |
| Completed primary vaccination series with maximum two doses 7–179 days before positive testc, d | 47 | 0.3 | 26 | 0.1 | 0.62 (0.31–1.24) |
| Completed primary vaccination series with maximum two doses ≥ 180 days before positive testc, d | 98 | 2.1 | 26 | 0.7 | 0.50 (0.25–1.01) |
| Vaccinated with three doses ≥ 7 days before positive testc | 55 | 3.7 | 20 | 0.6 | 0.19 (0.08–0.43) |
| Unvaccinated, but previously diagnosed with COVID-19 6–12 months before positive test | 0 | 0.0 | 0 | 0.0 | NAe |
CI: confidence interval; COVID-19: coronavirus disease; NA: not available due to small numbers and lack of discordant pairs; SARS-CoV-2: severe acute respiratory syndrome coronavirus 2.
a Proportions here are calculated using as denominator the respective population in the subgroup and variant as reported in Table 1.
b Risk for severe disease based on underlying comorbidities that are associated with a moderate or high risk of serious illness regardless of age. Details on the definitions used are provided in Supplement section 1.
c See Supplement section 1 for full details on how each vaccination status category is defined and data on vaccine type. Overall, 98% of vaccinated Omicron cases and 96% of Delta cases had received a homologous or mixed regimen of mRNA vaccines. The three-dose category predominantly includes cases who received their third dose as a booster dose, however it will also include cases who received their third dose as part of their primary series, for example those severely immunocompromised [14]. We were not able to clearly distinguish persons who had received a third dose as part of their primary series from those who had received a booster dose.
d When we used a recoded variable for vaccination status including both categories for those who had completed primary vaccination with maximum two doses, regardless of the time since last dose, we found an adjusted hazard ratio of 0.54 (95% CI: 0.34–0.87).
e We had 415 cases who were unvaccinated, but who had previously been diagnosed with COVID-19 6–12 months before positive test. None of these 415 required hospitalisation. This indicates that prior infection is associated with lower risk of hospitalisation than unvaccinated status, but we could not calculate estimates because of a lack of discordant pairs in our model.
Hazard ratios were estimated using Cox regression stratified by county of residence and date of sampling and further adjusted for variant, sex, age group, country of birth, underlying comorbidities and vaccination status at date of positive test.