Table 2. Odds ratio for the association between study period and mortality outcome.
Results of multivariate logistic models, with random intercepts for countries, on 14-day fatality risk are presented. Different models were fit that included different variables. Model III adjusts for all variables in the table, however due to missing data in the vaccination and comorbidity variables, less than a third of the study population was included in the estimation of that model; models I and II were thus fit that did not adjust for these variables and included more individuals. In model IV, a category for missing data was created for the variable on previous vaccination; individuals in that category had an odds ratio of 0.74 (0.69–0.80; reference group in this comparison is the non-vaccinated group). Note that similar results were obtained when finer categorisation of the age variable, 10-year intervals, was used. As previous SARS-CoV-2 infection has been shown to reduce severity of COVID-19 (Altarawneh et al., 2022), a multivariable model that also adjusted for this variable was fit; in that model, the odds ratio for the association between study period and fatality risk was 0.70 (0.61–0.80). As in other epidemiological studies, estimates for covariates other than the primary exposure (study period) should be carefully interpreted (Westreich and Greenland, 2013).
| Model | I | II | III | IV |
|---|---|---|---|---|
| Number of observations | 94,077 | 39,950 | 26,728 | 56,329 |
| Odds ratio (95% CI) | Odds ratio (95% CI) | Odds ratio (95% CI) | Odds ratio (95% CI) | |
| Variables | ||||
| Omicron period* | 0.65 (0.62–0.69) | 0.67 (0.61–0.75) | 0.68 (0.60–0.77) | 0.64 (0.59–0.69) |
| Sex (male) | 1.32 (1.26–1.38) | 1.33 (1.23–1.43) | 1.36 (1.24–1.49) | 1.33 (1.25–1.42) |
| Age | ||||
| Older than 60 years | Reference | Reference | Reference | Reference |
| Aged between 18 and 60 years | 0.26 (0.25–0.27) | 0.24 (0.22–0.26) | 0.27 (0.25–0.30) | 0.30 (0.27–0.32) |
| Younger than 18 years | 0.06 (0.05–0.07) | 0.06 (0.05–0.07) | 0.07 (0.05–0.09) | 0.06 (0.05–0.08) |
| Previous vaccination | - | 0.60 (0.55–0.65) | 0.53 (0.48–0.59) | 0.59 (0.54–0.65) |
| Comorbidities | ||||
| Hypertension | - | - | 1.29 (1.16–1.42) | 1.26 (1.17–1.35) |
| Diabetes | - | - | 1.22 (1.09–1.38) | 1.22 (1.12–1.32) |
| Chronic cardiac disease | - | - | 1.50 (1.31–1.71) | 1.51 (1.39–1.65) |
Odds ratio in univariate analysis 0.65 (0.61–0.69) (N=94,524).