Table 3. Risk of SARS-CoV-2 infection by disease history, severity of the first infection, and epidemic phase of the first infection in the pre-Omicron and Omicron BA.1 periods, Reggio Emilia province, Italy, 20 February 2020–28 February 2022 (n = 538,516 individuals).

Cox proportional hazards models	Period of SARS-CoV-2 infection
	Pre-Omicron 20 Feb 2020–20 Dec 2021		Omicron BA.1 1 Jan–28 Feb 2022
	HR	95% CI	HR	95% CI
Model 1: Disease history
No prior infection	1	NA	1	NA
Prior infection	0.08	0.07–0.10	0.42	0.41–0.44
Model 2: Severity of the first infection
No prior infection	1	NA	1	NA
Prior infection, no hospitalisation	0.08	0.07–0.10	0.43	0.41–0.45
Prior infection, hospitalisation	0.11	0.07–0.18	0.29	0.24–0.36
Model 3: Epidemic phase
No prior infection	1	NA	1	NA
Prior infection, wild-type	0.09	0.08–0.11	0.49	0.46–0.51
Prior infection, Alpha	0.07	0.05–0.09	0.38	0.36–0.40
Prior infection, Delta	0.03	0.01–0.12	0.37	0.33–0.42

CI: confidence interval; HZ: hazard ratio; NA: not applicable; SARS-CoV-2: severe acute respiratory syndrome coronavirus 2.

All models are adjusted by age, sex, number of COVID-19 vaccination doses and Charlson Comorbidity Index.