Table 3.

Natural protection among unvaccinated individuals against hospitalisation with a new SARS-CoV-2 infection following a prior infection at least 180 days earlier.

Analysis period	Population	Person-years of follow-up	SARS-CoV-2 associated admissions during follow-up*	Hospitalisation rate$ during follow-up	Adjusted‡hazard ratio (95% CI)	Estimated natural protection from prior infection
March 15 – June 30, 2021. Dominant variant: Alpha
No prior infection	2,889,876	549,870	732	1.33	-	-
Previously infected	113,702	12,634	2	0.16	0.134 (0.033; 0.537)	86.6% (46.3; 96.7%)
July 15 – November 15, 2021. Dominant variant: Delta
No prior infection	1,211,322	249,741	614	2.46	-	-
Previously infected	95,723	19,768	2	0.10	0.028 (0.007; 0.110)	97.2% (89.0; 99.3%)
January 1, 2022 – March 10, 2022. Dominant variant: Omicron
No prior infection	424,683	48,098	495	10.29	-	-
Previously infected	65,963	7058	18	2.55	0.302 (0.188; 0.485)	69.8% (51.5; 81.2%)

Participants (aged 2+ years) remained in follow-up until the date of their first vaccination, death, out-migration, a positive SARS-CoV-2 PCR test or the end of follow-up.

^⁎Hospital admission with associated primary ICD-10 diagnosis codes DB342A or DB972A occurring no earlier than two days before, and no later than 14 days after, a positive PCR test.

^$Rate of hospitalisations per 1000 person-years of follow-up.

^‡From a Cox regression model controlling for sex, age group, number of comorbidities and country region.