Table 2 Effectiveness of previous infection against symptomatic reinfection

Effectiveness	Cases^a		Controls^a		Effectiveness^b (%) (95% CI)^c
Effectiveness	Previous infection (n)	No previous infection (n)	Previous infection (n)	No previous infection (n)	Effectiveness^b (%) (95% CI)^c
(a) Effectiveness of a pre-Omicron infection against symptomatic^d reinfection with a pre-Omicron virus
Any previous infection	626	139,378	7,277	212,793	86.8 (85.7 to 87.9)
By time since previous infection
Subgroup analysis 1
3–<6 months	177	137,936	2,027	212,561	85.2 (82.8 to 87.4)
6–<9 months	164	138,182	2,375	212,594	89.4 (87.5 to 91.0)
9 months–<1 year	236	138,453	2,116	212,662	84.6 (82.4 to 86.6)
≥1 year	44	137,707	753	212,536	90.0 (86.5 to 92.7)
Subgroup analysis 2
<1 year	581	139,316	6,523	212,781	86.5 (85.3 to 87.6)
≥1 year	44	137,707	753	212,536	90.0 (86.5 to 92.7)
(b) Effectiveness of an Omicron infection against symptomatic^d reinfection with an Omicron virus
Any previous infection	3,211	49,371	7,629	62,670	45.4 (42.5 to 48.2)
By time since previous infection
Subgroup analysis 1
3–<6 months	119	47,753	1,125	61,367	86.3 (83.2 to 88.9)
6–<9 months	606	48,046	2,236	61,610	64.0 (60.1 to 67.5)
9 months–<1 year	834	47,835	1,781	61,696	29.8 (22.6 to 36.4)
≥1 year	1,481	47,952	2,222	61,971	4.8 (−3.8 to 12.7)
Subgroup analysis 2
<1 year	1,609	48,837	5,244	62,054	58.9 (56.1 to 61.5)
≥1 year	1,481	47,952	2,222	61,971	4.8 (−3.8 to 12.7)

^aCases (SARS-CoV-2-positive tests) and controls (SARS-CoV-2-negative tests) were matched exactly one-to-two by sex, 10-year age group, nationality, number of coexisting conditions, number of vaccine doses at time of the SARS-CoV-2 test, calendar week of the SARS-CoV-2 test, method of testing and reason for testing.
^bEffectiveness of previous infection in preventing reinfection was estimated using the test-negative, case–control study design¹⁵.
^cCIs were not adjusted for multiplicity and thus should not be used to infer definitive differences between different groups.
^dA symptomatic infection was defined as a SARS-CoV-2 PCR or rapid antigen test conducted because of clinical suspicion due to the presence of symptoms compatible with a respiratory tract infection.

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