Table 2.

Protection against BA.5 or BA.2 infection after a previous positive SARS-CoV-2 PCR test in Denmark, by variant strain

	Cases	Controls	OR	Adjusted OR*	Estimated protection
Protection against infection with BA.5
Previous omicron infection
Exposed	210 (2·4%)	33 972 (19·0%)	0·106 (0·092–0·121)	0·073 (0·063–0·084)	92·7% (91·6–93·7)
Unexposed	8468 (97·6%)	144 697 (81·0%)	1	1	..
Previous delta infection
Exposed	65 (0·8%)	3336 (2·3%)	0·333 (0·261–0·427)	0·266 (0·207–0·343)	73·4% (65·7–79·3)
Unexposed	8468 (99·2%)	144 697 (97·7%)	1	1	..
Previous alpha infection
Exposed	58 (0·7%)	1878 (1·3%)	0·528 (0·406–0·686)	0·388 (0·296–0·509)	61·2% (49·1–70·4)
Unexposed	8468 (99·3%)	144 697 (98·7%)	1	1	..
Protection against infection with BA.2
Previous omicron infection
Exposed	192 (0·7%)	33 972 (19·0%)	0·028 (0·024–0·032)	0·029 (0·025–0·034)	97·1 (96·6–97·5)
Unexposed	29 100 (99·3%)	144 697 (81·0%)	1	1	..
Previous delta infection
Exposed	100 (0·3%)	3336 (2·3%)	0·149 (0·122–0·182)	0·158 (0·129–0·193)	84·2 (80·7–87·1)
Unexposed	29 100 (99·7%)	144 697 (97·7%)	1		..
Previous alpha infection
Exposed	98 (0·3%)	1878 (1·3%)	0·259 (0·212–0·318)	0·262 (0·214–0·322)	73·8 (67·8–78·6)
Unexposed	29 100 (99·7%)	144 697 (98·7%)	1	1	..

Data are n (%), OR (95% CI), or estimated % (95% CI). All participants received three doses of mRNA COVID-19 vaccine. The outcome period was April 10–June 30, 2022. Cases were people infected with SARS-CoV-2 omicron subvariants BA.5 or BA.2 during the outcome period. Controls were people who tested negative for SARS-CoV-2 infection during the outcome period. Unexposed individuals had no positive PCR tests before the start of the outcome period on April 10, 2022.

^*Adjusted for age group, time of infection (week number), sex, region of residency, and comorbidities.