Table 1.

Comparison of infection and reinfection rates before and after first SARS-CoV-2 infection in 2020 in Denmark

		Population	Confirmed new infection during follow-up	Person-days of follow-up	Infection rate*during follow-up	Adjusted rate ratio (95% CI)†	Estimated protection (95% CI)
Main analysis of reinfection during the second surge
Positive during first surge		11 068	72	1 346 920	5·35	0·195 (0·155–0·246)	80·5% (75·4–84·5)
Negative during first surge		514 271	16 819	62 151 056	27·06	1 (ref)	..
Alternative cohort analysis with reinfection at least 90 days after first infection‡
Exposed periods		28 875	138	2 447 924	5·64	0·212 (0·179–0·251)	78·8% (74·9–82·1)
Unexposed periods		2 405 683	53 991	174 487 793	30·94	1 (ref)	..
Sensitivity analyses of reinfection during the second surge
In frequently tested nurses, doctors, social workers, and health-care assistants
	Positive during first surge	658	8	80 014	10·00	0·189 (0·094–0·379)	81·1% (62·1–90·6)
	Negative during first surge	14 946	934	1 798 184	51·94	1 (ref)	..
If the second surge was Aug 1 to Dec 31, 2020§
	Positive during first surge	11 068	87	1 687 700	5·15	0·233 (0·189–0·287)	76·7% (71·3–81·1)
	Negative during first surge	514 562	17 110	78 098 000	21·91	1 (ref)	..
If the second surge was Oct 1 to Dec 31, 2020§
	Positive during first surge	11 068	59	1 016 359	5·81	0·172 (0·133–0·222)	82·8% (77·8–86·7)
	Negative during first surge	513 025	15 573	46 739 367	33·32	1 (ref)	..

^*Rate of infection per 100 000 person-days of follow-up.

^†Adjusted for sex, age group, and test frequency, and, for the alternative cohort analysis only, start month of follow-up.

^‡Exposed periods are periods of follow-up time contributed by individuals with previous infection and unexposed periods are contributed by individuals without a previous infection.

^§For the sensitivity analyses exploring 2 months and 4 months of separation between the two surges, surge one was unchanged.