Table 1 Hazard ratios for the incidence of SARS-CoV-2 reinfection

From: Immune histories and natural infection protection during the omicron era

Epidemiological measure

Exposure cohort

Control cohort

a. Omicron double-infection cohort (Exposure cohort) versus omicron single-infection cohort (Control cohort)a

 Sample size

11,185

11,185

 Total follow-up time (person-weeks)

693,544

698,929

 Incident infections

675

498

 Incidence rate of reinfection (per 10,000 person-weeks; 95% CI)

9.7 (9.0–10.5)

7.1 (6.5–7.8)

 Unadjusted hazard ratio for SARS-CoV-2 reinfection (95% CI)

1.37 (1.22–1.53)

 Adjusted hazard ratio for SARS-CoV-2 reinfection (95% CI)b

1.27(1.13–1.43)

b. Omicron double-infection cohort (Exposure cohort) versus pre-omicron-omicron double-infection cohort (Control cohort)c

 Sample size

3357

3357

 Total follow-up time (person-weeks)

235,240

237,570

 Incident infections

268

185

 Incidence rate of reinfection (per 10,000 person-weeks)

11.4 (10.1–12.8)

7.8 (6.7–9.0)

 Unadjusted hazard ratio for SARS-CoV-2 reinfection (95% CI)

1.47 (1.22–1.78)

 Adjusted hazard ratio for SARS-CoV-2 reinfection (95% CI)d

1.37 1.13–1.65)

c. Pre-omicron-omicron double-infection cohort (Exposure cohort) versus omicron single-infection cohort (Control cohort)e

 Sample size

37,771

37,771

 Total follow-up time (person-weeks)

3,573,409

3,572,390

 Incident infections

2784

2726

 Incidence rate of reinfection (per 10,000 person-weeks)

7.8 (7.5–8.1)

7.6 (7.4–7.9)

 Unadjusted hazard ratio for SARS-CoV-2 reinfection (95% CI)

1.02 (0.97–1.08)

 Adjusted hazard ratio for SARS-CoV-2 reinfection (95% CI)f

0.97 (0.92–1.03)

  1. Hazard ratios comparing the incidence of SARS-CoV-2 reinfection between a the omicron double-infection cohort and the omicron single-infection cohort, b the omicron double-infection cohort and the pre-omicron-omicron double-infection cohort, and c the pre-omicron-omicron double-infection cohort and the omicron single-infection cohort.
  2. CI confidence interval, PCR polymerase chain reaction, SARS-CoV-2 severe acute respiratory syndrome coronavirus 2.
  3. aPersons in the omicron double-infection cohort were matched exactly one-to-one to persons in the omicron single-infection cohort by sex, 10-year age group, nationality, number of coexisting conditions, number of vaccine doses, vaccine type, in addition to testing method (PCR versus rapid-antigen testing), reason for testing, and calendar week of the SARS-CoV-2-positive test defining the reinfection with omicron for the omicron double-infection cohort and defining the primary infection with omicron for the omicron single-infection cohort. Each matched pair was followed from 90 days after the date of the reinfection for the individual in the omicron double-infection cohort.
  4. bAdjusted for sex, 10-year age group, nationality, number of coexisting conditions, number of vaccine doses, vaccine type, testing method, reason for testing, and calendar week of the SARS-CoV-2-positive test defining the reinfection with omicron for the omicron double-infection cohort and defining the primary infection with omicron for the omicron single-infection cohort, and testing rate.
  5. cPersons in the omicron double-infection cohort were matched exactly one-to-one to persons in the pre-omicron-omicron double-infection cohort by sex, 10-year age group, nationality, number of coexisting conditions, number of vaccine doses, vaccine type, in addition to testing method (PCR versus rapid-antigen testing), reason for testing, and calendar week of the SARS-CoV-2-positive test defining the reinfection with omicron for the omicron double-infection cohort and the pre-omicron-omicron double-infection cohort. Each matched pair was followed from 90 days after the date of the reinfection for the individual in the omicron double-infection cohort.
  6. dAdjusted for sex, 10-year age group, nationality, number of coexisting conditions, number of vaccine doses, vaccine type, testing method, reason for testing, and calendar week of the SARS-CoV-2-positive test defining the reinfection with omicron for the omicron double-infection cohort and for the pre-omicron-omicron double-infection cohort, and testing rate.
  7. ePersons in the pre-omicron-omicron double-infection cohort were matched exactly one-to-one to persons in the omicron single-infection cohort by sex, 10-year age group, nationality, number of coexisting conditions, number of vaccine doses, vaccine type, in addition to testing method (PCR versus rapid-antigen testing), reason for testing, and calendar week of the SARS-CoV-2-positive test defining the reinfection with omicron for the pre-omicron-omicron double-infection cohort and defining the primary infection with omicron for the omicron single-infection cohort. Each matched pair was followed from 90 days after the date of the reinfection for the individual in the pre-omicron-omicron double-infection cohort.
  8. fAdjusted for sex, 10-year age group, nationality, number of coexisting conditions, number of vaccine doses, vaccine type, testing method, reason for testing, and calendar week of the SARS-CoV-2-positive test defining the reinfection with omicron for the pre-omicron-omicron double-infection cohort and defining the primary infection with omicron for the omicron single-infection cohort, and testing rate.
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