Table 2.
Main effects of natural (previous infection) and induced (partial or complete vaccination) exposure to SARS-CoV-2 on the relative risk reduction (RRR) of the onset of any and symptomatic infections caused by delta and alpha variants, and corresponding 95% confidence interval
| Controls | Delta cases | Alpha cases | ||||
|---|---|---|---|---|---|---|
| N (%) | N (%) | RRR (95% CI) a | N (%) | RRR (95% CI) a | p value ‡ | |
| Any infection (symptomatic and asymptomatic together) | (N = 4960) | (N = 496) | (N = 496) | |||
| Previous infection | ||||||
| Unlike | 4411 (88.9) | 490 (98.8) | 0% (reference) | 487 (98.2) | 0% (reference) | |
| Ascertained | 549 (11.1) | 6 (1.2) | 90% (76% to 95%) | 9 (1.8) | 85% (70% to 92%) | 0.547 |
| Vaccination | ||||||
| No | 2650 (53.4) | 349 (70.4) | 0% (reference) | 402 (81.1) | 0% (reference) | |
| Partial | 876 (17.7) | 93 (18.8) | 29% (7% to 45%) | 65 (13.1) | 62% (48% to 71%) | 0.001 |
| Complete | 1434 (28.9) | 54 (10.9) | 75% (66% to 82%) | 29 (5.8) | 90% (85% to 94%) | 0.003 |
| Symptomatic (mild, severe, critical and fatal together) infection | (N = 3740) | (N = 374) | (N = 374) | |||
| Previous infection | ||||||
| Unlike | 3332 (89.1) | 371 (99.2) | 0% (reference) | 368 (98.4) | 0% (reference) | |
| Ascertained | 408 (10.9) | 3 (0.8) | 93% (79% to 98%) | 6 (1.6) | 87% (71% to 94%) | 0.410 |
| Vaccination | ||||||
| No | 1996 (53.4) | 269 (71.9) | 0% (reference) | 312 (83.4) | 0% (reference) | |
| Partial | 697 (18.6) | 71 (19.0) | 35% (12% to 51%) | 46 (12.3) | 66% (52% to 76%) | 0.005 |
| Complete | 1047 (28.0) | 34 (9.1) | 81% (71% to 87%) | 16 (4.3) | 93% (88% to 96%) | 0.004 |
a Relative risk reduction (RRR) calculated as 1- adjusted odds ratio. The latter was estimated with conditional logistic regression, adjusted for the number of previous contacts with the Regional Health Service, use of corticosteroids, drugs for chronic pain, oral anticoagulant agents and insulin, and the presence of anaemias, chronic respiratory disease, dyslipidaemia, depression, hypertension, coronary and peripheral vascular disease, hypothyroidism, epilepsy and recurrent seizures, psychosis, diabetes without insulin therapy, malignancies, other diseases of the respiratory system, other diseases of the digestive system, other diseases of the genitourinary system, gout, autoimmune disease, other diseases of the circulatory system, symptoms, signs and ill-defined conditions, diseases of the skin and subcutaneous tissues, arrhythmia, inflammatory bowel diseases, other mental disorders, heart failure, glaucoma and chronic kidney disease
‡ Chi-square testing the null hypothesis of between-variant homogeneity of the odds ratios