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
| Â | Controls | Delta cases | Alpha cases | Â | ||
|---|---|---|---|---|---|---|
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) | Â | ||
 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) | Â | ||
 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 |