Table 1.
Characteristics of included studies
| First author (year), country | Study design | Sample | Total number of participants | Incidence/frequency of COVID‐19 | NOS | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Unvaccinated | ≥14 days after dose 1 | Adjusted estimate | Unvaccinated | ≥21 days after dose 1 | Adjusted estimate | Unvaccinated | ≥7 days after dose 2 | Adjusted estimate | Unvaccinated | ≥14 days after dose 2 | Adjusted estimate | ||||||
| Andrews et al. (2021), England a | Retrospective, test‐negative, case‐control | Individuals aged ≥16 years who had reported symptoms and were tested for SARS‐CoV‐2 within 10 days after symptom onset in England | 5 233 372 | – | – | – | 337 142/806 829 (41.8%) | 21 535/57 509 (37.4%) | OR = 0.34 (0.33–0.35) | – | – | – | 337 142/806 829 (41.8%) | 24 328/124 934 (19.5%) | OR = 0.05 (0.04–0.06) | 8 | |
| Nasreen et al. (2021), Canada b | Retrospective, test‐negative, case‐control | Community‐dwelling Ontarians aged ≥16 years who had symptoms consistent with or a severe outcome attributable to COVID‐19, and who were tested for SARS‐CoV‐2 | 352 531 | – | – | OR = 0.30 (0.24–0.36) | – | – | OR = 0.31 (0.25–0.38) | – | – | OR = 0.05 (0.03–0.09) | – | – | OR = 0.06 (0.03–0.10) | 8 | |
| Tang et al. (2021), Qatar c | Retrospective, test‐negative, case‐control | Resident population of Qatar | 3660 | n = 1629/3218 (50.6%) | n = 11/62 (17.7%) | OR = 0.20 (0.11–0.39) | – | – | – | – | – | – | n = 1644/3175 (34.5%) | n = 22/157 (14.0%) | OR = 0.14 (0.09–0.22) | 8 | |
| Skowronski et al. (2021), Canada d | Retrospective, test‐negative, case‐control | Adults aged 50–69 years in British Columbia, Canada | 68 074 | – | – | – | n = 50/12 244 (0.4%) | n = 2/1322 (0.2%) | OR = 0.27 (0.06–1.14) | – | – | – | – | – | – | 8 | |
| Bruxvoort et al. (2021), USA e | Retrospective, test‐negative, case‐control | Individuals aged 18 years who were tested for SARS‐CoV‐2 in Kaiser Permanente Southern California | 12 162 | – | – | – | – | – | – | – | – | – | 1795/7342 (24.4%) | 232/4820 (20.8%) | OR = 0.13 (0.11–0.16) | 8 | |
Abbreviations: NOS, Newcastle‐Ottawa scale; OR, odds ratio; PCR, polymerase chain reaction.
Adjusted for age, sex, index of multiple deprivations, ethnic group, care home residence status, geographic region, period (calendar week), health and social care worker status, clinical risk group, clinically extremely vulnerable group.
Adjusted for age, sex, public health unit region, period of test, number of SARS‐CoV‐2 tests in the 3 months before December 14, 2020, presence of any comorbidity that increases the risk of severe COVID‐19, receipt of 2019/2020 and/or 2020/2021 influenza vaccination, Census dissemination area‐level quintiles of household income, proportion of persons employed as nonhealth essential workers, persons per dwelling, proportion of self‐identified visible minorities.
Adjusted for age, sex, nationality, reason for PCR testing, and calendar week of COVID‐19 test.
Adjusted for age, sex, epidemiological week, location of health authority.
Adjusted for smoking status, Charlson comorbidity score, frailty index, liver disease, pregnancy, history of COVID‐19 diagnosis, number of outpatient and virtual visits, preventive care, medical center area, month of specimen collection, specimen type.