Table 1.
Main characteristics of 36 included studies in the meta-analysis.
| Studies | Country | Sample size | Study design | Follow-up or investigated timespan | COVID-19 diagnostic approach | Vaccination identification | IVC (%) | Participants | Risk of bias |
| Yang et al[21] | US | 2005 | Retrospective cohort | 2020.3–2020.8 | Laboratory confirmed | Records | 10.7 | COVID-19 patients (>18 years) | Low |
| Wilcox et al[22] | UK | 6921 | Retrospective cohort | 2020.1.1–2020.7.31 | PCR | Records | 37.8 | Confirmed and suspected cases | Low |
| Umasabor-Bubu et al[23] | US | 588 | Retrospective cohort | 2020.3.12–2020.6.30 | PCR | Self-reported | 35.0 | COVID-19 hospitalized patients | Low |
| Taghioff et al[24] | US, UK, Italy, Germany, Israel, and Singapore | 37,377 | Retrospective cohort | Before 2021.1 | Laboratory confirmed | Records | 1.3 | SARS-CoV-2 positive patients | Low |
| Sánchez-García et al[55] | Mexico | 16,879 | Retrospective cohort | 2020.3.17–2020.8.16 | PCR | Records | 17.2 | COVID-19 positive patients (>18 years) | Low |
| Pedote et al[54] | Italy | 3872 | Retrospective cohort | 2020.2–2020.5 | PCR | Records | 28.7 | COVID-19 patients | Low |
| Pastorino et al[53] | Italy | 741 | Prospective cohort | 2020.3.1–2020.6.1 | PCR | Records | 32.4 | COVID-19 patients visited emergency room | Low |
| Paganoti et al[52] | Brazil | 1664 | Retrospective cohort | 2020.2.16–2021.5.1 | PCR or antigen | Records | 24.7 | Pregnant and postpartum women of a childbearing age (10–55 years old) with severe SARS-CoV-2 | Moderate |
| Massoudi and Mohit[51] | Iran | 261 | Case-control | 2020.3.10–2020.4.10 | PCR or pulmonologist-confirmed | Records | 34.5 | HCWs | Low |
| Kristensen et al[50] | Denmark | 35,168 | Prospective cohort | 2020.4.15–2020.5.7, 2020.6.2–2020.6.10, 2020.9.30–2020.10.7 | Serological test | Self-reported | 29.7 | HCWs | Low |
| Kowalska et al[49] | Poland | 5479 | Cross-sectional | 2020.8–2020.10 | Serological test | Self-reported | 16.5 | Residents | Low |
| Kline et al[48] | US | 149 | Retrospective cohort | 2020.3.20–2020.5.10 | Not mention | Records | 65.8 | COVID-19 positive patients | Low |
| Kissling et al[47] | European | 1701 | Test-negative case-control | 2020.3.1–2020.8.31 | PCR | Records or self-reported | 25.3 | Patients with acute respiration infection | Moderate |
| Huang et al[45] | US | 55,667,977 | Cross-sectional | 2020.1.15–2020.6.30 | Not mention | Records | 23.4 | Older adults (≥65 years) | Low |
| Green et al[44] | Israel | 22,563 | Cross-sectional | 2020.2.1–2020.4.30 | PCR | Records | 10.2 | Health maintenance organization members | Low |
| Greco et al[42] | Italy | 952 | Retrospective cohort | 2020.3.15–2020.6.13 | PCR | Records | 40.0 | COVID-19 patients (≥18 years) | Low |
| Fink et al[43] | Brazil | 39,745 | Retrospective cohort | 2020.1.1–2020.6.23 | Clinical COVID-19 diagnosis | Self-reported | 31.2 | Hospitalized COVID-19 cases | Low |
| El-Qutob et al[37] | Spain | 255 | Retrospective cohort | 2020.2.26–2020.5.20 | PCR | Records | 42.0 | Hospitalized COVID-19 cases (≥18 years) | Low |
| Diallo et al[41] | French | 819 | Retrospective cohort | 2020.3.10–2020.3.31 | PCR and/or clinically/radiologically diagnosis | Self-reported | 45.8 | People with diabetes hospitalized for COVID-19 | Moderate |
| Conlon et al[40] | US | 27,201 | Retrospective cohort | 2020.2.27–2020.7.15 | Laboratory confirmed | Records | 47.8 | Hospitalized patients | Low |
| Arce-Salinas et al[39] | Mexico | 560 | Age-matched case-control | From 2020.4 | PCR | Records | 33.4 | Hospitalized COVID-19 cases (≥18 years) | Low |
| Xiang et al[38] | UK | 30,835 | Prospective cohort | 2020.1.31–11.3 | Laboratory confirmed | Records | NA | Record in UK Biobank (40–69 years) | Low |
| Candelli et al[73] | Rome | 602 | Retrospective cohort | 2020.3.1–2020.6.30 | PCR | Not mentioned | 24.9 | COVID-19 patients admission in emergency department | Moderate |
| Scozzari et al[35] | Italy | 8761 | Prospective cohort | 2020.5.4–2020.11.20 | Serological test | Self-reported | 18.0 | Workers at the University Hospital and University | Low |
| Pawlowski et al[74] | US | 25,582 | Retrospective cohort | 2020.2.15–2020.7.14 | PCR | Records | NA | Suspected SARS-CoV-2 infected cases | Low |
| Ilic et al[33] | Serbia | 107 | Retrospective cohort | 2020.3.20–2020.4.22 | PCR | Self-reported | 70.1 | COVID-19 confirmed HCWs | Moderate |
| Fernández-Prada et al[32] | Spain | 188 | Test-negative case-control | 2020.2.28–2020.5.8 | PCR | Not mentioned | 43.1 | Patients in a regional hospital | Low |
| Debisarun et al[75] | Netherlands | 6856 | Cross-sectional | 2020.3–2020.6 | PCR | Records | 53.3 | HCWs | Low |
| Noale et al[30] | Italy | 6680 | Cross-sectional | 2020.4–2020.6 | Not mention | Self-reported | 21.0 | Adults (≥18 years) | Low |
| Ragni et al[29] | Italy | 17,608 | Test-negative retrospective cohort | 2020.2.15–2020.5.22 | PCR | Records | 30.8 | Residents underwent nasal and oropharyngeal swab test | Low |
| Martínez-Baz et al[28] | Spain | 8935 | Prospective cohort | 2020.3.1–2020.5.31 | PCR or serological test | Records | 34.1 | Tested HCWs | Low |
| Olivar-López et al[26] | Mexico | 510 | Cross-sectional | 2020.3–2020.6 | PCR | Not mentioned | 23.3 | Patients under 18 years of age who came for a consultation with a clinical picture compatible with SARS-CoV-2 and who underwent RT-PCR testing | Low |
| King et al[46] | US | 1736 | Test-negative case control | 2020.6.1–2020.9.30 | PCR | Records | 65.1 | Patients with an acute illness | Low |
| Ortiz-Prado et al[25] | Ecuador | 9468 | Retrospective cohort | 2020.2.27–2020.4.18 | PCR | Self-reported | 0.9 | COVID-19 positive patients | Moderate |
| Belingheri et al[27] | Italy | 3520 | Cross-sectional | Before 2020.5 | PCR | Records | 23.2 | HCWs and medical residents | Low |
| Tayar[56] | Qatar | 2576 | Test-negative case-control | 2020.9.17–2020.12.31 | PCR | Records | 28.0 | HCWs | Low |
HCWs: Healthcare workers; IVC: Influenza vaccine coverage; NA: Not available; PCR: Polymerase chain reaction; SARS-CoV-2: Severe acute respiratory syndrome coronavirus 2.