Table 1. Characteristics of included studies.
| Study | Study design | Characteristics of included patients. | Country | Study period | Time of follow-up | Predominant SARS-CoV- variants. | Funding |
|---|---|---|---|---|---|---|---|
| Ganatra et al., 2022 [13] | retrospective cohort | Adults aged 18 or older who were vaccinated and subsequently contracted COVID-19 at least 1 month after vaccination and were not hospitalized. | United States | 1 December 2021 to 18 April 2022 | 30 days | Not reported | Not reported |
| Yip et al., 2022 [14] | retrospective cohort | Outpatient patients, regardless of vaccination status, who attended one of the selected clinics | China | 16 February 2022 to 31 March 2022 | 30 days | Omicron | None declared |
| Wai et al., 2023 [17] | retrospective cohort | Hospitalized and non-hospitalized patients aged 60 years or older or with at least one chronic disease with mild to moderate COVID-19. | China | 22 February 2022 to 15 April 2022 | 30 days | Omicron | The Tung’s Foundation, Innovation and Technology Comission of Hong Kong |
| Hedvat et al., 2022 [18] | retrospective cohort | Non-hospitalized adult solid organ transplant recipients with asymptomatic, mild, or moderate COVID-19. | United States | 16 December 2021 to 19 January 2022 | 30 days | Omicron (BA.1) | Not reported |
| Dryden-Peterson et al., 2022 [19] | retrospective cohort | Outpatient patients aged 50 years or older with COVID-19. | United States | 1 January 2022 to 17 July 2022 to | 14 days 28 days |
Omicron (BA.1.1, BA.2, BA.2.12.1 y BA.5) | U.S. National Institutes of Health. |
| Wong et al., 2022 [20] | retrospective cohort | Outpatient patients with mild clinical presentation of COVID-19 and high risk of disease severity. | China | 26 February 2022 to 26 June 2022 | 28 days | Omicron (BA.2.2) | Health and Medical Research Fund |
| Arbel et. al, 2022 [21] | retrospective cohort | Outpatient patients aged 40 years or older with mild clinical presentation of COVID-19 and high risk of disease severity. | Israel | 9 January 2022 to 31 March 2022 | 35 days | Omicron | None declared |
| Schwartz et. al., 2023 [16] | retrospective cohort | Outpatient patients aged 18 years or older with COVID-19. | Canada | 4 April 2022 to 31 August 2022. | 30 days | Omicron | Ontario Ministry of Health (MOH); the Ministry of Long-Term Care (MLTC); Public Health Ontario |
| Aggarwal et al., 2023 [24] | retrospective cohort | All non-hospitalized patients within the Colorado healthcare system with a positive test result for SARS-CoV-2. | United States | 26 March 2022 to 25 August 2022 | 28 days | Omicron (BA.2/BA2.12.1) | U.S. National Institutes of Health |
| Najjar-Debbiny et al., 2022 [22] | retrospective cohort | Patients ≥18 years old with COVID-19 who are not hospitalized and have at least one comorbidity or condition associated with high risk of severe COVID-19. | Israel | 1 January 2022 to 28 February 2022 | 28 days | Omicron (BA.1) | Not reported |
| Qian et al., 2022 [15] | retrospective cohort | Patients ≥18 years old with COVID-19 and a diagnosis of systemic autoimmune rheumatic disease. | United States | 23 January 2022 to 30 May 2022 | 30 days | Omicron | Rheumatology Research Foundation |
| Shah et al., 2022 [23] | retrospective cohort | Patients aged ≥18 years with COVID-19 who are not hospitalized and have at least 1 comorbidity or condition associated with a high risk of severe COVID-19. | United States | 1 April 2022 to 31 August 2022 | 30 days | Omicron | Not reported |
| Bajema et. al., 2022 [25]* | retrospective cohort | Non-hospitalized veteran patients with at least one risk factor, clinical presentation of COVID-19, and high risk of disease severity. | United States | 1 January 2022 to 28 February 2022 | 30 days 31–180 days |
Omicron (B.1.1.529 y BA1.1) | Veterans Health Administration Health Services Research & Development (HSR&D) |
| Lewnard et. al., 2023 [28]* | retrospective cohort | Outpatient patients aged 12 years and older with COVID-19 within Kaiser Permanente, Southern California healthcare system. | United States | 8 April 2022 to 7 October 2022 | 30 days 60 days |
Omicron (BA.2; BA.4 y BA.5) | US Centers for Disease Control & Prevention National Institute for Allergy and Infectious Diseases of the US National Institutes of Health. |
| Zhou et. al., 2022 [29]* | retrospective cohort | Outpatient patients aged 12 years and older with COVID-19 within Optum repository, with >700 hospitals and 7000 clinics from all states in the US. | United States | 22 december 2021 to 8 May 2022 | 15 days 30 days |
Omicron | Pfizer Inc. |
| Patel et al., 2022 [26]* | retrospective cohort | Outpatient patients, aged ≥12 years at study initiation, and diagnosed with COVID-19 | England | 1 december 2021 to 31 May 2022 | 28 days | Omicron (BA.1, BA.2 y BA.5) | GlaxoSmithKline Pharmaceuticals Ltd. |
* preprint study.