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Adopting a population-based approach, we conducted a systematic review to explore: (1) the etiological role of multimorbidity and frailty in developing SARS-CoV-2 infection and COVID-19-related short-term outcomes; and (2) the prognostic role of multimorbidity and frailty in developing short- and long-term outcomes. This review presents the state of the evidence in the early years of the pandemic. It was conducted within the European Union Horizon 2020 program (No: 101018317); Prospero registration: CRD42021249444.<\/jats:p>\n              <\/jats:sec><jats:sec>\n                <jats:title>Methods<\/jats:title>\n                <jats:p>PubMed, Embase, World Health Organisation COVID-19 Global literature on coronavirus disease, and PsycINFO were searched between January 2020 and 7 April 2021 for multimorbidity and 1 February 2022 for frailty. Quantitative peer-reviewed studies published in English with population-representative samples and validated multimorbidity and frailty tools were considered.<\/jats:p>\n              <\/jats:sec><jats:sec>\n                <jats:title>Results<\/jats:title>\n                <jats:p>Overall, 9,701 records were screened by title\/abstract and 267 with full text. Finally, 14 studies were retained for multimorbidity (etiological role, <jats:italic>n<\/jats:italic>\u2009=\u20092; prognostic, <jats:italic>n<\/jats:italic>\u2009=\u200913) and 5 for frailty (etiological role, <jats:italic>n<\/jats:italic>\u2009=\u20092; prognostic, <jats:italic>n<\/jats:italic>\u2009=\u20094). Only short-term outcomes, mainly mortality, were identified. An elevated likelihood of poorer outcomes was associated with an increasing number of diseases, a higher Charlson Comorbidity Index, different disease combinations, and an increasing frailty level.<\/jats:p>\n              <\/jats:sec><jats:sec>\n                <jats:title>Discussion<\/jats:title>\n                <jats:p>Future studies, which include the effects of recent virus variants, repeated exposure and vaccination, will be useful for comparing the possible evolution of the associations observed in the earlier waves.<\/jats:p>\n              <\/jats:sec>","DOI":"10.1007\/s40520-023-02685-4","type":"journal-article","created":{"date-parts":[[2024,2,14]],"date-time":"2024-02-14T10:02:01Z","timestamp":1707904921000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":6,"title":["Multimorbidity and frailty are associated with poorer SARS-CoV-2-related outcomes: systematic review of population-based studies"],"prefix":"10.1007","volume":"36","author":[{"given":"Tatjana T.","family":"Makovski","sequence":"first","affiliation":[]},{"given":"Jinane","family":"Ghattas","sequence":"additional","affiliation":[]},{"given":"St\u00e9phanie","family":"Monnier-Besnard","sequence":"additional","affiliation":[]},{"given":"Lisa","family":"Cavillot","sequence":"additional","affiliation":[]},{"given":"Monika","family":"Ambro\u017eov\u00e1","sequence":"additional","affiliation":[]},{"given":"Barbora","family":"Va\u0161inov\u00e1","sequence":"additional","affiliation":[]},{"given":"Rodrigo","family":"Feteira-Santos","sequence":"additional","affiliation":[]},{"given":"Peter","family":"Bezzegh","sequence":"additional","affiliation":[]},{"given":"Felipe Ponce","family":"Bollmann","sequence":"additional","affiliation":[]},{"given":"James","family":"Cottam","sequence":"additional","affiliation":[]},{"given":"Romana","family":"Haneef","sequence":"additional","affiliation":[]},{"given":"Brecht","family":"Devleesschauwer","sequence":"additional","affiliation":[]},{"given":"Niko","family":"Speybroeck","sequence":"additional","affiliation":[]},{"given":"Paulo Jorge","family":"Nogueira","sequence":"additional","affiliation":[]},{"given":"Maria Jo\u00e3o","family":"Forjaz","sequence":"additional","affiliation":[]},{"given":"Jo\u00ebl","family":"Coste","sequence":"additional","affiliation":[]},{"given":"Laure","family":"Carcaillon-Bentata","sequence":"additional","affiliation":[]}],"member":"297","published-online":{"date-parts":[[2024,2,14]]},"reference":[{"key":"2685_CR1","doi-asserted-by":"publisher","first-page":"e290921196908","DOI":"10.2174\/1871526521666210929121705","volume":"22","author":"E Mehraeen","year":"2022","unstructured":"Mehraeen E, Oliaei S, SeyedAlinaghi S et al (2022) COVID-19 in Pediatrics: a systematic review of current knowledge and practice. 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