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subscription/membership/fee required Elsevier Science https://linkinghub.elsevier.com/retrieve/pii/S0269-7491(21)01999-0
COVID-19 (Coronavirus Disease 2019) free resource MedlinePlus Health Information https://medlineplus.gov/covid19coronavirusdisease2019.html
free resource PubMed Central https://www.ncbi.nlm.nih.gov/pmc/articles/pmid/34743966/
free resource Europe PubMed Central https://europepmc.org/abstract/MED/34743966