Links to curated collections from various publishers and organizations:
LitCovid: Curated Literature Hub from NCBI: PubMed references about COVID-19.
World Health Organization coronavirus global research database
Clinical Trials Relating to COVID-19 from ClinicalTrials.gov
National Academies of Sciences, Engineering, and Medicine: Coronavirus Resources: topics relating to COVID-19/SARS-CoV-2.
PubMed Central Coronavirus & COVID-19 Collections: Collection of coronavirus-related publications and associated data deposited by over 50 publishers.