Historically, preprints have not been included in major indexes of biomedical literature. However, the COVID-19 pandemic, as well as the increasing acceptance of preprints by funding agencies as evidence of research output, have prompted some producers of journal indexes to include preprints in their databases.
You can now find preprints that report on the SARS-CoV-2 virus and COVID-19 in PubMed and PubMed Central. This pilot project, which began in 2020, includes preprints which NIH support identified in the iSearch COVID-19 Portfolio tool developed by the NIH Office of Portfolio Analysis. This tool includes content from the preprint servers with the highest volume of papers relating to the SARS-CoV-2 virus and COVID-19. These include medRxiv, bioRxiv, ChemRxiv, arXiv, Research Square, and SSRN. Only preprints that have NIH-affiliated authors or acknowledge NIH support in the preprint are included. Learn more about the NIH Preprint Pilot Project .
(Image Source: https://icite.od.nih.gov/covid19/search/ )
Europe PMC, a member of the PubMed Central International Network, also includes a growing international preprint library from a variety of preprint servers. Learn more about Europe PMC .