Some time ago, the NLM launched the NIH Preprint Pilot Project. This is intended to test the searchability of preprints in PubMed Central (PMC) and thus the findability in PubMed. The current status has now been announced in an entry in the Technical Bulletin:

"Ten months into the NIH Preprint Pilot, more than 2,100 preprints reporting NIH-supported research on COVID-19 are now discoverable in PubMed Central (PMC) and PubMed. Through early April 2021, these records have been viewed more than 1 million times in each of these databases (1.4 million in PMC; 1 million in PubMed). Of the preprints included in the pilot, ~60% are currently discoverable only as a preprint version, having not yet been linked to a published article. All articles are clearly identified as preprints. Preprints may be selected or excluded in searches by using the preprint filter."

For more information, see the full news release.