To cite medrxivr in publications use:

  Luke A. McGuinness & Lena Schmidt (2020). medrxivr: Accessing and
  searching medRxiv and bioRxiv preprint data in R. Journal of Open
  Source Software, 5(54), 2651. https://doi.org/10.21105/joss.02651

A BibTeX entry for LaTeX users is

  @Article{,
    title = {medrxivr: Accessing and searching medRxiv and bioRxiv preprint data in R},
    author = {Luke A. McGuinness and Lena Schmidt},
    journal = {Journal of Open Source Software},
    year = {2020},
    volume = {5},
    number = {54},
    pages = {2651},
    doi = {10.21105/joss.02651},
    url = {https://doi.org/10.21105/joss.02651},
  }