The search interface on medRxiv has limited functionality. You have two additional search choices:
Google Advanced Search - Using Google Advanced Search you can specify the URL or domain you want to search, for example medrxiv.org. You can find Google Advanced Search here, and see an example search using Google here. You can also just type in your keywords into Google Search using this syntax paramedic OR ambulance site:https://www.medrxiv.org/
medrxivr - Using medrxivr, a custom built search that allows
* Basic Search - a simple keyword search
* Advanced search using Boolean terms (AND OR NOT)
* Export results as a csv file
* visualise your results as a graph or as a word cloud