I curate a massive list of USA-based ArcGIS server addresses. I just posted an update that includes a section listing the public-facing FeatureServer layers I know about that meet the following two criteria.
1. COVID-19 related
2. Cover the USA
If you know of any MapServer or FeatureServer or WMS layers that meet *both* of the above criteria, please let me know and I will add them to the list. I will include a note identifying the source of the data or the party doing the data collection.
The COVID-19 layer list starts at page 7 of this PDF file:
https://mappingsupport.com/p/
I included a link to the GISsurfer map I produce that can display these layers.
Joseph Elfelt
Redmond, WA
Twitter: @mappingsupport
https://mappingsupport.com
(posted on the GISCO listserve 3/31/2020)