Except when interferred with by the White House, CDC recommendations have been consistent about masks, social distancing, etc. Serious medical studies to nail down how the virus operates take a lot longer. Ad hoc, quick ‘studies’ can produce bogus results like the ones supporting hydrochloroquinine. This is still a new virus and the science is still evolving.
S Korea is using extraordinary contact tracing as its main tool to control the virus with good results.
https://www.thelancet.com/journals/landig/article/PIIS2589-7500(20)30251-X/fulltext