yep, just going to the sauna already kills all the viruses and toxins in your body. i mean in the end we don't know whats the deal behind this virus. mayb its men made in a lab or someshit that cant be cured.
Also, only Ultraviolet sun light can destroy that kind of virus and there are many factors that affect the level of ultraviolet radiation(time of the day, geographical position, sea level etc).
here is good picture from WHO that explains it: prnt.sc/r8ifpi
Hence, it'll be still dangerous in many parts of the world where UV radiation won't be high in most of the days. That is why there are many epidemics in China in summer.