Azithromycin also helps the body's cells absorb zinc, and zinc interferes with viral replication.
I don't know if doxycyclin does this as well, but the proof that it works, regardless of mechanism, is the survivors. One could argue that the "control group" is the other area nursing homes that didn't have the same high survival rates.
azithro is being used because it is thought to have anti inflammatory properties in the lungs