on a mild, moderate and severe scale, moderate is still manageable with a moderate expectation of survival but highly symptomatic: high fever, unrelenting diarrhea, vomiting, coughing, mucus, requires supplemental oxygen, muscle swelling, moderate to extreme pain in joints and spine, splitting headaches, breathing is painful, still manageable organ damage, etc. - meds that need constant monitoring of experienced personnel
serious is a lower expectation of survival to life threatening - unabated fever, more than a little organ damage, opportunistic infections, knocked out/intubated, etc.
i’d say that moderate is severe, and that severe is mortal...
but you are some using a clinical definition that while true in that sense, not as a general rule in common knowledge