COVID is a terrible bioweapon. It's fragile, with an outer lipid membrane that has to be preserved. It's not very lethal to people of military age, and spreads readily between people making collateral damage inevitable.
One of the best bioweapons is Tularemia, and part of that reason is its general lack of lethality. It temporarily renders the exposed population incapable of putting up a good fight, but it kills relatively few. Escape isn't that big a deal (because again, it's not very lethal), and it's easy enough to inoculate one's own troops against the particular flavor being used.
