[tinmandespot]

If we proceed on the assumption that exposure to higher viral load correlates to more severe symptoms and fatality, I think the trend can be attributed to the drop in viral load averaged over those being infected.

1. At the start of the pandemic, the older and more vulnerable were affected disproportionately. These patients had a high viral load.

2. Patients with more severe symptoms are now isolating at home or at hospitals. Previously they may have been contributing to serious super-spreader like situations.

3. PPE/Mask usage is more common now and these are possibly protecting most people against high exposure

4. Antivirals could be reducing the length of stay at hospitals and protecting workers from higher exposure

In other words, the virus is spreading but in a less severe form. However, I don't think fundamentally different (less virulent) mutations are the contributing to the lower death rate.