This only works for diseases where symptoms and being contagious are concurrent. In the case of Covid-19 we know that humans can be contagious without experiencing negative symptoms. The potentially fatal symptoms appear later on.
The danger with that is that there is no reason for evolutionary processes to favor a less deadly strain as the disease has a window to spread before it starts damaging you. The evolutionary pressure towards being less deadly only happens because a disease that kills people too fast doesn't have time to spread as far as a less lethal strain.
In the case of Covid the disease could easily evolve the opposite direction: to be 100% fatal after two weeks of non-symptomatic contagion. It would still be able to spread just fine using its initial two week period, so there would be little to no evolutionary pressure toward lower fatality rates.
This only works for diseases where symptoms and being contagious are concurrent. In the case of Covid-19 we know that humans can be contagious without experiencing negative symptoms. The potentially fatal symptoms appear later on.
The danger with that is that there is no reason for evolutionary processes to favor a less deadly strain as the disease has a window to spread before it starts damaging you. The evolutionary pressure towards being less deadly only happens because a disease that kills people too fast doesn't have time to spread as far as a less lethal strain.
In the case of Covid the disease could easily evolve the opposite direction: to be 100% fatal after two weeks of non-symptomatic contagion. It would still be able to spread just fine using its initial two week period, so there would be little to no evolutionary pressure toward lower fatality rates.