[Jefro118]

I believe one difference with Covid is that people often have issues across multiple organs (I believe this is because the virus binds to ACE2 receptors which are present over the body, not an expert though) whereas with flu/pnemuonia I think it's more likely to be just generic fatigue and lung capacity as longer term effects.

I think the sheer range of long lasting symptoms is quite unique to Covid, although other diseases like Ebola or Smallpox would leave more severe damage.