Hits all at once. Without natural immunities and a vaccine those at great risk would all face a higher rate of death. In a very short window.
I think you are right, 80% of us would be unaffected but 15% would face a struggle and 5% would be in the hospital. That 20% would seriously strain the system.
80% put on ventilators die anyway, but could we handle all of them at the same time?