Same question went through my mind this morning (if the swine flu infected so many, how come our health care system didn't collapse even though extreme social distancing measures were not taken?).
Did a little digging, and turns out that not only was a vaccine able to be delivered within a few months, but about 30 percent of ppl (in particular those over the age of 60) already had partial immunity due to prior exposure to similar viruses.
Also, as far as I can tell, the case fatality rate of swine flu is about 0.02 percent, which is an order of magnitude or two lower than what covid-19 appears to be, so hospitals wouldn't have been experiencing the same stress levels.