When the virus first hit in 1918 it was the usual victims that died-the young and elderly. In the second wave the healthy strong demographic you described began dropping like flies. Recent thought I’ve read indicates it may not have been a mutating virus after all. Debate is ongoing on many aspects of the Spanish Flu pandemic.
Ive spoken to survivors in years past, they all were children at the time, all went through hell, all lost friends and family. One of them was 10 years old at the time, and his parents were told to leave some water and close the door on the bedroom and do not open the door for 5 days. And if he was alive at the end of it they were to be lucky. Obviously he survived it but that’s how it went for him . He said his hair was completely white and it took almost a year for his natural hair color to come back. It really did kill many.
Look up the PBS documentary on youtube.. the survivors stories are shocking.