https://newsgranularity.com/2020/05/11/covid-19-vs-spanish-flu-a-societal-comparison/
Overall, the Spanish Flu is likely to be deadliest epidemic in the history of world. Estimates are that 1-3% of the world’s population died from the Spanish Flu. So many younger people died in the US in 1918 that the average US life expectancy was reduced by 10 years.
It is not clear why the second wave of the virus was so much more lethal than the first. There is some speculation that there may have been a mild and deadly version of the virus, but this has not been definitively confirmed. In the ‘developed’ world, the mortality rate was generally believed to be about 2%. In other counties, the mortality rate has been estimated to have caused up 14% of a population (Fiji islands) to die.
Eventually, toward the end of 1918 the number of deaths caused by the virus began to decrease. This is believed to be because there were so many people that had already been infected and/or the virus may have mutated again to be less invasive to the lungs. It eventually ‘devolved’ to be part of the seasonal flu. There was never a vaccine developed for the Spanish Flu.