COVID-19 “might be” about three times more deadly than the 1918 “Spanish Flu”:

I say “might be” because with COVID-19 there may be large numbers with mild symptoms who are being left out of the infected count.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5911181/

Sample quote

The 1918–1921 influenza pandemic killed an estimated 0.8% of the Arizona population in three closely spaced consecutive waves. The mortality impact of the fall 1918 wave in Arizona lies in the upper range of previous estimates reported for other US settings and Europe, with a telltale age distribution of deaths concentrated among young adults.

End quote

My grandfather lived through the 1918 Flu as a WW-I soldier in training at training Camp Zachary Taylor in Louisville KY where several hundred of his fellow soldiers died.

About fifty yards away from the house I grew up in was a cemetery for a family that had previously owned the land. I started mowing that cemetery’s grass when I was about 9 years old. I noticed that half the graves were from the fall of 1918. My grandfather later explained to me about the 1918 Flu.

In 1976 when I was in college there was the Swine Flu danger when one odd H1N1 flu strain infected US soldier in training died after a 5 mile march. My grandfather long distance telephoned me over and over asking if I had gotten the vaccine yet. I was one of 48 million who got the Swine Flu shot and had zero bad reactions. Of the 48,000,000 vaccinated USA citizens a bit over 4000 filed a adverse reaction damage claim with the Federal government. Of this 4000 only 25 were for actual deaths.