No virus is 100% lethal.
I am starting to think that the original SARS-COV was the precursor to COVID-19, though and it was TOO lethal - too lethal to spread, in any event.
I thought it was interesting in Tucker Carlson’s interview of Putin, that when the subject of scientific advancements came up, Putin spoke only of the very old-fashioned concept of an engineered super-athlete or soldier. This is much more science fiction than the very real scientific advances in the creation of bio-weapons.
“No virus is 100% lethal.”
Over a long enough time frame, everything is 100% lethal. :-)