the most recent pandemic has the casualty of 0.1% total population with astounding number of 7mil. In retrospect: that's 3 died per 3000 population city.
Now we come back to the worst pandemic: black plague dead toll is 5mil and casualty rate is 50%. That's 1 in 2 would die. A family of 4 would lose 2 workhands. Which cause more dead by famine in later years, which lead to more dead by the weaken population.
At 99%, even with natural immunity, there will be no one to maintain the infrastructure, farming, services. It will be a total collapse of humanity as we know it. No but, no if, just total collapse.
Yamane kept developing it because he could, never to wonder if he should.