[NhanHo]

I've been trying to understand the number from the Spanish flu and it seems utterly confusing for some reasons.

The consensus seems to be 30% population of the world was infected, with a case-fatality rate of 2.5% (which is another commonly quoted number), it doesn't add up to 5% of the world's population. The case-fatality rate will have to be around 20% for it to kill that many people. This happens even in the same academic paper (for example: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3291398/ )

> An estimated one third of the world's population (or ≈500 million persons) were infected and had clinically apparent illnesses (1,2) during the 1918–1919 influenza pandemic. The disease was exceptionally severe. Case-fatality rates were >2.5%, compared to <0.1% in other influenza pandemics (3,4). Total deaths were estimated at ≈50 million (5–7) and were arguably as high as 100 million (7).

?!

Can anyone more knowledgable explain what is going on with all those numbers?

[russholmes]

They seem to be confusing mortality rate (MR) with case fatality rate (CFR). Mortality rate is deaths per thousand per year applied to the whole population. Despite its name, CFR is the proportion of actual cases who die, and is a probability rather than a rate.

The CFR is estimated at between 10 - 20 %, i.e. a symptomatic member of the population (a case) has a 10 - 20 % chance of dying. 30 % of the global population of 1.8 billion were infected over the several year course of the epidemic, which equates to 600 million cases leading to 60 - 120 million deaths.

MR was >2.5 %, so more than 2.5 % of the global population died per year during the epidemic. With a population of 1.8 billion, this equates to >45 million deaths per year over several years.

[yread]

I think Spanish flu doesn't tell us much about how this will play out - different virus, different connectedness of the world, no air travel, lots of the dead were from secondary bacterial infections and there were no antibiotics back then. Too many things are different

[dharma1]

I think majority of the deaths relating to Spanish flu were actually from secondary infections (bacterial pneumonia) rather than the virus itself