Australian govt. states that even during Spanish flu, only "about 25 per cent of the world’s population was infected".

This means herd immunity was attained at very low levels.

Why do people keep chanting 60% for COVID-19?

https://www1.health.gov.au/internet/main/publishing.nsf/Content/about-pandemic-history

Confirmation from Epidemics by Geoffrey Marks, William K. Beatty (1976) - extract from google books.

I suspect these surveys missed out the asymptomatic, so the true infection rate was probably higher.

Ordinary flu infects around 5% of the adults in a given year.

A "bad flu" like COVID-19 could perhaps infect 20%, even 30%, but why did Neil Ferguson estimate 80%?!

Sources: https://www.who.int/biologicals/vaccines/influenza/en/

https://bmcinfectdis.biomedcentral.com/articles/10.1186/s12879-014-0670-5

And for H1N1 (2009), the infection rate was 24%.

75% didn't get the virus **no matter how badly they tried** (there were no masks, no lockdowns - and lots of normal behaviour and regular partying).

https://www.cidrap.umn.edu/news-perspective/2013/01/study-puts-global-2009-pandemic-h1n1-infection-rate-24