The H2N2 flu of 1957-58 is 'credited' with causing 33,000 deaths in UK. and around 1m worldwide. The UK sickness benefit payments system was hit for £10m in extra outgoings (goodness knows what that is in 2020 £!) and the Dow lost 15% of its value in late 1957.
The H3N2 flu of 1968-69 killed slightly more (numbers are a bit variable, but I've seen estimates from 30k to 80k in UK, and 1m to 4m worldwide). In UK, it was prevalent over quite a long time - about 15 months - and had a significant impact on younger people. In both outbreaks, it seems that more people were ill to an 'inconvenient' (ie off work) level than in the Covid-19 outbreak. As I say, I remember a significant number of businesses being closed for a while in 1968-69, and a little research shows that there were ad-hoc infirmaries like the Lighthouse hospitals popping up around the place, but that the peak death rates were nothing like what we are currently experiencing.