“The World Health Organization estimates that 2–3% of those who were infected died (case-fatality ratio).
[...]
In Iran, the mortality was very high: according to an estimate, between 902,400 and 2,431,000, or 8% to 22% of the total population died.” — source, Wikipedia, see [0].
I wonder if it’s genetics (immune system reacts differently?), or cultural (habits - kissing on cheeks, handshaking, large religious or non-religious gatherings) or climate, or a mix.
References: [0]: https://en.wikipedia.org/wiki/Spanish_flu
Edit: admittedly, it’s too early to draw conclusions. It’ll take a few weeks to have more realistic numbers.