~20-50K deaths in the US according to the CDC in the last few months alone [1] and 95K worldwide [2] targeting up to 650K/yr from the CDC and WHO. And I did answer the question you asked: there's no hard and fast rule but it's fair to say that we should respond to threats based on the threat they pose, and use similar threats as a baseline.

[1] https://www.cdc.gov/flu/about/burden/preliminary-in-season-e...

[2] https://www.cdc.gov/media/releases/2017/p1213-flu-death-esti...

[3] https://www.who.int/mediacentre/news/statements/2017/flu/en/