It it's going to go horribly wrong, how long after the onset of symptoms?

[HallFloor]

I'm on day 7 and it's been like a heavy cold.Headache the worst bit.

DH started feeling "fluey" yesterday. He isn't in any offical vulnerable category but he's had a torrid year for health issues that make him seem vulnerable iyswim.

How long until I know he'll be Ok?

I'm sure they say by around the 10th day

This says Day 6 on average. But it's dated Aug 20. we may have better knowledge now.