Still a bit confused about self isolation rules

[SourAppleChew]

Well, I'm not confused as such. I just don't understand the logic.

Was meant to meet up with a mate yesterday who's been living abroad. They called me last night to say their partner's family who they met a few days ago have tested positive.

The NHS advice says that you still need to isolate for ten days even with a negative result as symptoms can show later, but that people you live with can stop self isolating if they show no symptoms.

What I don't understand is that surely there's a risk of the people you live with spreading it if they go about their business and you then start showing symptoms the next day (i.e. had it but was in the incubation stage so wasn't yet showing).

This has probably been done to death but it's a first for me. My friend thinks they can resume business as usual if they get a negative result.

There is a very small risk, but the benefits of asking those people to isolate don't outweigh the costs to society as a whole - you'd end up with lots more people isolating and it would have major issues on healthcare, education etc.

The risk is very small because say person A is identified as a close contact and needs to isolate. They live with person B who does not need to isolate. A is incubating the virus and does not know, they become contagious a couple of days before showing symptoms and give it to B. A couple of days later A develops symptoms so B isolates but was still incubating the virus but not yet contagious as the incubation period is quite long. B goes onto get symptoms 3-4 days later whilst already isolating and so they have no close contacts of their own.

Thing is, if you think that 600,000 school children were all off school as close contacts last week (Which doesn’t include adults or early years children who were isolating), if all of their families were also isolating, That would be at least 2 million people isolating, not attending education and not going or work. It would be like lockdown again, society wouldn’t be able to function. We’ve got schools closed Round here because there aren’t enough staff to safely open them, the nhs is apparently under huge pressure because of the amount of staff isolating and I’m sure many other employers are reporting similar issues.

What I don't understand is that surely there's a risk of the people you live with spreading it if they go about their business and you then start showing symptoms the next day (i.e. had it but was in the incubation stage so wasn't yet showing).
Not that many people are asymptomatic, which is what you seem to be assuming.
When you get symptoms, that is when your household isolate, because then you might infect them and they might become contagious 3-14 days after exposure, usually 5 days.

A plausible model to work it through is that you get advised of contact on day 3 of exposure, then infectious on day 4 and symptoms on day 5. Your household isolate from day 5, but they don't get infectious until day 9 and don't get symptoms until day 10. They were isolating until day 15 but restart the clock to day 20 due to now having symptoms.