To be confused about Coronavirus Testing

[Ilovemypantry]

I know that the government is being bashed for not carrying out enough testing but I am confused by how helpful it would actually be.
If a front line NHS worker was tested on Monday and it came back negative, they could be infected by Tuesday. I can understand if they have been self isolating after experiencing symptoms as if they have had the virus, it would be safe (as far as we know) for them to go back to work. But if they are just testing all frontline workers, surely they would need to test them all every day?
Am I missing something here or is the testing not as useful as everyone seems to think?

What if you're self-isolating bc someone you live with has a cold, or you have cold symptoms that are so mild that you would normally go to work, anyway, but feel you dare not this time.

Also, because many people are asymptomatic it could help them not to come to work and spread it.

YANBU I think it will be most helpful when they have a test to see if you’re immune. That way we will know we’re able to go to work safely. Will also give a much better idea of who’s already had the infection and be able to guide relaxation of the social distancing rules.

@Pindlesandneedles There are plenty of illnesses where you do not get immunity unless vaccinated, so that may not really work.