UK: Why impose quarantine after a negative test result?

Those arriving in the UK must stay in a quarantine hotel for at least 10 days. But they are given a coronavirus test on the second day of their quarantine. Why is a negative test result not enough to release them?

The UK government website says:

“You will not be allowed to shorten your quarantine period if you receive a negative test result, as you may still develop COVID-19.”

I thought a negative test result was a definitive finding that, at that moment, the person did not have the coronavirus. Isn’t it?

(I’m not arriving in the UK anytime soon – I just want to understand.)

You may have contracted the virus and have low enough levels that it doesn’t show on the test. It takes time for the virus to multiply to detectable levels.

I know that New Brunswick, Canada was finding that people who were entering the province and quarantining were testing negative on day two but positive on day 5. It was enough that they updated the procedures for essential workers coming back to the province a few months ago.

…nope. And IMHO the 10 day quarantine period isn’t long enough. (In NZ quarantine is for 14 days to match the typical incubation period of the virus)

From the NZ guidance:

For those that are interested here is the Managed Isolation Welcome Packs that are given to all new arrivals. (Warning PDF)

NZ tests everyone in Managed Isolation at least twice (and for people coming from high risk countries three times). They test on Day Zero, Day Three and Day Twelve. People often have a negative test on Day Zero and Day Three but test positive on Day Twelve.

So a negative test isn’t a definitive finding that you don’t have covid. And the type of test and even who is doing the testing matters. This story highlights this.

TLDR: a group of really smart people gathered for a conference on AI, Robotics, etc. They attempted to set up an “immunity bubble”. They tested everyone 72 hours before the event and had only one person test positive. They tested everyone on arrival and then every morning throughout the production. All members got tested five times over four days. They didn’t require everyone to wear masks.

At the end of the production a staff member tested positive. Eventually 12 out of 30 members contracted Covid, 4 our of 9 faculty contracted covid, 5 out of 10 staff contracted covid, but none of the 35 audio visual team (who all wore masks throughout the production) tested positive.

The author of the article who contracted Covid-19 (he tested positive using the PCR test) also tested himself with the Rapid PCR and the Rapid Antigen twice per day over several days. None were positive.

The author seems to come to the conclusion that testing didn’t really work. I think he misses the point. Even after all of this he still doesn’t understand where they went wrong.