Chat about Novel Coronavirus - 2019-nCoV - COVID-19

https://youtu.be/yl78rcVww4E?t=1690

Should link to the right timestamp for the question, but it's 28:10 if that hotlink doesn't work, and goes on until 30:01.

Rough transcription follows:

"
Q: I had COVID at the beginning of November and was told by Test and Trace not to take a lateral flow test for 91 days as the result may not be accurate. So how is it now reliable to use the results of these tests on days 6 & 7 to reduce the isolation period? Does the 91 day period for not using lateral flow tests no longer apply?

Amanda from Cheshire

Johnson: Er, Chris, I'm going to defer to you on this one.

Witty: Thank you Prime Minister. The answer is that the key thing the lateral flow is really good at ... is the fact that it ... is a pretty good guide, a very good guide actually, to whether someone is at that moment infectious. And so the reason that we feel that it is a useful tool to allow on day 6 and day 7 someone has isolated because they know they've got COVID day 5 and a negative test on day 6 and a negative test on day 7 we have confidence they are much less likely to be in any way infectious to other people if they then leave isolation than if they had not done those tests. So that's the reason why adding those tests on allowed ministers to decide to move from 10 days isolation down to 7, but the last two you do the lateral test because they are very predictive of how infectious someone is, obviously if they still positive they do need to stay in isolation 'til it goes negative.

Witty: The PCR tests, which are the other way of testing, they can remain positive for a long time after someone has had an infection, including beyond the point where they are infectious. So the reason that we find the PCRs extremely good because they are very sensitive and they are extremely good because they tell you which type of variant of Covid, it's got many advantages but the lateral flows are really good at helping to determine whether someone is infectious at that point to other people.
"

So, there you go. If you test positive on a lateral flow on either day 6 or day 7 of your isolation you should continue to isolate until you test negative.

Also he seems pretty confident that they are a very good indicator of whether someone is infectious or not at that point in time, and that applies near the end of an infection too.

I'll leave it to others to challenge Prof Sir Chris Whitty for his sources.

And, PERSONAL OPINION ALERT I think it's pretty clear given what he said that he doesn't think that people shouldn't take lateral flow tests within 90 days of originally testing positive, otherwise why would he be advocating the current advice of people taking them on day 6 and day 7 to end isolation "early".

And, again, PERSONAL OPINION ALERT, I think the 90 day thing only ever really was meant to apply to PCR tests but somewhere along the way lateral flows got bundled in there too which just confused things. Further conjecture by me would probably say it was a deliberate and cynical move by the Government to avoid spotlights on the shortages of tests early last year, and the staff shortages (in healthcare, care homes and schools) that would almost certainly have occurred if many of the staff had been actively testing frequently in those long periods where Delta was on the rise. Instead they let it circulate amongst the much less statistically vulnerable.

Also I've no idea why certain individuals continue to test positive on lateral flow tests but negative on PCR tests. But you shouldn't use the fallacy of perfection to throw away the wide sweeping benefits of LFDs and PCRs just because they fail in odd ways for a very small minority.