Hi @ScienceShared and @mugevic!

Let's say the reproduction interval is about 4 days. That's the interval between the onset of symptoms in case 1 and the onset in case 2 (on average).
1/10
Patients are most infectious before the onset of symptoms. So, if case 1 develops symptoms, they will probably already have infected at least half of the secondary cases.

And some of them will already be infectious and have infected tertiary cases.
2/10
Few people will get tested on the day of onset of symptoms. Most of the time the onset of symptoms is identified retrospectively. After all, symptoms are mostly common to many other minor viral infections.
3/10
So it's usually at least 24-48 hours before people decide they need to get tested - by which time, most of the secondary cases have been infected, and some will already be transmitting disease to tertiary cases.
4/10
If you then await the test result (as we do in the UK) before telling contacts to self-isolate, you will clearly be shutting the stable door after the horse has bolted.
5/10
I'm not saying that TTIS cannot prevent any transmission. Just that it will not prevent most of it.

And it is enormously resource intensive.
6/10
When we dealt with the first UK citizen in Brighton, there were a huge number of contacts (and secondary cases). Contacting and advising all of them was possible because we only had a single case.
7/10
When case numbers reached double figures, we were overwhelmed, and could not do TTIS rigorously.
8/10
We have recruited many call centre workers to work for so-called-NHS test and trace; but even so, case numbers would have to be an order of magnitude lower for rigorous TTIS to be possible.
9/10
And, as I have explained, TTIS is unlikely ever to be able to prevent most of the transmission.
10/10

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More from @petermbenglish

4 Dec
@sweetanimo @patricia_stoop @abcdiagnosis I have seen some of your conversation on Twitter about the use of Covid-19 vaccines in people who are immune-suppressed, and thought I might be able to help.
1/11
First, the UK guidance, including on contraindications etc., is conveniently all in one place - in the public domain - in the "Green Book", via gov.uk/government/pub…
2/11
But first, let's consider how and why immunosuppression might be an issues with a vaccine.
3/11
Read 12 tweets
3 Dec
Thisk, on lateral flow testing "false positives" is from DH guidance, and strikes me as extremely stupid. assets.publishing.service.gov.uk/government/upl…
@deeksj @jackiecassell

1/4 Image
It says the test is "99.6% specificity (meaning four false positives per thousand tests)". (It doesn't mean that actually: it's four false positives per thousand tests that should have been positive - not the same thing at all. Confusing sensitivity and predictive values.)
2/4
But that isn't the stupidest thing.

It then says you can use a test with sensitivity of <70% (in the real world >30% of true positive cases give a false negative result) as a confirmatory test, and cease isolation if the second test is negative.
3/4
Read 4 tweets
30 Nov
We keep banging on about "test and trace" for #Covid-19.

But it's pointless.

With a reproduction interval of only 4-6 days, if you wait for a test result before isolating contacts, you will do it too late. They will already have spread the infection.
1/2
And many cases are asymptomatic, so you'll miss them, anyway.

Testing is too unreliable. Too many false negatives, and too short a time from infection to infectiousness.

The only thing which works is keeping people apart so they can't transmit it.
2/2
"given the
potential delays in isolation of patients, even the early
detection and isolation strategy might not be fully effective
in containing SARS-CoV-2."

thelancet.com/journals/lanmi…

@mugecevik et al
Read 4 tweets
30 Nov
I seem to have been saying this internally within @PHE_uk and predecessor organisations forever.

Every time there's a major event and they ask for feedback, we tell them "Email does not work. It gets /we get overwhelmed. We can't keep up and feel guilty. It's stressful."
1/6
We frequently get the same message once from the original sender, and then four more times from people concerned we might have missed it; and then we have to try to work out if we're looking at an updated version...
2/6
And sometimes the delays mean that we receive older versions after newer ones... It is a disaster!
3/6
Read 6 tweets
30 Nov
Hi @ajpollard1 @OxfordVacGroup !

What does "ChAdOx1 vector is modified to be replication deficient" mean?

Can it induce production of antigen proteins if it doesn't replicate at all?

Does the lack of replication reduce the immune response to the vector?
1/3
Are ChAdOx1 vector vaccines contraindicated where live virus vaccines are contraindicated (and why [not]?)

Where can we find a New Scientist level explanation of these things?
2/3
If it is "replication deficient", how do you produce it? Can you culture it?

Parenthetic thought:
Are viruses ever "live", and where does this vector vaccine fit? Is it a Schrödinger's virus, alive or dead, depending on your reason for asking?
3/3
Read 4 tweets
27 Nov
Editor of @bmj_latest @KamranAbbasi on Facebook (& the Danish mask study):

"A disagreement among experts, especially about interpretation of a study, is a common occurrence. It is the usual business of science. Only, Facebook didn’t see it that way…
bmj.com/content/371/bm…
1/7
"The social media platform that allows statements about injecting bleach to prevent covid-19, [and] calls to behead the leading US expert on pandemics, decreed… Heneghan & Jefferson should be censured for misinformation after they reposted their Spectator article on [FB]…
2/7
"It is possible to disagree with Heneghan and Jefferson about the robustness and interpretation of the DANMASK-19 trial—which I do—and still believe it is wrong that their opinion of it was marked as “false information.”…
3/7
Read 7 tweets

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