If - and it’s still a big if - exposure notifications from the NHS covid app/T&T were a major factor in driving the recent UK case decline, it’s worth considering what might happen next... 1/
It’s plausible that a rapidly growing epidemic + rapid testing availability + exposure notifications (both formal via T&T/app and informal among friends) has led to large numbers of people who would have been involved in transmission instead quarantining. 2/
If exposure risk is concentrated in time (e.g. because of Euro matches), quarantine timing would also have been concentrated in the period immediately after. Which means we can make a testable hypothesis about what might happen next… 3/
If exposure notifications have led to a ‘mini-lockdown’ among younger groups with lots of contacts, with testing identifying cases within these groups, it may create an analogous dynamic to the large scale testing effort in Slovakia last autumn: science.sciencemag.org/content/372/65… 4/
Cases in Slovakia dropped by about 50% then started rising again as people came out of isolation/quarantine. If above is what’s happening in UK (again, big if), then post-Euro final quarantine periods (i.e. effectively mini-lockdowns) will have ended around 21 July... 5/
UK isn’t in exactly same situation because widely available rapid tests will continue to pick up infections (and interrupt transmission) – plus there's host of other factors at play (weather, school holidays, rising vaccine coverage) that will be contributing to dynamics... 6/
But if above is what’s going on, there’s still lot of susceptibility out there - so if adherence to quarantine drops off, could see another rise in cases in same groups very soon. 7/7

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More from @AdamJKucharski

15 Jul
Still see 70% quoted as level of vaccination required for 'herd immunity'. Important to note it's now likely to be much higher. The standard (albeit rough) calculation for herd immunity threshold is (1/E) x (1-1/R) where E is vaccine effectiveness in reducing transmission... 1/
In scenario where R is 6 (plausible for Delta in susceptible populations without any restrictions), and vaccination reduces infection/infectiousness such that onwards transmission reduced by 85%, above calc suggests would need to vaccinate (1-1/6)/0.85 = 98% of population. 2/
If transmission reduction is less than this (which is likely the case for some vaccines against Delta), or R higher, then herd immunity wouldn't be achievable through current vaccines alone. This leads to three possibilities... 3/
Read 6 tweets
12 Jul
One argument put forward for July 19th UK reopening is to bring infections forward to reduce winter wave. To be honest, I’ve always found idea that we could tailor a pandemic to get 'better' sized future waves a bit absurd - whether in spring 2020 or now. A few thoughts... 1/
For me, main issue now is medium term disruption vs medium term epidemic size. Many people now seem OK with R>1 in countries with relatively high vaccination % (at least implicitly, given they aren’t advocating for the strong measures required to guarantee R<1). 2/
Given R>1, much of Europe faces large epidemics likely to end with accumulation of immunity in next few months - much of it from infections. Reopening would accelerate this, but won't be difference between epidemic & no epidemic (unlike, say, reintroducing measures to get R<1) 3/
Read 10 tweets
11 Jul
In discussions of Delta in UK & much of Europe, it's worth remembering that to avoid a large number of future COVID-19 cases at this point, countries would need to dramatically curtail social mixing - otherwise they've still got a rising epidemic, just with a flatter peak. 1/
Look at recent action in Singapore, for example, which has now fully vaccinated 40% of population. Article from May: straitstimes.com/singapore/heal… and last week: straitstimes.com/singapore/heal… 2/
Big difference with UK, of course, is case numbers. Given current case level in UK, if test & trace was suddenly omniscient with full adherence, millions of people would now be in quarantine. In terms of disruptions, it would be somewhat equivalent to a snap ban on gatherings. 3/
Read 6 tweets
4 Jul
I've always found it very unhelpful that 'self-isolation' is used to refer to both isolation and quarantine, but the distinction is now becoming increasingly important... 1/
To recap, isolation is for people who are confirmed to be infected; quarantine is for people who currently seem healthy but may be infected. A stay-at-home order is basically a large, untargeted quarantine (some countries even call it 'community quarantine'). 2/
As vaccines reduce infections/transmission, countries are re-evaluating approaches to disruptive quarantine, whether for travellers or contacts of cases (e.g. in US: cdc.gov/coronavirus/20…). However, we need to be careful about jumbling isolation and quarantine together... 3/
Read 5 tweets
2 Jul
Schools, workplaces, pings from COVID app… Having high UK case numbers over summer will have huge implications for quarantine burden. A few thoughts… 1/ bbc.co.uk/news/business-…
Because vaccines reduce onwards transmission, contacts are becoming less risky on average - which means that for a given value of R, each case will typically have far more contacts than they would have had last year. 2/
Under pre-pandemic contact patterns, a typical case will have 25+ contacts while infectious (thelancet.com/journals/lanin…). That’s a lot of people who could potentially be quarantined per case. 3/
Read 4 tweets
18 Jun
Any discussion of daily testing vs quarantine for contacts of cases in schools needs to address the key epidemiological question: if a child in a school tests positive, what do you do next? 1/
Encouraging ventilation etc. to reduce transmission risk is important, but you still have to decide what to do about a positive result. Do you quarantine their contacts or not? 2/
If you decide to abandon quarantine because you think ventilation etc. has sufficiently reduced risk, then this still means accepting higher transmission risk than if quarantine had remained in place. 3/
Read 8 tweets

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