I’ve been away for a few days, and have come back today with a fresh eye. A couple of observations: 1. What was a handful of hotspots before, now looks like more broadly-based growth 2. Whereas growth last week was led by the u20s, it’s now the 20-30s leading the way. 1/5
To evidence the latter, look at the growth rates by age category last week: 2/5
And now this week: 3/5
Putting both on the same chart (orange is more recent), you can see the shift. 4/5
It’s hard to disentangle the effects of the new variant and “Step 3” on 17th May, but it’s plausible that opening up would create the biggest shifts in the mostly-unvaxxed 20-somethings, and the single-vaxxed 30-50s. Schoolkids were already in school, so less affected. 5/end
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Some thoughts on where we are, in the form of a “zigzag” thread where I offer alternating good and bad news, and end on a question mark. Let’s start with some bad news: 1/11
This morning’s threads from @alexselby1770 and @TWenseleers, as well as last week's PHE data on secondary attack rates , are all pointing towards ~70% higher transmission for the Delta variant (B.1.617.2) vs Alpha (B.1.1.7) 2/11
BUT (good news) my previous analysis showed 55-60% higher R0 wasn’t a disaster, and could be kept under control with a combination of baseline controls and cautious behaviour (eg. continued WFH). A quick model run suggests 70% isn't very different. 3/11
It’s even better than that: it’s brilliant news. By my calcs, Hancock’s statistic (10% of people in hospital have had 2 doses of vaccine) implies the vaccine is having at least a 95% protective effect – and probably more like 98%, or maybe even higher. Let me explain… 1/n
To start with, we have to recognise that for this calculation, not all people are equal – some have more hospitalisation risk than others. So vaxxing those people will have more of an effect. Fortunately, we have targeted our vaccines on those with highest risk. 2/n
By applying some broad weighting factors to different JCVI categories, I can estimate that c. 71% of the (pre-vaccine) hospitalisation risk is in people who have now had a second dose of the vaccine, and a further 22% is in people who have had a first dose. 3/n
When I tweeted my initial reaction to the PHE data release last night, I promised you some model scenarios to help understand the impact of B.1.617.2 on the roadmap, and in particular whether opening up on 21st June still looked possible. 1/
I’ve done some analysis, and I’ll warn you in advance it’s a bit of a mixed bag: at the more optimistic end of assumptions, things look not too bad. But at the more pessimistic end, we’re back to facing a mid-sized exit wave, which calls the timing of Step 4 into question. 2/
Before that, a quick word on assumptions: while the PHE data last night was very helpful, it still leaves quite a lot of questions unanswered, and some of the data is confusing, or appears to contradict things we thought we knew. See for example: 3/
for those of you too distracted by Eurovision to read the PHE's technical briefings, here's a quick summary thread. first, the bad news: 1) as per press reports earlier today, B.1.617.2 appears to have a moderate degree of vaccine 'escape' particularly after 1 dose 1/n
2) looking at secondary attack rates, B.1.617.2 appears to have a transmission gain of around 50-60%; some of this will be due to the vaccine escape, but as I noted earlier, that's not enough to explain 50-60%, so there's probably an uplift in R0 also 2/n
but there's also quite a bit of good news, some obvious and some less so: 1) the vaccine escape is much smaller after 2 doses than after 1 (so once we're fully vaxxed, much less of an issue) 2) there's no sign yet of large numbers of re-infections, suggesting that immunity.. 3/n
having played around with some numbers, I think it is plausible that a moderate immunity escape by B.1.617.2 (as per JBM thread) could account for a ~15-30% apparent gain in transmissibility for the new variant, given the UK's current distribution of immunity.
some workings for those following at home: I'm assuming (rough numbers, allowing for lags) we have about 35% of total population with an effective 2nd dose, and another 25% with an effective 1st dose. and about 30% of the unvaxxed will have a prior infection.
I'm then applying transmission reduction factors of 80% for a first dose, 90% for 2 doses, and 85% for a prior infection. note these factors include the VE vs. infection, with an assumed 40-60% reduction in transmissibility of breakthrough infections (more after 2nd dose).
My summary thoughts on B.1.617.2: 1. The data on increased transmissibility is concerning, but it’s good that a significant vaccine escape looks unlikely 2. My model suggests that we can probably still exit with only a small wave if we’re prepared to extend baseline controls,
… and vaccinate teenagers in July. 3. We need to keep watching the data closely, and be prepared to take further action, but at present I don’t see a compelling case to alter the roadmap.
A long thread follows, with model graphs to illustrate the various scenarios/actions.
Sadly I’m as confused as everyone about the data on transmissibility – but after playing around with some regression analyses, I have no reason to doubt the SAGE conclusion that a ~50% increase in transmission is a “realistic possibility”. 3/ assets.publishing.service.gov.uk/government/upl…