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22 May, 8 tweets, 2 min read
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).
the net result of all that is we have about 65% effective transmission reduction (vs. B117) today, or about 35% effective susceptibles.
now with B.1.617.2, I'm assuming the transmission reduction factors fall to 65-70% (from 80%) for 1st dose, stay at 85-90% (from 90%) for 2nd dose, and fall slightly to 75-80% (from 85%) for prior infection. these falls may look small, but it's based on an assumption of a ...
... larger reduction in VE vs. infection, but with little change in the reduced transmissibility of breakthrough infections, which therefore acts to mute the effects of the vaccine escape on transmission.
now applying those reduced transmission reduction factors to the (unchanged) numbers of people with 1 dose / 2 dose / infection, implies we would have 55-60% total transmission reduction (down from 65%). this means the number of susceptibles is up from 35% to 40-45%.
and so the apparent gain in transmission would be a ratio of +15-30% (=+5/35 to +10/35). @DevanSinha @andrew_lilico (& others) welcome critique on this.

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More from @JamesWard73

James Ward Profile picture
23 May
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/
Read 31 tweets
James Ward Profile picture
22 May
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
Read 7 tweets
James Ward Profile picture
16 May
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…
Read 42 tweets
James Ward Profile picture
9 May
HI thread Mark II: after I tweeted last week about Herd Immunity (HI), quite a lot of people got in touch to point out that my model was either over-estimating or under-estimating the HI threshold, because it treated people as all being the same. 1/n
And the thing is, they’re right, in both directions. Differences between groups of people, and between individuals within those groups (so-called “heterogeneity”), make it likely that I’m over-estimating the effect of vaccines, and under-estimating the effect of infections. 2/n
Let’s see if we can correct for both of these effects, or at least estimate how wrong I might be. We’ll start with the age group effect on vaccines, because that’s easier to sort out. The problem here, as @AdamJKucharski noted in his reply, 3/n
Read 33 tweets
James Ward Profile picture
1 May
A thread on herd immunity. In summary:
- We’re not there yet
- Full herd immunity will probably arrive in late June, once we’ve vaxed all 18+
- From early June we’ll likely be in a partial-HI state where R will stay <1 with just baseline controls and cautious behaviour. 1/n
There’s been a lot of wild claims about herd immunity in recent weeks, varying from “we’re already there” (fil.ion.ucl.ac.uk/spm/covid-19/f…) to “it’s impossible” (nature.com/articles/d4158…). Both are demonstrably wrong, and new data this week (from ONS and PHE) makes this clear. 2/n
First, the ONS-based analysis (ox.ac.uk/news/2021-04-2… ) showed us that a single dose of either AZ or Pfizer reduces covid infections by 65%. (and after 2 doses by 70%, although that’s probably an underestimate). 3/n
Read 22 tweets
James Ward Profile picture
16 Apr
Reflecting on this week’s “lockdown vs. vaccines” debate, while the binary logic of “it's one or the other – which is it?” is clearly wrong, my initial reaction of “it’s a bit of both, the balance changes over time” was also slightly too reductive. 1/
Surely the bigger strategic point is this: to get to the relatively good place we’re in now, we needed both lockdown AND vaccines. Essentially, lockdown kept the virus pinned down until the vaccines were ready to come and knock it on the head. 2/
This wouldn’t be a proper thread without some numbers from the model to illustrate it, so here goes. First, let’s start with a world where there is no lockdown, and no vaccine. The tricky bit here is what to do instead of lockdown: I think the most realistic alternative… 3/
Read 12 tweets

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