Thread by @JamesWard73

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

https://twitter.com/alexselby1770/status/1400043619738869768?s=20

https://twitter.com/TWenseleers/status/1399989400063426560?s=20

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

https://twitter.com/JamesWard73/status/1396616545745395713?s=20

BUT (bad news) increased transmission of Delta isn’t just due to higher R0, but is probably a combination of that with:
a) some escape of immunity from vaccines & infection
b) other environmental factors.
I’d like b) to be true but can’t rely on it, so will focus on a). 4/11

For the UK, the more immunity escape we have, the worse it is. This is because losing some of the immunity we’ve already built up pushes us back on the road towards herd immunity, AND makes it harder to move forwards (because each vaccination or infection counts for less). 5/11

BUT (good news) it looks from PHE data so far that the escape of immunity is much greater against the 1st dose than it is against the 2nd (and unclear vs. prior infection). This offers us a way out: get a high % of the population to have their second dose of the vaccine. 6/11

BUT (bad news) to get to herd-immunity levels of protection on second doses, with a variant that also has very high R0, means we need to get the 18-40s second-dosed. And (depending on how high R0 is) maybe teenagers as well. 7/11

And because we have limited supply of mRNA vaccines (Pfizer and Moderna), and have decided the 18-40s should take those in preference to AZ (due to side-effect risks), it’s not easy to speed up the second-doses for those groups in the way we’ve done for the over-50s. 8/11

This means we may face a choice: delay Step 4 until (say) September, when we have a much higher % of the population second-dosed. Or go ahead on roughly the current schedule, and risk a significant exit wave over the summer. 9/11

I don’t have enough data yet to offer a clear view on the size of the exit wave; it’s all very assumption-dependent, and I’m hoping the next PHE Technical Briefing will help to solidify the data on the extent of the immunity escape, and hence on the R0 for Delta. 10/11

I don’t expect the new data will make the trade-off decision for us, but it might change the balance of risks. To answer one frequent question: I’ve seen nothing so far that makes me think we might need to reverse Step 3, this is all about the timing & content of Step 4. /end