Thread by @marcburleigh

The Delta variant (B.1.617.2) is upending #coronavirus projections, esp. in the UK, which is by far the country in Europe most affected (91% of cases, vs 0.5% in France where the Alpha (UK) variant is dominant).

This is important because the UK vaccination mix is different

/1

The UK, of course, had a vaccination headstart on its erstwhile EU peers. But its strategy - heavy dependency on AstraZeneca and 2nd doses spaced out by 12 weeks - is under pressure from the Delta variant in ways different to the continent.

/2

Early studies suggest 2 doses of AZ is only 60% effective against Delta, meaning more than one fully vaccinated person in the UK in three is is susceptible to infection.

With Pfizer, there is 88% efficacy (roughly one in 10 susceptible to infection from Delta).

/3

I'm unable to find what proportion of what vaccine is in the UK roll-out. A couple of months ago I saw one report that it was 75% AZ and 25% Pfizer.

Oddly, @ourworldindata (Oxford Un!i) doesn't give that for the UK but does for several other countries

/4

@OurWorldInData In France, for ex. Pfizer accounts for 77% of doses, (with AZ just 13% and Moderna and J&J both less than that, per @ourworldindata and covidtracker.fr/vaccintracker/)

/5

@OurWorldInData For illustration purposes, let's say the UK used only AZ on its 30m fully vaccinated people, and France used only Pfizer on its 15.6m fully vaccinated.

And see what a Delta-only scenario would look like....

/6

@OurWorldInData In the UK, that would equate to just 18m people being immune to the Delta variant.

In France, it would be 13m.

If we look at a 75%-25% AZ-Pfizer split in the UK and a 75%-25% Pfizer-AZ split in France, we get...

/7

@OurWorldInData UK: 20.1m immune to Delta

France: 12.6m

The UK gov keeps putting both types of vaccine together to reassure that "full vaccination" gives 80% effectiveness to Delta. But the breakdown matters.

/8

@OurWorldInData That is probably why we now have the deputy chair of the JCVI suggesting people in UK over 50 vaccinated with AZ should see their 2nd dose gap narrowed to 8 weeks (but Pfizer can still be spaced out up to 12 weeks).

/9

@OurWorldInData Given the UK govt's full-throated backing of AZ, it may explain why we don't see vaccine comparisons against Delta: most Brits have AZ and the govt doesn't want its strategy spoiled by people demanding Pfizer.

/10

@OurWorldInData Of course Delta is almost certainly going to spread in the EU. It's tiny now but climbing. The 6-week lag to the UK's situation means Europeans will likely get in a summer break by the Med first though.

Then we should also start seeing how different vaccines cope.

/11

@OurWorldInData It wouldn't be surprising if the UK also takes a headstart on booster shots, offering 3rd jabs late this year. (Other small studies suggest mix-and-match Pfizer and AZ shots - heterologous prime-boost - could be way more effective.)

/ENDS

@OurWorldInData PS this Lancet study out today (based on data from Scotland) gives a lower efficacy for Pfizer against the Delta variant: 79% (and AZ 60% efficacy)

thelancet.com/journals/lance…