if you're using PHE's latest analysis of Secondary Attack Rates to work out an advantage for Delta vs. Alpha, it's worth looking at table 7, as well as table 5. while table 5 suggests the advantage has fallen to +35%, table 7 suggests it's probably still 40-45%. why is this?
https://twitter.com/kallmemeg/status/1408348154039353346
table 7 splits out the data into household contacts (which imply an advantage for delta of 40%) and non-household contacts (advantage 43%). If you're wondering how the combined (household and non-household) figure can show an advantage of only 35%, it's because delta has... 
...a higher proportion of non-household contacts, and these tend to have lower SARs. the most obvious explanation for this is that delta's cases have a lower average age than alpha's, and hence are more sociable, with more non-household contacts.
So I think the stratified data (in table 7) probably gives us a more reliable view on delta's transmission advantage, and I wouldn't place too much reliance on the overall benefit (in table 5) appearing to fall to 35%. I'll continue to use 40-45% as a central view in my models.
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