The Winton Centre link (“which can be seen here”) is interesting. (Mea culpa, I missed that in the Guardian article, as it looks like that link describes who they are (when in fact it takes you to an article by them) so I skipped it.)
It has limitations because they have to make assumptions to base their calculations on, but I like that it highlights how relative risk (clots vs severe COVID) varies with age (though I can’t see how they can have a good handle on blood clot risks by age cut, based on the relatively small number of reports available. But anyway.)
What I wanted to add was this: you really need to have similar data on severe adverse events for other vaccines in order to make meaningful judgments about how much you should worry about blood clots. For example, lets say the thing you really need to worry about with the Pfizer vaccine is anaphylaxis. If that was really rare in under 30s, rarer than blood clots with AZ (or at least easy to manage if it does occur) then it makes sense to vaccinate under 30s with Pfizer instead of AZ. This sort of data isn’t present in the Winton link, which is really frustrating. As the UK Govt is nudging towards recommending mRNA vaccines for under 30s, I guess this calculation may have been done - but not made public.
As I said upthread, the risk/benefit of a medicine is a relative concept - it depends on the group of patients you’re considering (age under 30s in this case); and the relative risk of alternative treatments within this patient group. It may be that AZ is a relatively bad bet for a 25 year old (because of a better alternative for them), but a much better bet for a 65 year old.
(We would also have to consider the relative risk of anaphylaxis with Pfizer in other age groups; other serious adverse events of the different vaccines within age groups; and, in the current situation of limited supply, the overall value of prioritizing certain vaccines for certain groups. But hey, I was just trying to illustrate a principle here.)