06/04/2021

It's all still quite uncertain, but more evidence has been accumulating that there is a *very rare* link between the Oxford/AstraZeneca vaccine and a combination of blood clots throughout the body + low platelets (the cells involved in clotting, so this seems contradictory and is quite weird).

By rare, I mean something like 1 per 100,000 vaccinated people seem to develop this reaction. That proportion is also quite uncertain; it could be closer to 1 in 20,000 or 1 in 1,000,000. A high proportion of the small number of cases studied have died. This may be in part because the blood thinners sometimes used to treat symptoms such as these could actually make this particularly rare condition worse. So understanding this means that we may be able to save many more people who develop this reaction by treating more appropriately.

Even if this reaction turns out to be caused by the vaccine, the risk of dying of Covid is way, way higher - maybe thousands of times higher for the elderly. And vaccinating people also prevents Covid spread. So giving the Oxford/AstraZeneca vaccine to people (especially old people) definitely saves many lives, and I still think Europe should not have stopped using it while we figured this out. And that we should keep using it in much of the world where better options are not available.

But there are places (New Zealand, Australia, etc) where case loads are so low that it's reasonable to pass on this vaccine and wait for better ones, assuming this risk is real. For young people in other places, the risk of dying from Covid is so low that it's also somewhat reasonable to skip this vaccine in favour of others. But if other vaccines are not available right now and skipping this means they have to wait, it will likely lead to more infection spread and more deaths overall; a tough choice.

^This is all heavily hedged because we still don't know this stuff with much confidence.