Does the AstraZeneca vaccine (AZV) cause rare and dangerous types of blood clot? Probably yes, and AZV is now not recommended for people under 30. Let’s look at the overall picture.
Out of 4.4 million people confirmed to have Covid-19, over 150,000 have died. That’s about 34,000 per million confirmed cases. Confirmed cases are likely to be an underestimate of actual cases but, taking the highest estimate, the fatality rate is still at least 10,000 per million cases or about 1%. In principle, mass vaccination should eventually be able to reduce Covid infection rates, and therefore deaths, to almost zero. However, if the vaccines themselves carry a risk, they may cause some illness and deaths. This is just what has been claimed for AZV, with reports of blood clotting (venous thromboembolism — VTE) and deaths.
Here, it might be useful to look at the problem of inappropriate blood clotting in general. Clots form to prevent us from bleeding to death from cuts but if a clot forms inside a blood vessel (typically a leg vein), bits can break off and lodge in a narrow vein somewhere else, blocking blood flow (mostly VTEs). If in the lungs, this causes pulmonary embolism (PE).
NICE estimates 2,000 cases of VTE per million occur each year, of which about 750 per million are cases of PE (other estimates are available!). The risk is far higher for older and far lower for younger people, 5,000 per million for over-85s versus 100 per million for under-40s.
We would expect some cases of VTE to occur naturally during the time following a vaccination. Let’s say that the relevant time period for the immune response is a month: for 22.6 million people, we might expect one-twelfth of a year’s cases to occur, about 3,600 for the average population but more for the older population that made up the majority of the vaccinated. In fact, yellow card (adverse event) reports for AZV total a bit less than this, at around 3500 for thrombosis-related events (though not all such events will have been reported).
The question of risk has particularly arisen with AZV, where rare cases of abdominal and brain blood clotting have occurred. AZV seems to be linked to the incidence of blood clots in connection with thrombocytopenia (low platelet count in the blood). In particular, AZV is linked to a rare type of clot in the brain, cerebral venous sinus thrombosis (CVST). There have so far been 242 cases of clotting with thrombocytopenia, including 49 deaths, among 22.6 million first doses of AZV (latest government figures on 28 Apr). This gives a risk of about 11 cases (with 2 deaths) per million doses. Of these, 140 were cases of CVST, including 16 deaths. This is about 6 cases (with 0.7 deaths) per million doses. People affected ranged in age from 18 to 79 years, two-thirds women, whereas Covid-19 deaths are skewed towards men and much older groups.
The question now arises of how many cases of CVST might normally be expected. Estimates vary, partly because it’s difficult to diagnose CVST (a brain scan with a specialised form of magnetic resonance imaging is needed), but boil down to around 0.7 per million per month so, on the face of it, the number of cases with AZV at 6 per million doses seems rather higher. In view of this, despite the small number of cases, it makes sense to offer an alternative vaccine to the younger age groups, whose risk of serious illness and death from Covid-19 is very low.
Overall, however, we should remember that Covid-19 brings a risk of death of about 10,000 per million cases, with a greater risk of lengthy hospitalisation and a variety of long-term symptoms (not least PTSD) which can be very debilitating. (The experiences of children’s author Michael Rosen illustrate this well).
We also know that Covid-19 itself comes with a serious risk of blood clots, either from the effects of the virus (causing severe inflammation, triggering the clotting system) or from patients being immobile at home or in hospital, especially in intensive care. Immobility raises the chance of deep vein thrombosis (DVT), which accounts for some 40,000 hospital deaths per year (nearly half preventable by better care such as mobilisation of legs, anticoagulant drugs or wearing compression stockings).
Covid sufferers have had life-threatening clots in legs, lungs, hearts and brain. These have led to heart attacks, strokes, pulmonary embolisms or amputations; if not fatal, these can lead to long-term debility.