Great Britain

How big are the blood-clot risks of the AZ jab? | David Spiegelhalter and Anthony Masters

Last Wednesday, the European Medicines Agency stated there was a plausible link between the Oxford/AstraZeneca (Vaxzevria) vaccine and rare types of blood clotting, which the MHRA estimates may happen in one in 100,000 young adults who get the vaccine.

It is challenging to think of such low risks: when we have to count the zeros, all intuition goes. So what else has roughly a one in 100,000 chance for a young adult? We could choose from the risk of dying when under general anaesthesia, or in a skydiving jump, or, on the positive side, winning the Lotto jackpot if you bought 450 tickets, or guessing the last five digits of someone’s mobile phone number.

Perhaps more pertinently, it’s roughly the risk of a young woman on the contraceptive pill having some form of blood clot in one week.

But think how you react to these risks: do they seem negligible or important? Risk is as much a matter of feeling as analysis. For the briefing last week, the Winton Centre (which DS chairs) constructed a comparison of benefits and risks in different age bands, balancing avoiding intensive care with Covid-19 against getting one of these specific clots. When there is not much virus circulating, these may be finely balanced in younger people, as they do not tend to get severely ill with Covid-19.

But this analysis leaves out important potential benefits of vaccination, such as preventing other risks from Covid, including blood clotting. Then there’s long Covid – around 12% of people aged 17 to 24 reported symptoms 12 weeks after infection.

Vaccination is also not just for the person who gets jabbed; it looks like it will help prevent symptomless infections and viral spread and so benefit all those they meet.

Life is neither safe nor unsafe: risk is a spectrum. There are acceptable risks, which are so low people do nothing in response. There are risks so high we desist our activities.

The third kind is tolerable risks: where we seek to reap the benefits while mitigating downsides. For most people, when there is virus circulating, the risks of Covid-19 outweigh the minimal risks from the vaccines.

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