The reports of cases of life-threatening blood clots in blood vessels alongside low numbers of blood platelets (small blood cells that assist blood clots to form) in very small numbers of people among millions who have received the AZ COVID-19 vaccine are again causing concern and have led to a decision to stop immunising younger adults with the vaccine in Germany similar to action taken in France last week.

It remains uncertain whether the vaccine caused these cases and the mechanism by which these blood clotting abnormalities come about and why they affect this very small proportion of individuals has still not been properly worked out.

What seems clear is that the risks to individuals in age groups currently being targeted for vaccination in the UK of death and of dangerous blood clots are greatly reduced by receiving this vaccine, because it reliably prevents severe COVID-19 which definitely causes many deaths and blood clots and which poses a much larger and clearer risk than any possible rare vaccine side effects that may exist.

There are other vaccines that we use routinely which cause rare, severe side effects. In using them we accept that the benefits greatly outweigh the risks. We need to learn to think about all vaccines and medicines in this way. We would all prefer to have drugs that are 100% safe but they don’t exist. Right now the biggest risk to our lives and livelihoods in this country and throughout the world is COVID-19. We need to stay focussed on the need to prevent it taking millions more human lives before it is brought under control and the only effective way to do that is through vaccination. We need to be able to use every dose of the vaccines that we currently have available in order to achieve that urgent goal.

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The way to tell if cases are caused by vaccination is to look to see if there is an excess of cases in people who have been vaccinated.

If you look for excesses of cases in a wide range of rare conditions, you will find some, due to the play of chance. The cases are uncommon enough that the number of excess cases will be small, which can easily happen by chance; and if you look at multiple conditions, then of course one or more of them will have an excess, just by chance.

There is no evidence that these cases are caused by vaccination, rather than simply associated with it, in people who would have had such conditions anyway. And it is hard to postulate a biological reason mechanism which could explain how a clotting disorder in which clots form in a particular part of the body could be more common, when clotting, in general, is not more common.

Of course, we need to investigate this to be sure there is no causal link; and this will require prospective data linkage. (It is worth noting that the recently reported AstraZeneca data looked specifically at this, and found no excess of cases).

Vaccination is being undertaken as fast as possible; and the bottleneck – at least in the UK –  is the availability of vaccines, and I think this is also true in Germany. This means that if the use of vaccines that are available are withheld, people will go unvaccinated, or be vaccinated later, leaving them at risk of disease for longer.

Right now, in the middle of a pandemic, with case numbers rising alarmingly, any risk from vaccination has to be set against the risk of disease. A decision may be thought of as “precautionary”; but if people are not vaccinated, because use of the vaccine is suspended, or because of a drop in confidence in the vaccine caused by the decision, some of them will get ill from Covid-19; and some of them will die. Any decision to withhold the vaccine will directly cause excess, avoidable Covid-19 deaths.

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