When a vaccine is administered to millions of people, it is inevitable that some adverse events – that would have happened anyway – will happen shortly after vaccination. When deciding whether the events are a coincidence, or caused by the vaccine, you have to consider biological plausibility and whether there are more events than you would expect without vaccination. We refer to the number of events observed after vaccination as O, and the number you would expect in that population without vaccination, as E (Observed and Expected). If the ratio – O/E – is not greater than 1, then the vaccine is not likely to be the cause of the events.

Authorities in various countries have carefully considered the evidence, and concluded that there is no excess of clotting events in people who have been vaccinated – even with the increased awareness of the possibility of such events (and the increased likelihood of reporting them if they should occur that inevitably follows such increases in awareness.)

It is most regrettable that countries have stopped vaccination on such “precautionary” grounds: it risks doing real harm to the goal of vaccinating enough people to slow the spread of the virus, and to end the pandemic.

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It would seem the move has been done following reports of serious blood clotting events in adults in Norway.

These events were said by the Norwegian Institute responsible for delivering vaccines to be “they have had a reduced number of blood platelets. Blood clots and subsequent brain haemorrhages are a rare condition.”

However, what these authorities do not make clear is that these coagulation disorders are very common in patients with Covid-19. Unless we are sure that those who suffered these unfortunate events very definitely did not have COVID-19 then it would seem to be premature to suggest it was the vaccine that caused these events.

It has been well known for over a year that coagulation disorders, both clotting causing strokes, and bleeding (thrombocytopenia, which is a reduced number of platelets in the blood), are very common in patients with COVID-19.  Early reports from China noted over 30% of patients reaching hospital had thrombocytopenia.

It is also a principle of regulatory action that when action is taken in regard to a particular product, that the alternatives definitely do not have the same problem. There was a case of thrombocytopenia in the US trial for the Pfizer vaccine, but detailed investigation made it clear that the vaccine was not the cause. The publicly available information on the AstraZeneca vaccine lists a total of 35 cases of thrombocytopenia reported on “Yellow Cards” in the UK up to 8th March 2021. This is a very small proportion of the yellow card reports which total over 54,000 in the context of nearly 10 million vaccinations delivered. For the Pfizer vaccine there are a total of 22 reports of thrombocytopenia out of 33,000 reports and well over 10 million vaccination doses. It is clear that the proportion of reports for this bleeding disorder is not different in the two vaccines.

Covid definitely causes coagulation disorders and each of the vaccines prevents Covid disease, including more severe cases. Therefore, it is extremely likely that the benefit of the vaccine notably outweighs any risk for coagulation disorders and the vaccine prevents other consequences of Covid including deaths from other causes.

It is entirely reasonable that detailed studies are done on the vaccines in regard to coagulation disorders, but it seems a step too far in taking precautions that would stop people getting vaccines that would prevent disease.

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Given the clear evidence we now have of real-world effectiveness of this vaccine against severe COVID-19 and against SARS CoV2 infection and the continuing widespread circulation of the virus in most if not all European countries, the potential public health consequences, both direct and reputational, of taking a stop-start approach need to be considered very carefully. In order to use vaccines effectively to help gain control over the pandemic there needs to be vaccine supply, vaccine delivery and vaccine acceptance. Getting all three lined up and in place at the same time is not easy and cannot be taken for granted while the need for rapid progress is obvious. If clear evidence of serious or life threatening side-effects emerges that will have important consequences. However so far it hasn’t and it’s highly undesirable to disrupt a complex and urgent programme every time people develop illnesses after receiving vaccine that may be coincidental and not causally related. Making the right call in situations like this is not easy but having a steady hand on the tiller is probably what is needed most.