The absolute risk of blood clots after the Oxford/AstraZeneca Covid-19 vaccine are “small”, a new study examining the vaccine in Denmark and Norway has concluded.
But the researchers cautioned that the risks linked to the jab should be “interpreted in the context of the benefits of Covid-19 vaccination”.
And one UK scientist said that for most people the probability of surviving the year is “much greater for people who accept any vaccine when offered than if they decline it”.
The study, published in The BMJ, suggests that the vaccine was linked to increased rates of vein blood clots, including clots in the veins of the brain, compared with expected rates in the general population.
But the risk of such events is “small”, the authors said.
The UK medicines regulator, the Medicines and Healthcare products Regulatory Agency, previously concluded that there is a “possible link” between the AstraZeneca jab and extremely rare blood clots.
It said that the benefits of vaccination continue to outweigh any risks, but it has recommended that adults under 30 should be offered an alternative to the vaccine – such as the Pfizer or Moderna jabs.
The new study, led by researchers in Denmark, assessed the rates of heart and blood clot “events” in the first 28 days after people were jabbed with the Oxford/AstraZeneca vaccine in Denmark and Norway.
Both countries have stopped using the vaccine as part of their immunisation programmes.
The team of researchers compared data on 281,264 people – about four fifths were women – and compared this with data from the general population.
The researchers found 59 blood clots in the veins compared with 30 expected in the general population.
This equates to 11 excess blood clotting events per 100,000 vaccinations, including 2.5 additional blood clots in the brain per 100,000 vaccinations.
They found no increase in the rate of arterial clots, such as heart attacks or strokes.
“Among recipients of ChAdOx1-S (the Oxford /AstraZeneca vaccine) increased rates of venous thromboembolic events, including cerebral venous thrombosis, were observed,” they said.
“Absolute risks of events were small and should be interpreted in the context of the benefits of Covid-19 vaccination at both the societal and the individual level.”
In a linked opinion piece, Professor Paul Hunter at the University of East Anglia said the new study improves understanding of the population risk of thrombotic events after the Oxford-AstraZeneca vaccine, but it does not change the conclusions of the UK and European medicines regulators that the benefits of the Oxford/AstraZeneca vaccine “far outweigh its risks for most age groups”.
He added: “It remains the case that for most age groups, the probability of surviving the year is much greater for people who accept any vaccine when offered than if they decline it.”
Prof Hunter continued: “Those countries that delayed their own vaccination programmes at a time of high transmission rates by declining to use available Oxford/AstraZeneca vaccines should know that their decision will have contributed to an increase in the number of avoidable deaths from Covid-19.”