Patients may have a higher risk of developing blood clots, including in the brain, after receiving the Oxford/AstraZeneca vaccine, but the absolute risk is small, a new large study has concluded.

Researchers based in Denmark and Norway studied national health records for 280,000 people aged 18-65 who received a first dose of the AZ vaccine from February and until 11 March this year.

They looked for rates of events including heart attacks, strokes, deep vein blood clots and bleeding events within 28 days of receiving a first dose of the vaccine.

The study, published in the BMJ, identified 59 blood clots in the veins compared with 30 expected in the general population, corresponding to 11 excess events per 100,000 vaccinations.

This included a higher than expected rate of blood clots in the veins of the brain – cerebral venous thrombosis – at a rate of 2.5 events per 100,000 vaccinations.

They found no increase in the rate of arterial clots, such as heart attacks or strokes. 

The study concluded that among recipients of the AZ vaccine. ‘increased rates of venous thromboembolic events, including cerebral venous thrombosis, were observed’.

However, it added: ‘The absolute risks of venous thromboembolic events were, however, small, and the findings should be interpreted in the light of the proven beneficial effects of the vaccine, the context of the given country, and the limitations to the generalisability of the study findings.’

‘For the remaining safety outcomes, results were largely reassuring’ and although there were ‘slightly higher rates of thrombocytopenia/coagulation disorders and bleeding’, these ‘could be influenced by increased surveillance of vaccine recipients’, the paper said.

The researchers also pointed out that the study was observational so could only establish correlation not cause. They also said there was a lack of data on underlying risk factors for clotting and a possibility that the results do not apply to patients of all ethnicities.

Following the reports of rate blood clotting events related to the AZ vaccine, a large number of European countries have restricted its use. In the UK, the NHS will be administering vaccine alternatives to patients aged under 30, on recommendation from the JCVI.

Commenting on the Scandinavian research study, Professor Paul Hunter, a professor of public health protection at the University of East Anglia, said the benefits of the vaccine far outweigh its risks for most age groups. 

He commented: ‘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’.

Dr John Ashcroft, a Derbyshire GP with an interest in cardiovascular medicine told Pulse of incidents of patients refusing the AZ vaccine even when advised it was safe, asking instead to have the Pfizer vaccine.

He said: ‘Many patients will be happy just to get the vaccine, whichever it is, but some will not want the Oxford/AstraZeneca [one]. The problem is that if we give choice many will want the Pfizer,  if we give no choice then some will decline.’

The MHRA has advised that vaccinated individuals should ‘seek immediate medical attention if four or more days after vaccination they develop new onset or worsening severe or persistent headaches with blurred vision, which do not respond to simple painkillers’.

This story first appeared on our sister title, Pulse.