Dr Michael Head, senior research fellow in Global Health, University of Southampton, said that while the link is not proven, it is now thought to be increasingly plausible.
He said: “We have seen an update from the UK and EU regulators, suggesting that these thrombotic events may have been a causal, but rare, adverse event from the Oxford AstraZeneca vaccine.”
He added: “It’s important to emphasise that adverse events happen with all medicines, and vaccines are no exceptions.
“Safety surveillance is vital in picking up and assessing signals that emerge from the data.”
What are the side effects of the Oxford/AstraZeneca vaccine?
The AstraZeneca vaccine lists the following side-effects that can occur after the jab: tenderness, pain, warmth, itching or bruising where the injection is given, generally feeling unwell, feeling tired, chills or feeling feverish, headache, feeling sick (nausea), joint pain or muscle ache.
Ian Douglas, professor of pharmacoepidemiology at London School of Hygiene and Tropical Medicine, explains that these side-effects are “pretty common” and occur in more than one in 10 people who are given the vaccine.
When should I see a doctor?
While some people will experience side-effects from the jab, experts have said that certain symptoms may be a sign of a more serious condition which needs immediate medical attention.
Prof Beverley Hunt, medical director at the charity Thrombosis UK, said thrombosis in the head can present as an extremely bad headache.
“We’ve seen patients who have been presenting with thrombosis in the head or abdomen from about day four after the vaccine,” she said.
“I think it’s very important to tell people that lots of people get side effects from the AstraZeneca vaccine and they usually settle down by day four or five.
“What we’ve seen is people presenting with the worst headache they have ever had on day four and they have proved to have thrombosis in the large vein of the head.”
What factors increase the risk of blood clots?
Blood clots are rare in young, healthy people. You’re more likely to get them if you:
- Are staying in or recently left hospital – especially if you cannot move around much (like after an operation)
- Are overweight
- Are using combined hormonal contraception such as the combined pill, contraceptive patch or vaginal ring
- Have had a blood clot before
- Are pregnant or have just had a baby
- Have an inflammatory condition such as Crohn’s disease or rheumatoid arthritis
What other vaccines are available?
The UK is currently using two vaccines, Pfizer/BioNTech and Oxford/AstraZeneca, while a third coronavirus vaccine, the Moderna jab, began its rollout in Wales on Wednesday.
Meanwhile, preliminary results announced on Tuesday from the trials of the Valneva Covid-19 vaccine, which is set to be manufactured in the UK, have shown it produces a “strong immune response”, paving the way for a phase three clinical trial.