In June 2021, regulators around the world concluded that very rare cases of a particular type of blood clot may be linked to the AstraZeneca/Oxford University vaccine.
However, the risk of the rare clot from the vaccine is eight times less than the risk of a clot caused by Covid-19, according to an Oxford University study from April 2021.
Regulators, including the Medicines and Healthcare products Regulatory Agency (MHRA) in the UK and the European Medicines Agency, say the overall benefits of the vaccine outweigh the risks for all age groups.
As of December 2 2021, researchers believe they have sourced the link between blood clots and the AstraZeneca/Oxford University vaccine.
Although the blood clots led to 73 deaths out of 50m administered doses of the Covid jab, the side effect led to the AstraZeneca jab not being offered to people under 40, due to the risks.
Scientists at Cardiff University now believe to have found the "trigger" of the blood clots: they found that a protein in our blood can bind to part of the vaccine - which can lead to clotting.
The reaction is not caused by the Covid-19 particles contained in the vaccine itself but, in fact, the system delivering it to the body.
Unfortunately, the new hybrid protein-virus can cause confusion to the body's immune system, which creates new antibodies which then stick to the proteins - causing the blood clots.
It's a rare reaction and process, which is why only a few people who received the AstraZeneca vaccine were affected.
The UK continues to use the the Pfizer/BioNTech jab the Moderna vaccine and Johnson & Johnson's Janssen vaccine - as well as AstraZeneca (for those who are unable to receive the former two vaccines).
Prime Minister Boris Johnson has said people should follow the regulator's advice and keep getting their jabs, stating that "everyone must play their part" in getting vaccinated.
Read on to find out the latest about the Oxford/AstraZeneca vaccine, and to have your questions answered by our expert, The Telegraph's Global Health Security Editor Paul Nuki, below.
What do we know about any link between the AstraZeneca vaccine and blood clots?
Researchers at Cardiff University claimed to have found the "trigger" of the blood clots after people received their doses of the AstraZeneca vaccine on December 2 2021.
According to the team of scientists, the issue was a rare attraction between a weakened version of the common cold virus, known as adenovirus, and a protein called "platelet factor four".
The AstraZeneca vaccine contrains coronavirus genetic material inside the adenovirus and confuses the immune system which then leads to your body forming dangerous blood clots.
At the time of the discovery of the clots in June 2021, despite the side effect resulting in an increase in the number of serious blood clots reported after vaccination, the Government's regulatory agency continued to maintain that the benefits of receiving the AstraZeneca vaccine continued to outweigh the risks.
The specific side-effect is known as vaccine-induced thrombosis and thrombocytopenia (VITT), which means blood clots and low platelets after a jab. The clots were largely found in the brain - cerebral venous sinus thrombosis - but there were also clots in other parts of the body.
"On the basis of this ongoing review, the advice remains that the benefits of the vaccine outweigh the risks in the majority of people," the MHRA said in June 2021. The European regulator, the European Medicines Agency, has come to the same conclusion. As discussed below, for younger people - particularly when Covid rates are low - the equation is slightly different.
We asked Telegraph readers for their questions on the vaccines that have been rolled out in the UK. Our expert, The Telegraph's Global Health Security Editor Paul Nuki, answered them on April 8 2021. Above, we have updated the article based on emerging information. Below, you can read his answers from that date.
Read on for a selection of the best questions from the Q&A.
What are the blood clot risks of the different vaccines currently used in the UK?
Q: What are the blood clot risks of the used vaccines apart from the AstraZeneca vaccine? How do they compare?
A: The other vaccines have not been associated with a raised incidence of clotting. With AZ there does seem to be a raised risk but the problem remains extremely rare.
The MHRA has said up to March 31, it had received "two reports of blood clots (thromboembolism) reported with thrombocytopenia for the Pfizer/BioNTech vaccine".
By this date, approximately 11 million first doses and 3.5 million second doses of the vaccine had been given, the MHRA said.
There is no data on the Moderna vaccine as it is not yet being used widely in the UK.
I've had one dose of AstraZeneca, but I'm under 30, what vaccine will I get for my second dose?
Q: If the AstraZeneca vaccine is banned in an age group what happens for my second dose if l had AstraZeneca for my first?
A: The AZ vaccine is not being banned in any age group. It is just that those under 30 are being offered a choice if an alternative is available. If you have had one dose of the AZ jab the very clear advice is that you should have your second. Any risk is tiny and the upside enormous, especially if we get another surge in infections later in the year –which is likely.
Is there evidence to suggest that we should have different vaccines for different doses?
Q: What evidence is available showing one jab of AstraZeneca vaccine and one jab of Pfizer vaccine provides better efficacy? Is it possible to get this treatment here in England?
A: I’m not aware of there being any evidence yet that mixing jabs has any additional benefit and there may be risks. It’s possible but we will only find out once proper trials have been done. If you search online you may be able to enter one.
Why should I risk getting a vaccine if I have already had Covid?
Q: Having had Covid (no symptoms, but loss of smell and taste), I fail to see why people not at risk, such as myself, are required to take the additional risk of vaccination. Why not stop after all the vulnerable are vaccinated?
A: The natural immunity you got when you first had Covid may not last long, leaving you exposed in future. Even in the short term, the immunity you have may not protect you against some of the new variants. There are now plenty of examples of people who have caught the virus twice, some seriously. Also, the risk of having the vaccine is tiny - about one in 100,000 according to the bigger dataset from the EU. Many things you do day in day out have a higher risk than that – driving, for example.
Could the AstraZeneca vaccine have caused my husband's blood clot?
Q: My 85-year-old husband died from a blood clot on the brain and the hospital said he had a stroke but he didn't have any symptoms of a stroke. He had his first vaccination three months ago and was due to get his second this Friday. Could the AstraZeneca vaccine have caused the blood clot?
Paul: If the clot occurred three months after his inoculation it seems unlikely there is any connection but you might talk to your GP to get their view if you have not already. It’s important to remember that clots and strokes become much more common with age and, at 85, are far from unusual.
Do you have a question about the AstraZeneca vaccine or any of the other vaccines currently in use in the UK? Leave your question below
What did regulators say at the time?
A review by EMA safety committee, at the time of the discovery of the clots in June 2021, concluded that "unusual blood clots with low blood platelets should be listed as very rare side effects" of the AstraZeneca vaccine.
Emer Cooke, executive director of the EMA, said: "The risk of mortality from Covid is much greater than risk of mortality from these side effects."
The MHRA always said that there were still huge benefits of the vaccine in preventing Covid-19 and serious disease but added that due to a very small number of blood clots in younger people, those under the age of 40 were to be offered Pfizer or Moderna jabs instead.
Dr June Raine, MHRA chief executive, added: "Anyone who has symptoms four days after vaccination or more should seek prompt medical advice - a new onset of a severe or persistent headache or blurred vision, shortness of breath, chest pain, leg swelling, persistent abdominal pain or indeed unusual skin bruising or pin-point spots beyond the injection site."
How did other countries react?
Germany, the Netherlands, Sweden and Canada all restricted the use of the AstraZeneca vaccine in younger people. Italy and Spain stopped the use of the jab in the under-60s.
Denmark announced on Apr 14 2021 that it would stop administering Oxford/AstraZeneca's Covid-19 vaccine entirely following its link to very rare cases of blood clots, a decision based on the low levels of Covid-19 in the country and caution over the rare side-effect.
The decision, which at least for now removes the shot from Denmark's vaccination scheme, could delay the country's vaccine rollout by up to four weeks, based on previous statements by health bodies.
Also in Apr 2021, France said under 55s who have had a first AstraZeneca dose should take a different vaccine for the second. Olivier Véran, the country's health minister, said the new advice will be that the Moderna and Pfizer vaccines should be used for their second dose.
Spain began studying the effects of mixing different coronavirus vaccines, government researchers said on Apr 19 2021, responding to shifting guidelines on the safety of the AstraZeneca's shot.
Also, Australia doubled its order of the Pfizer Covid-19 vaccine, as the country began its race to overhaul its inoculation plan over concerns about the risks of blood clots with the AstraZeneca vaccine.
Many of the decisions to restrict the use of the jab also have to be seen in the context of a country having access to other jabs, and low rates of Covid.
What was AstraZeneca's response?
In March 2021, AstraZeneca said it was analysing its database to understand "whether these very rare cases of blood clots associated with thrombocytopenia (low blood platelet count) occur any more commonly than would be expected naturally in a population of millions of people".
Meanwhile on Apr 6, a trial of the Oxford/AstraZeneca vaccine on children was paused, but the scientists involved said there were no safety concerns with the trial itself and they were waiting for further information from the MHRA.
In response to these concerns, an Oxford University study examined the incidence of blood clotting on the brain in coronavirus patients and AstraZeneca recipients, finding that the occurrence of brain clots from coronavirus was eight times greater than the risk presented by the AstraZeneca jab.
Sir John Bell, Oxford University's Professor of Medicine, stated that he expected all vaccines to have "some background level of clotting issues". Prof Sir Bell went on to say that the data on this issue was still being collected for further study.
What about the Johnson and Johnson vaccine and blood clots?
Regulators in the United States paused the roll out of the Johnson and Johnson vaccine after six people suffered blood clots two weeks after receiving the jab - one person died. Regulators say the decision is based on an "abundance of caution", and have since resumed use of the jab after a safety review.
The decision is already being felt more widely: the company delayed shipments of the vaccine to Europe. And South Africa, which decided to stop using the AstraZeneca vaccine because of lack of efficacy against a new variant, has also put its campaign on pause.
On Apr 20, the European Medicines Agency (EMA) said there was a possible link between Johnson & Johnson's Covid-19 vaccine and rare blood clots but said the benefits of the J&J vaccine far outweighed the risks and that further investigations would continue.
The single-dose Johnson & Johnson vaccine was approved for use in the UK as of May 28 2021. The government has ordered 20 million doses, which will be used to target "hard-to-reach" groups in the vaccine rollout, such as those who may be reluctant to come forward for two jabs. Additionally, the Janssen vaccine will potentially be used as part of a booster programme later in the year.
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. Below is a list of factors that can make them more common, although it is not clear if these factors are involved in the post-vaccination events, which are a specific kind of clot occurring alongside low platelets. In fact, haematologists have said there is currently no evidence these groups are more at risk of this side-effect.
Generally, you are more likely to experience a blood clot if you:
What other vaccines are available?
The UK is currently administering all four vaccines: Pfizer/BioNTech, Moderna, Janssen and Oxford/AstraZeneca (for those over the age of 40 and for anyone who might experience side effects from the other three jabs).
The Valneva Covid-19 vaccine was set to be manufactured in the UK as preliminary test results showed it produces a "strong immune response", paving the way for a phase three clinical trial.
However, the UK dropped its deal to receive approximately 100 million doses in September 2021, after the UK Health Secretary Sajid Javid told the House of Commons that the vaccine would not have passed UK regulations.