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Home Risk and the Oxford/AstraZeneca Vaccine

Risk and the Oxford/AstraZeneca Vaccine

There has been a lot of discussion recently about the risk of blood clotting in people who have been given the Oxford-AstraZeneca (AZ) vaccine. Now new evidence has emerged about the risk the vaccine actually poses. So how risky is it? Read on for more…



Since the roll-out of the AZ vaccine, there have been 79 cases of rare blood clots reported in the UK. All of these cases occurred after the first dose of the vaccine had been given, and 19 of the cases resulted in death.1 While this sounds scary, it is important to remember that over 20 million doses of the AZ vaccine have been given in the UK alone.2 This means that the odds of experiencing blood clotting is around 4 in a million, or approximately a 0.0004% chance – nearly as improbable as winning the lottery.3

Of the cases of blood clotting observed, 51 were in women and 28 were in men, all aged between 18 and 78. While this appears at first glance to suggest that women are more susceptible to clotting, this is NOT the case – more women have been vaccinated against Covid-19 than men, and therefore it makes sense that more cases of clotting would be seen in women.4



As explained above, the risk of blood clotting due to receiving the AZ vaccine is extremely low. No vaccine is without risk5, and so unexpected side effects are not unusual in the roll-out of large-scale medical treatments. According to the European medicines regulator, the EMA, there are no specific risk factors for blood clotting.6 This means that there are no medical conditions or personal traits (age, sex, race etc) that make it more or less likely that you will experience clotting. Therefore, the chance that you may have blood clots is the same regardless of who you are. Remember, the chance of suffering blood clots as a result of the vaccine is extremely low. In addition, Covid-19 itself causes blood clotting if you are infected, in a much higher percentage of the population (between 7% and 11%, as opposed to 0.0004% who received the AZ vaccine).7 Therefore the risk from the vaccine is much lower than the risk from the virus it protects against.

The scientific consensus is that the benefits of the AZ vaccine continue to outweigh the risks of clotting. As was pointed out by the UK’s medicines regulator, the evidence that the vaccine and the blood clotting are linked is causal, but not proven beyond all doubt.8 In addition, it should be remembered that in an average population sample of 20 million people, there would be far more than 79 cases of blood clotting of the same type expected due to general health conditions.9,10,11 As such, the danger posed by the vaccine is extremely small.


The UK government and vaccine regulators have decided to err on the side of excessive caution in their reaction to this situation. They have made several recommendations, which are set out below.

If you have been given a dose of the AZ vaccine already, you should accept the second dose unless you experienced blood clotting as a result of the first dose.12

If you are under the age of 30, you are likely to be offered an alternative dose of another vaccine if one is available.13 While this makes sense to alleviate the fears of people, it is unlikely to make much (if any) difference to the risk of experiencing blood clotting due to the minimal risk they pose. In addition, every Covid-19 vaccine has side effects.14 There is therefore a chance that by opting to have a different vaccine, you trade one set of side effects for another and therefore do not profit from the situation.

The Government has stressed that these changes will not result in a delay to the vaccine program.15 This means that you should still be offered and receive a vaccine at the same time as you would before this announcement. This is also why the decision has not been taken to withdraw or reduce the roll-out of the AZ vaccine while further tests are done – the risk of clotting is far outweighed by the benefits, and would actually be more damaging to the population than continuing to give the vaccine.

In addition, it is vital to remember that vaccines are a crucial way to protect people from Covid-19, and are essential in order to be protected. Therefore, it is important to continue to get your vaccine when it is offered, unless you have been specifically recommended not to by your GP or another qualified medical professional.



The European Medical Agency (EMA) recommend that if you experience any of the following symptoms of blood clotting in the weeks after your vaccine, you should speak to a medical professional immediately. These symptoms are: shortness of breath, chest pain, leg swelling, persistent abdominal (belly) pain, neurological symptoms (such as severe and persistent headaches or blurred vision), and tiny blood spots under the skin beyond the site of the injection.16

Remember, the risk of experiencing these side effects is extremely low! However, if you do experience them, please seek medical help immediately.



If you are offered a dose of the AZ vaccine, you should accept it unless you have been specifically advised not to by your GP or another medical professional. If you are offered a dose of any vaccine, you should accept it. If you have already received a dose of the AZ vaccine, you should accept the other dose as recommended by the UK medicines regulator. Remember, the risk from the vaccine is extremely low.

If you still have concerns about the risk the vaccine may pose, it is advised that you speak to your GP as they will be best able to advise you on the best course of action. This is especially important for people with a past history of blood clots and pregnant women.17 This is because in pregnancy, there is a slightly elevated (but still low) risk of blood clotting. Remember, the most important thing is that you are healthy and safe.



To see all BuDS’ articles about Covid-19, use this link: https://buds.org.uk/articles-about-covid-19/

To learn more about the vaccines currently in use, as well the answers to some frequently asked questions about them, use this link: https://buds.org.uk/vaccine-update-and-faqs-04-03-21/

To learn more about Long Covid and its impact, use this link: https://buds.org.uk/long-covid-in-children-and-young-adults/

To learn more about the people who are more likely to die or have serious illness if they catch Covid-19, use this link: https://buds.org.uk/how-dangerous-is-covid-19-if-you-catch-it/

To learn more about how face coverings and face masks can protect you, and read our recommendations about using them, click this link: https://buds.org.uk/information-about-face-coverings-and-masks/



Please share this article on social media, but always credit BuDS. If you need help or support or you’re anxious about Covid-19, BuDS is here for you. Please e-mail [email protected], call 01494 211179 (voicemail) or message us and we’ll do all we can to help.




[1] https://www.gov.uk/government/news/mhra-issues-new-advice-concluding-a-possible-link-between-covid-19-vaccine-astrazeneca-and-extremely-rare-unlikely-to-occur-blood-clots

[2] Ibid.

[3] Ibid.

[4] Ibid.

[5] As stated by Dr June Raine, the Chief Executive of the MHRA https://www.gov.uk/government/news/mhra-issues-new-advice-concluding-a-possible-link-between-covid-19-vaccine-astrazeneca-and-extremely-rare-unlikely-to-occur-blood-clots

[6] https://www.ema.europa.eu/en/news/astrazenecas-covid-19-vaccine-ema-finds-possible-link-very-rare-cases-unusual-blood-clots-low-blood

[7] https://www.bbc.co.uk/news/health-56665517

[8] https://www.gov.uk/government/news/mhra-issues-new-advice-concluding-a-possible-link-between-covid-19-vaccine-astrazeneca-and-extremely-rare-unlikely-to-occur-blood-clots

[9] According to the CDC in the USA, the average percentage of the population who experience blood clotting due to non-vaccine related conditions is 0.27%. This is far, far higher than the 0.0004% who experience clotting due to the AZ vaccine. For a sample size of 20 million, therefore, around 54,000 cases of blood clotting would be statistically expected due to normal medical reasons. https://www.cdc.gov/ncbddd/dvt/features/blood-clot-risk.html

[10] For another perspective, it is about the same risk as dying in accident during a 250 mile car journey, as reported here https://www.bbc.co.uk/news/health-56665517

[11] https://www.gov.uk/government/news/mhra-issues-new-advice-concluding-a-possible-link-between-covid-19-vaccine-astrazeneca-and-extremely-rare-unlikely-to-occur-blood-clots

[12] Ibid.

[13] Ibid.

[14] https://www.nhsinform.scot/covid-19-vaccine/the-vaccines/side-effects-of-the-coronavirus-vaccines

[15] https://www.bbc.co.uk/news/health-56665517

[16] https://www.ema.europa.eu/en/news/astrazenecas-covid-19-vaccine-ema-finds-possible-link-very-rare-cases-unusual-blood-clots-low-blood

[17] https://www.gov.uk/government/news/mhra-issues-new-advice-concluding-a-possible-link-between-covid-19-vaccine-astrazeneca-and-extremely-rare-unlikely-to-occur-blood-clots