After this fact-check was first published on 18 March 2021, reports began emerging of a different rarer form of blood clotting linked to the vaccine. While the original verdict remains the same, we updated our check on 15 April 2021 to add further information and context.
Countries across Europe began pausing the roll-out of Oxford-AztraZeneca’s Covid-19 vaccine in March 2020 over fears it may have caused blood clots in a small number of people.
Austria, Denmark, Iceland and Norway were among the first to temporarily suspend use of the vaccine, followed by France, Germany, Ireland, the Netherlands and Spain. They began restarting its use after the European Medicines Agency (EMA) confirmed there was no link between the common form of clotting and the vaccine
In April, a number of countries suspended AstraZeneca vaccine use after reported links between the vaccine and what the UK’s Medicines and Healthcare Products Regulatory Agency called “extremely rare, unlikely to occur blood clots”.
After the European Medicines Agency (EMA) released its initial statement on 15 March saying the vaccine was safe, many people took to social media to note that the clotting risk is higher in contraceptive pills, which are deemed safe for use.
One claim that was shared more than 40,000 times and liked well over 200,000 times on Twitter said suggested that the contraceptive pill was in fact much more likely to result in blood clots than the AstraZeneca vaccine.
Ferret Fact Service looked at the claim and found it Mostly True.
On 10 March the EMA issued a statement noting that the Austrian medicines regulator had suspended the use of one batch of the vaccine after one person was diagnosed with blood clots and died 10 days after being inoculated. Another had been hospitalised with a pulmonary embolism, but was recovering.
The Danish Health Authority also reported the blood-clot death of someone who had received the Oxford-AstraZeneca jab.
In Norway, health officials said they had been informed “about an unexpected death from a brain haemorrhage” in a person who had received the jab as well as “three more reports of severe cases of blood clots or brain haemorrhages in younger people who have received the AstraZeneca vaccine”.
How many cases of blood clots were there?
In a statement released on 14 March, AstraZeneca said that as of 8 March its vaccine had been given to 17 million people across the UK and Europe. From that number, there had been 15 reports of deep-vein thrombosis (DVT) and 22 of pulmonary embolism.
That is a significantly lower incidence rate than would be expected in the general population.
The National Institute for Health and Care Excellence in England says around two people in every 1,000 will suffer a DVT each year and seven to eight per 10,000 will be diagnosed with pulmonary embolism. Based on those figures, it could be expected that in a population of 17 million, about 34,000 would develop a DVT and between 11,900 and 13,600 would be diagnosed with pulmonary embolism.
After reports of blood clots started to emerge the EMA began an investigation to, it said, “determine whether the vaccine might have contributed or if the [clot] is likely to have been due to other causes”.
The investigation was carried out in co-operation with AstraZeneca, experts in blood disorders, and individual countries’ health authorities, including the UK’s Medicines and Healthcare products Regulatory Agency (MHRA), which shared the data collected from the administration of around 11 million Oxford-AstraZeneca doses.
On 18 March, the EMA released the results of that investigation. Its executive director, Emer Cooke, said the investigating committee had come to “a clear scientific conclusion” that the vaccine is “safe and effective” and not linked to an increased risk in blood clots.
That means that, while in a very limited number of cases there has been a temporal link between receiving the vaccine and developing a clot, no causal link has been found.
Why have countries started suspending the vaccine again?
After the EMA made its 18 March announcement reports started emerging of a new, rarer form of blood clot being linked to the vaccine.
On 30 March Germany suspended use of the AstraZeneca jab in people under the age of 60 after 31 cases of the rare brain clot cerebral venous sinus thrombosis (CVST) were reported. The EMA says that CVST affects between two and five people per million every year, although that could be an underestimate due to a lack of studies.
Of the cases reported in Germany, all had received the AstraZeneca jab in the preceding 16 days. Nine of the people affected died. Of the 31, 29 cases occurred in women aged between 20 and 36.
Also on 30 March, Canada suspended use of the vaccine in those age 55 and under after the country’s National Advisory Committee on Immunization said there was “substantial uncertainty” about the benefit of giving it to people in that age group. The organisation pointed to reports of CVST cases in Europe as the reason behind its decision.
At the beginning of April the MHRA said that 30 cases of those rarer blood clots had been identified among the 18.1 million people who had received the jab in the UK. Of that number, 22 of the cases were CVST.
On 7 April, the EMA said there was “a possible link” between the vaccine and CVST and splanchnic vein thrombosis, which occurs in the abdomen. It said most cases had occurred in women under the age of 60, but that there was not enough data available to confirm any specific risk factors.
On the same day the MHRA said there was a possible link between the vaccine and “extremely rare, unlikely to occur blood clots”, but did not recommend age restrictions in its use.
Is it still safe to use?
Both the EMA and the MHRA were clear that, even though a possible causal link has been found between the AstraZeneca jab and rare blood clots, it is still safer to have than not.
The EMA said: “The benefits of the vaccine continue to outweigh the risks for people who receive it. The vaccine is effective at preventing COVID-19 and reducing hospitalisations and deaths.”
MHRA chief executive Dr June Raine said “no effective medicine or vaccine is without risk”, adding that “it is still vitally important that people come forward for their vaccination when invited to do so”.
What about the contraceptive pill?
The NHS warns that anyone using combined hormonal contraception such as the combined pill, contraceptive patch or vaginal ring has a heightened risk of developing blood clots.
These contraceptives all contain oestrogen hormones, which Thrombosis UK says makes the blood “more sticky”. That in turn heightens the risk of clots developing.
A meta-analysis published in the British Medical Journal (BMJ) in 2013 found that all combined contraceptive pills increase the risk of venous thrombosis, but that the effect size depends on how much progesterone is also present.
A further study was published in the BMJ two years later. It found that the annual number of extra clotting cases that could be attributed to the use of combined oral contraceptives varied from six per 10,000 for those that contain the progestin levonorgestrel to 14 per 10,000 for those that contain progestins desogestrel and cyproterone.
Ferret Fact Service verdict: Mostly True
The contraceptive pill has been found to increase the risk of blood clots and the Oxford-AstraZeneca vaccine has not. That means it is not strictly possible to say one has “a higher potential blood clot rate” than the other. By definition, however, the contraceptive pill does have the potential to cause more blood clots than the vaccine.
Photo thanks to iStock/nensuria.
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Note: This article was updated on 23 March 2021 to clarify that the presence of progestins alongside oestrogen caused increased clotting rates and not the progestins alone.