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Comparing the risk of thrombosis with vaccines, the contraceptive pill or smoking

Verificat, April 29

An infographic comparing the risk of blood clots after receiving AstraZeneca/Oxford vaccines with the risk caused by smoking, taking birth control pills and for getting infected by COVID-19 has gone viral in recent weeks.

The information is misleading because although some cited data are correct, the numbers are not comparable between them because they refer to different types of clots.

The datum ensuring that suffering from coronavirus involves a 16.5 per cent likelihood of developing thrombi could not be further from the truth because, while other numbers refer to the population in general, this one is valid only in the hospitalised patients and due to a concrete type of clot.

“The risk of suffering blood clots is of 0.0004 per cent with the AstraZeneca/Oxford vaccine, and between a 0.05 and a 0.12 per cent with the contraceptive pill, of 0.18 per cent in smokers and of 16.5 per cent due to COVID-19 infection.”

“The comparison cannot be made,” explains Carlos Rodrigo, Clinical Director of Paediatrics at Germans Trias Hospital and vaccinologist, to Verificat. It is very delicate to equate the different types of thrombi without mixing them up, and, in each case, the infographic takes a different one.

Blood clots are formed when blood cells get glued together and constitute a regular procedure in our daily life. When we suffer an injury, the body sends platelets and a series of proteins at the site of the injury in order to coagulate the blood and stop the haemorrhage. Sometimes, however, the procedure occurs at other sites and the clots are formed in the veins and arteries’ walls, becoming dangerous.

These blood clots are divided into two main families: venous and arterial, depending on the blood vessel in which they develop. Each one is further subdivided into thrombosis and embolisms.

The former is static, while the latter break off and travel to other parts of the body such as the lungs (causing a pulmonary embolism). Both conditions block the passage of blood and can cause serious damage and even death.

The contraceptive pill: 0.05 and 0.12 per cent

The datum that is indeed correct is the development of blood clots in people taking the contraceptive pills, but only for the concrete case of venous thromboembolism: the range varies from 500 to 1200 cases per million of users, as the Spanish Agency for Medicines and Health Products (AEMPS, in Spanish) publishes.

The risk, even so, “is considered very low”. It varies depending on the drug and is “higher during the first year, at reinitiating its use after a minimum stop of four weeks and in women with risk factors”. The pill is not associated with arterial problems, explained José Román Escudero, Director of Angiology, Vascular and Endovascular surgery from Hospital de la Santa Creu i Sant Pau, to Verificat.


The numbers of risk tied to smoking come from a study about cases of venous thromboembolism (deep venous thrombosis and pulmonary embolisms). There is evidence, however, pointing that smoking is also a risk factor to suffer arterial clots, which is not contemplated in this viral graphic.

In this sense, Rodrigo explains that the risk of suffering a clot not only depends on the fact of smoking, but also on variable factors, such as the intensity of consumption, or personal, such as age or physical condition. For this, it is difficult to assert that smoking has a concrete percentage of causing clots.

Clots, vaccines,

and COVID-19

The datum of infections by COVID-19 (16.5 per cent) refers to patients who have developed a pulmonary embolism. Of these, 42.4 per cent also developed deep venous thrombosis.

In total, 14.8 per cent of COVID-19 hospitalised people have suffered this type of thrombosis, according to a pooled analysis of 27 studies with more than 3,000 patients worldwide.

In the case of AstraZeneca/Oxford vaccine, the European Medicines Agency (EMA) is only studying “thrombotic events and cerebral venous sinus thrombosis (a rate type of thrombosis in the brain)” and analyse them combined with low platelets levels (thrombocytopenia). These cases are therefore different from those studied cases of severe COVID-19.

Although at first instance, EMA first raised the alarm due to thrombotic and thromboembolic episodes in general, after analysing the data the focus of attention changed towards the number of combined cases with thrombocytopenia.

“The frequency at which they occur in the population is difficult to estimate due to its rareness,” explained Adelaida Sarukhan, immunologist and scientific writer at ISGlobal, to Verificat.

The UK considers these events as “extremely rare” and the most updated cases denote about five cases per million of vaccinated people with AstraZeneca. In Spain, 12 cases related to the injection among the 2.5 million of vaccinated people are being studied, as Fernando Simón, Head of the Health Alerts and Emergencies Coordination Centre, explained in a press conference.

These numbers would also give an incidence of about five cases per million of immunised people. At a global level there are between 1 and 10 cases per million.

Thrombi generated by Janssen’s vaccine are even rarer. The United States reported eight cases of the rare thrombotic events combined with low platelet levels among the seven million people vaccinated on April 13.

After an emergency meeting with the Pharmacovigilance Risk Assessment Committee, the EMA has confirmed the numbers and the similarity to the AstraZeneca vaccination cases.

Half of thrombi are due

to unknown causes

45 per cent of thrombi have unknown causes, according to the Riete register, a database with more than 90,000 patients in 24 countries which is updated month by month.

Rodrigo explains that this lack of knowledge about what the common trigger may be has made it possible to put the spotlight on the vaccine. “There have been millions of vaccinated people in a short time and with more surveillance,” he points out. “This means that there is extraordinary surveillance. If we did so with more drugs, we would probably find a common trigger,” he explains.

After the unknown cases, the more usual reasons causing clots are immobilisation, cancer, surgery, hormone treatment, travelling or pregnancy, according to Riete numbers.