Clots due to AstraZeneca vaccine – a reality check

Recent reports of a temporary pause in use of AstraZeneca vaccine due to cases of venous thromboembolism (VTE) in some European countries (Austria, Denmark, Norway and Iceland) have been going viral across the world. India is primarily using this vaccine manufactured locally by Serum Institute of India (Covishield) and hence this issue is of importance to us directly. Let us therefore consider this issue in some detail.

The hallmark of COVID-19 as a disease is the micro-thrombosis it causes in various organs, leading to severe conditions such as strokes, deep vein thrombosis and heart attacks. As the virus is known to cause thrombo-embolic phenomena, it is natural to suspect whether a vaccine could trigger the body and cause similar occurrences. However, one must also consider that Covishield is a genetically modified non-replicating type of vaccine, meaning it cannot multiply in the body. As a result, the chances of it spreading throughout the body to cause such phenomena are extremely remote.

If the vaccine is to be accused of causing thrombo-embolism, a simple way to find out would be to compare the incidence of VTE in vaccinated persons against the incidence of VTE in general population. And we do have large data available for this comparison.

The incidence of venous thrombo-embolism in patients of European ancestry is 104 to 183 per 100,000 patient-years. It is a major health hazard in the West with over a million cases annually in just six countries of the European Union. A 2018 study of VTE in France found that the incidence was as high as 184 per 100,000 population.

The European Medicines Agency has reported that as of 9th March there were 22 cases of VTE in three million vaccinated population and that there was no specific issue with the AstraZeneca vaccine. The BBC has given a number of 30 cases of VTE in five million vaccinated persons in Europe. This translates to 30 cases per 50,00,000 population, or just 0.6 per 100,000 population – considerably lower than the incidence of VTE in the general population. Even if we were to assume that this number is for just three months and multiply it by four to account for a year, the number barely reaches 2.4 per 100,000 population.

But this is not all. We actually have precedents to what is happening with clots and the AstraZeneca vaccine. Influenza and its H1N1 variant are known to increase the risk of developing VTE. As cases of VTE following influenza vaccination had been reported, the influenza vaccine was extensively studied for potential link to occurrence of VTE due to flu vaccination. The results are a great disappointment for anti-vaxxers. Studies found that flu shots actually brought down the incidence of VTE due to long distance travel by 26%. Another study found no difference in the incidence of VTE after studying 1488 cases of VTE following flu shots including H1N1 variant and comparing the incidence in vaccinated and non-vaccinated population. They did find an increased risk in smokers (not surprising as smoking is a known risk factor for developing VTE).


All available data at present suggests that there is no cause for alarm. The incidence of venous thromboembolism in population is, if anything, lower in the vaccinated group rather than higher. And this is the reason why, despite pausing vaccination with AstraZeneca vaccine, the authorities of various states have reiterated that the vaccine is safe, and that they are just being cautious. The International Society on Thrombosis & Haemostasis (ISTH) has also issued a statement that the continued use of AstraZeneca vaccine heavily outweighs any potential risk with regard to thromboembolism.

Therefore, it is no surprise at all that India has ignored this controversy and is continuing with its mass vaccination campaign in full gusto. My recommendation (after taking two doses of Covishield myself and getting all senior citizens in my family vaccinated with Covishield) is to please go ahead with the shot. As far as clots are concerned, the risk is negligible and I am quite certain that the countries that have paused their programs will restart it within a very short time.

Update: since many are curious about how the incidence of VTE can be 97% lower than general population, I offer this simple explanation: it’s most likely an artefact. 50-60% of cases of DVT are asymptomatic. If we factor this into the equation, the number of asymptomatic VTE following vaccination that was missed / not taken into account can be increased by 60%. Even then, the numbers would fall well within the range for general population.

Differing opinions are welcome as long as you keep it clean, so go for it :)