Are COVID vaccines still safe? Blood clots, the mystery behind them, and the solution.

The light at the end of this dark tunnel (the year 2020) is finally visible, as large scale vaccinations are in progress. Whether one stayed at home or worked endlessly to bring this pandemic to an end, our team work has contributed to end the terror. Unfortunately, a new hurdle arose in the battle against COVID19- the blood clot formation as a side effect of some vaccines. First reports of the blood clots came from Europe where Oxford-Astra Zeneca vaccine was being administered on a large scale. Despite of being an extremely rare side effect, the occurrences of blood clots promptly led to pauses and changes in vaccination strategies in many EU nations. Recently, Johnson & Johnson vaccines were also found to have a similar side effect (again, extremely rare) in the US too. Both these vaccines are different from the two other titans (Pfizer and Moderna) in their mechanisms. The Pfizer and Moderna vaccines are mRNA vaccines while Astra Zeneca and Johnson & Johnson are viral vector vaccines delivering DNA encoding an altered version of the spike protein.

Viral vector vaccines make use of modified viruses that cannot actively multiply inside our bodies. Though not harmful, the presence of a foreign substance like the virus carrying a target DNA leads to activation of the immune system (the main goal of vaccination) which is sometimes accompanied by side effects like tiredness, headaches, fever, and nausea. These symptoms were experienced especially after the second dose of the AstraZeneca vaccine. These symptoms in a way signify that the immune system is responding to the vaccinations and should not be feared since they subside in a few days.

On the other hand, the blood clots formed as a side effect are very rare, peculiar, and as of now, unexplained. Since the first reports, scientists across the world have started investigating possible outlooks and mechanisms which connect the vaccines with this side effect. Scientists are puzzled by the appearance of multiple clots in such unexpected regions, especially the brain. A recent report published in NEJM that has become the main references for this problem found that the clotting events observed were mainly cerebral venous sinus thrombosis (CVST).

CVST is caused when the clot is in the veins that drain the blood out of the brain. This blockage bursts blood cells out of the veins and spills them into the region, leading to hemorrhage. The report found that these thrombotic events were accompanied by lowered platelet levels in the patients. Hence, these very rare cases of side effects are being referred to as “Vaccine-induced Immune Thrombotic Thrombocytopenia (VITT)” where ‘thrombotic’ refers to the formation of clot and ‘thrombocytopenia’ refers to the loss of platelets.

The published study also determined that loss of platelets is due to Heparin-Induced Thrombocytopenia (HIT): a disorder usually found in patients who have been previously treated by a largely popular blood thinner called heparin. In this disorder, one’s body develops an immune response against heparin and produces antibodies specifically against it. These antibodies also have a region which is specific towards certain proteins expressed on platelets. These antibodies destroy the platelets and release enzymes involved in clotting that predispose the patients to thrombosis. Surprisingly, the people who got vaccinated did not have history of administering heparin, meaning although the process is similar, the initiating cause is unclear and unexplained.

The governments and health authorities are increasingly facing problems with misinformation and risk of reduced trust in vaccines and are supposed to take precautions by pausing and altering vaccine strategies in critical times like these, to avoid panic. But as far as statistics are concerned, for Johnson & Johnson only 6 women out of 7 million vaccinated individuals developed CVST and for the Astra Zeneca vaccine, 169 out of 34 million vaccinated people developed some form of clotting side effect. Recent study also showed that the risk of developing the same condition after suffering from COVID19 is 8-10 times higher. Normally, CVST affects 5 in 1 million people every year (a ratio higher than that reported after COVID vaccination) and is treatable if diagnosed in time. All this data coupled with the fact that the vaccine induced clots can be detected in the 6-14 day after vaccination window point to its preventability. As many densely populated countries enter third waves, we can only hope that vaccinations on a large scale continue despite these setbacks; as that is the only way to prevent bleak consequences in the future.


Thrombotic Thrombocytopenia after ChAdOx1 nCov-19 Vaccination | NEJM

How could a COVID vaccine cause blood clots? Scientists race to investigate (

U.S. Calls for Pause on Johnson & Johnson Vaccine After Blood Clotting Cases – The New York Times (

Higher risk of clots from COVID-19 compared with vaccines: Study | Coronavirus pandemic News | Al Jazeera

Vaccine-induced Immune Thrombotic Thrombocytopenia –

Cerebral Venous Sinus Thrombosis (CVST) | Johns Hopkins Medicine

Heparin-Induced Thrombocytopenia (

What we know about the super-rare blood clots linked to the J&J vaccine (

Article author: Kevin Merchant. Kevin is a MS student at LMU Munich, Germany, who is passionate about Immunology and writing. He aims to simplify latest research so that it becomes accessible to all.

Editor: Sutonuka Bhar. Sutonuka is a PhD candidate at the University of Florida. Her work focuses on host immune responses against viruses and bacterial membrane vesicles.

Check out Antibuddies’ blog post “Are COVID vaccines still safe? Blood clots, the mystery behind them, and the solution.”


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