The U.S. on April 13 paused the use of Johnson and Johnson’s vaccine following adverse reactions in a few vaccinated persons while investigations are on to understand the mechanism of the rare reaction better. This followed the fact that six people in about 7 million vaccinated with J&J’s vaccine developed blood clots in the days following vaccination. A similar adverse reaction to AstraZeneca’s vaccine has been observed in rare cases. In this situation, a German and Austrian group, led by Andreas Greinacher of University Medicine Greifswald, in Germany, has announced a partial understanding of this mechanism and a possible method of treatment.
Age limits
Since the incidence of clotting was observed mostly in younger women, several countries had set age limits for the use of the AZ vaccine. Canada, for example, recommended limiting the use of this vaccine to those above 55 years of age.
Researchers have come closer to identifying the reason for the blood clotting events, seen in rare cases, following vaccination with the AstraZeneca vaccine. In two separate papers published in The New England Journal of Medicine , a group of researchers from Norway and another from Austria and Germany have outlined how this adverse reaction resembles a reaction to heparin — a blood thinner. Heparin can induce a condition where the platelet number dips and blood clots form. This is known as heparin-induced thrombocytopenia (HIT). This is unusual because it is the platelets that help in clotting blood and, for example, preventing damage at injury sites.
The mechanism
After vaccination, the vaccinated person develops specific antibodies which can bind to the platelets or thrombocytes and activate them to form clots which could eventually block the blood vessel (thrombosis). There is an accompanying decrease in the free platelets (thrombocytopenia).
The researchers are not sure whether it is the vaccine that is causing the reaction or it is due to some factor in the person’s constitution.
The researchers differentiate the blood clots arising from vaccination from HIT and have also outlined a way to test patients exhibiting the worrying symptoms and to manage the condition. They have developed a screening assay to determine whether the person has developed these particular antibodies.
The symptoms
Most people experience flu-like symptoms and pain for a day or two after the vaccination. This is not a cause for concern. Symptoms of the adverse side-effect include dizziness, headache, visual disturbance, nausea, shortness of breath, acute pain in chest, abdomen, or extremities accompanied by thrombosis, and these occur approximately between five and sixteen days after vaccination. This needs medical attention. The researchers advise first ruling out heparin-induced thrombocytopenia and then testing for vaccine-induced immune thrombotic thrombocytopenia (VITT) followed by specific treatment of the condition with immunoglobin.
The J&J vaccine is of the same type as the AstraZeneca’s vaccine, in that both use an adenovirus to convey the instruction to human cells to make and recognise the spike protein of the SARS CoV-2 virus. This, in turn, teaches the immune system to guard against invasion by the virus. However, at the time of writing this report, researchers are not sure whether this commonality can be extended to infer that the blood clot reaction is due to the common factors in the vaccines themselves.
