The Oxford–AstraZeneca COVID-19 vaccine was developed in Britain by AstraZeneca in Cambridge and the University of Oxford. It was approved for use in the UK vaccination programme on December 30, 2020.

In the last couple of weeks, over 20 European countries have stopped the use of this vaccine due to an unusual reaction that has ensued among some recipients—a clotting disorder characterized by a strange combination of blood clot and thrombocytopenia (low platelet count). Platelets are originally the blood cells that help blood clot. This rare disorder has been confirmed to share some similarities with a syndrome called heparin-induced thrombocytopenia (HIT), in which the body produces antibodies against its own platelets as a result of the immune system being so strongly activated. These rare clots of concern also appeared in unusual parts of the body, such as the brain and abdomen.

The safety committee of the European Medicines Agency (EMA) swung into action and concluded, after a couple of weeks of investigation, that though there truly is a possible link between the vaccine and the clots, they are extremely rare side effects. “The risk of mortality from COVID is much greater than the risk of mortality from these side effects.” Emer Cooke, the EMA’s executive director, told reporters.

As at March 22, around 25 million people had received the vaccine, according to the EMA, and 86 cases of blood clots with low platelets had been reported. This equates to around one case per 290 000 people vaccinated. Most of the cases occurred in women under the age of 60 within two weeks of vaccination, according to the EMA’s findings. It is however not conclusive that women are at higher risk.

In order to solve this puzzle and reach a conclusion on the relationship that exists between the vaccine and blood clotting, as well as glean further salient information about risk factors, the EMA is asking AstraZeneca to conduct necessary investigations and laboratory studies. The agency is also supporting studies by an academic consortium led by Erasmus University Medical Centre, Rotterdam and another led by the University Medical Centre Utrecht and Utrecht University, both in the Netherlands.

The investigations and studies are aimed to answer a number of pertinent questions:

• What are the effects of coronavirus on blood coagulation?

• Are there cases of HIT among cases of blood clots following vaccination?

• Is there a tendency that the already small risk can be cut even further?

• Is this problem restricted to certain populations or geographical locations?

• Is there really a definite association between the vaccine and the syndrome?

Through clinical trials and data collection, the agency hopes to obtain some results from the projects within the next two months.

Behnood Bikdeli, a cardiologist at Brigham and Women’s Hospital in Boston, Massachusetts, stressed that as the hypothetical information regarding a possible relationship between the syndrome and the vaccine becomes amplified, there could be a false increase in the perceived rate at which the syndrome occurs. How? There would be a rise in the reporting rates among those who are vaccinated compared with those who are not, and this could lead to flawed conclusions.

There is therefore a big need to be very careful and accurate in getting our information in order to reach an incontrovertible conclusion.