Oxford–AstraZeneca Vaccine Not Associated With Overall Risk of Thromboembolic Events, Says EMA

The European Medicines Agency (EMA)'s Pharmacovigilance Risk Assessment Committee (PRAC) has shared a statement after concluding its review of the Oxford–AstraZeneca COVID-19 vaccine following reports of thromboembolic events occurring in a small number people.

As of March 16, 20 million people in the UK and the European Economic Area had received the Oxford–AstraZeneca COVID-19 vaccine. At this time point, seven cases of blood clots in multiple vessels (known as disseminated intravascular coagulation, or DIC) and 18 cases of cerebral venous sinus thrombosis (CVST) had been reported to the EMA.

PRAC has confirmed that the benefits of the COVID-19 vaccine continue to outweigh the risk of side effects, and that the vaccine is not associated with an increase in the overall risk of thromboembolic events. The statement also emphasized that there is no evidence of any problems related to a specific batch of the vaccine or issues with manufacturing sites.

PRAC worked alongside blood disorder experts to conduct this review, according to the statement. It found that the majority of  reported cases had occurred in females that were under the age of 55 years. "Because these events are rare, and COVID-19 itself often causes blood clotting disorders in patients, it is difficult to estimate a background rate for these events in people who have not had the vaccine," the EMA said.

Based on pre-COVID-19 figures, the committee determined that less than one reported case of DIC might have been expected by March 16 within 14 days of receiving the Oxford–AstraZeneca COVID-19 vaccine in individuals under the age of 50 years. Five cases had been reported. In contrast, an average of 1.35 cases of CVST might have been anticipated in the same age group, but there had been 12. PRAC states that the demonstrated efficacy of the vaccine in preventing death and hospitalization with COVID-19 outweighs the "extremely small" likelihood of developing DIC or CVST.

"This is a sign of a well-prepared response to the vaccine role out which was always going to be accompanied by safety worries. What matters is that they are thoroughly investigated and that the findings are communicated as soon as possible," said Ian Douglas, professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine.

The initial reports of blood clotting-related issues had led certain European leaders to halt the vaccine rollout in countries such as Ireland and Germany. Scientists have expressed their concerns over the potential impact this decision may have on vaccine hesitancy. Dr Michael Head, senior research fellow in global health at the University of Southampton, said, "It is absolutely right that countries investigate safety signals as soon as they are observed. This is a hugely important part of any vaccine campaign. However, I’m still not clear why the decision of several countries was to withdraw the vaccine from use. It is entirely possible to continue the rollout whilst adverse events are investigated."

“Withdrawing a vaccine during a pandemic will have other consequences, including leaving people unprotected against COVID-19 when they may otherwise have been immunized. There may also be a rise in vaccine hesitancy, and that may be difficult to overcome in the short to medium term," he added. 

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