AstraZeneca’s COVID-19 vaccine-related woes grow

European drug regulators are recommending that a class of rare and unusual blood clots be added to the list of possible side effects of AstraZeneca’s COVID-19 vaccine.

The recommendation comes even as the European Medicines Agency (EMA) stresses that the two-dose vaccine is still considered safe. AstraZeneca has also defended the vaccine’s safety but is nonetheless pausing clinical trials in children while UK drug regulators review the link between the blood clots and the injection.

The blood clots are uncommon: of the roughly 25 million people in the European Union and the UK who have received AstraZeneca’s vaccine, 86 have reported developing blood clots. Of the 86, 18 have died. And they are unusual: while most blood clots are rich in platelets, these clots are not.

Instead, they are part of an immune response and similar to blood clots that are sometimes seen in people being treated with heparin, an anticoagulant, EMA executive director Emer Cooke said at a press conference April 7. Most of the post-vaccine blood clots have occurred in women under the age of 60.

“This case clearly demonstrates one of the challenges posed by large-scale vaccination campaigns,” Cooke said. “When millions of people receive these vaccines, very rare events can occur that were not identified during the clinical trials.”

AstraZeneca had previously said that incidences of blood clots during trials were not out of the ordinary compared with what would be expected in the general population, and that fewer blood clots were found in people who had received the vaccine than in those who took a placebo.

The vaccine is one of four that have been authorized for use in the EU. AstraZeneca has not yet applied for an emergency use authorization from the US Food and Drug Administration. The vaccine, developed at Oxford University, is a weakened adenovirus carrying the genetic instructions for the SARS-CoV-2 spike protein.

The EMA’s decision comes as AstraZeneca is criticized for multiple gaffes in the testing of its vaccine, including dosing errors, halting a trial without much public disclosure, and using an incomplete data set to calculate the vaccine’s efficacy.