The European Medicines Agency (EMA) has concluded there is a link between AstraZeneca’s (AZ) Covid-19 vaccine and “very rare,” unusual, but dangerous blood clots, which in some cases have killed. A safety committee of the agency announced today that “unusual blood clots with low blood platelets should be listed as very rare side effects” of the vaccine.
At a press conference, the EMA put its best face forward and highlighted the benefits of the vaccine, which in a large U.S.-based study was recently shown to be 76% effective at preventing Covid infections after two doses. It also appears that the EMA plans to continue to use the vaccine. Emer Cooke, the EMA’s executive director, commented, “This vaccine has proven to be highly effective to prevent severe disease and hospitalization, and it is saving lives.”
The AZ vaccine is not licensed in the United States and likely won’t be necessary given the other types of vaccine we have available. The other U.S.-available COVID-19 vaccines, from Pfizer, Moderna, and Johnson and Johnson, are not associated with this problem despite being used in many more individuals. In contrast, The AZ vaccine is a mainstay for the EU COVID-19 vaccine program.
Blood clots in legs are common and can lead to breathing problems, heart attacks, and strokes. These can come from many sources like an injury or hardening of the arteries. But the types of clots that have been seen with the AstraZeneca vaccine occur in different places, like the brain and liver, and have been associated with low levels of platelets — the blood cells that normally cause clotting.
The types of clots seen include cerebral (head) venous sinus thrombosis, also called CVST, and splanchic vein clots in the abdomen. There were 62 cases of cerebral venous sinus thrombosis and 24 cases of splanchnic vein thrombosis reported in the European Union’s drug safety databases as of March 22. Eighteen of these cases were fatal. The cases came from the reporting systems of the European Economic Area and the United Kingdom, where 25 million people had received the vaccine.
From early on, the development of AstraZeneca’s vaccine has been problematic at best. But the tie to rare blood clots could have particularly far-reaching implications, both in terms of the vaccine’s use and vaccine hesitancy overall.
The adverse events resemble a condition known as heparin-induced thrombocytopenia, or HIT, a rare phenomenon in which people who are given the anti-clotting drug heparin actually develop widely disseminated clotting problems.
Why would this vaccine cause this problem that was not seen with the other RNA vaccines? The AstraZeneca vaccine is unique in that it uses a whole, chimpanzee virus to deliver SARS-CoV-2 spike protein tor the vaccine. It may be that an immune reaction to parts of the Chimp virus yields antibodies that destroy platelets.
While rare, this problem highlights the need to do long term safety studies with all vaccines. It also suggests that giving a whole, animal virus as a vaccine for another animal virus is not an optimal approach.