Doctors say they are honing in on the cause of blood clots that may be linked with certain coronavirus vaccines, and add their findings have important implications for how to treat the condition, regardless of whether vaccines cause it.
Even though the link is not firm yet, they’re calling the condition vaccine-induced immune thrombotic thrombocytopenia or VITT. It’s characterized by unusual blood clotting combined with a low number of blood-clotting cells called platelets. Patients suffer from dangerous clots and, sometimes, hemorrhaging at the same time.
It’s been linked most firmly with the AstraZeneca coronavirus vaccine, which is in wide use in Europe and the UK.
The US Centers for Disease Control and Prevention and the Food and Drug Administration are checking to see if Johnson & Johnson’s Janssen vaccine also might cause the blood clots. Both AstraZeneca’s vaccine and the J&J vaccine use common cold viruses called adenoviruses as a carrier and some experts suspect the body’s response to those viral vectors might underlie the reaction. AstraZeneca’s vaccine is not authorized in the US.
The FDA and CDC have asked for a pause in giving out the J&J vaccine while they investigate.
A team led by Dr. Marie Scully, a hematologist at University College London Hospitals, studied 22 patients who developed the syndrome after receiving AstraZeneca’s vaccine, and found they had an unusual antibody response. These so-called anti-PF4 antibodies had only been seen before as a rare reaction to the use of the common blood thinner heparin.
The findings support a theory that an immune reaction might underlie the rare blood clots, but the findings don’t yet explain it, Scully and colleagues reported in the New England Journal of Medicine Friday. What may be going on is a reaction by the immune system with platelets to cause uncontrolled clotting.
If vaccines cause it, it’s still very rare and unusual, they wrote. It might not even be happening any more often in recently vaccinated people than among the population in general.
“The risk of thrombocytopenia and the risk of venous thromboembolism after vaccination against SARS-CoV-2 do not appear to be higher than the background risks in the general population, a finding consistent with the rare and sporadic nature of this syndrome,” they wrote.
“The events reported in this study appear to be rare, and until further analysis is performed, it is difficult to predict who may be affected. The symptoms developed more than five days after the first vaccine dose,” they added.
“In all cases reported to date, this syndrome of thrombocytopenia (low platelet count) and venous thrombosis (blood clot) appears to be triggered by receipt of the first dose of the (AstraZeneca) ChAdOx1 nCoV-19 vaccine. Although there have been a few reports of patients with symptoms consistent with this clinical syndrome after the receipt of other vaccines against SARS-CoV-2, none have yet been confirmed to fulfill the diagnostic criteria,” they added.
But if vaccination can cause the condition, it would be important to recognize that and treat it appropriately — because the usual treatment for blood clots is not recommended for VITT.
