Studies suggest possible mechanism
On April 9, the New England Journal of Medicine published a detailed evaluation of the 11 patients in Germany and Austria who developed the rare clots after their Vaxzevria vaccines.
The study detected rare antibodies to a signaling protein called platelet factor 4, which helps to coordinate clot formation.
These same type of antibodies form in some people given the blood thinning drug heparin. In those reactions, which are also exceptionally rare, the same type of syndrome develops, leading to large, devastating clots that consume circulating platelets.
It’s not yet clear whether people who develop reactions to the vaccines already have some platelet factor 4 antibodies before they are vaccinated, or whether the vaccines somehow spur the body to make these antibodies, which then launch a kind of autoimmune attack.
The researchers on the paper gave the syndrome a name, vaccine-induced thrombotic thrombocytopenia (VITT).
It’s also not clear why more cases seem to be in women than in men. Andrew Eisenberger, MD, an associate professor of hematology and oncology at Columbia University, New York, said the most common causes of cerebral venous sinus thrombosis have to do with conditions that raise estrogen levels, like pregnancy and hormonal contraception.
“Estrogen naturally leads to changes in several clotting proteins in the blood that may predispose to abnormal blood clotting in a few different sites in the body,” he said. “The clotting changes we are encountering with some of COVID-19 vaccines are likely to be synergistic with the effects of estrogen on the blood.”
No matter the cause, the CDC on April 13 alerted doctors to keep a high index of suspicion for VITT in patients who have received the Johnson & Johnson vaccination within the last 2 weeks. In those patients, the usual course of treatment with blood thinning drugs like heparin may be harmful.
Symptoms to watch for include severe headache or backache, new neurologic symptoms, severe abdominal pain, shortness of breath, leg swelling, tiny red spots on the skin, or easy bruising.
Grappling with evidence
The CDC’s Advisory Committee on Immunization Practices will meet today in an emergency session to review the cases and see if any changes are needed to use of the J&J vaccine in the United States.
Last week, for example, the United Kingdom restricted the use of the AstraZeneca vaccine in people aged younger than 30 years, saying the risks and benefits of vaccination are “more finely balanced” for this age group.
With cases of COVID-19 rising again in the United States, and the Johnson & Johnson vaccine currently the most convenient form of protection against the virus, the committee will have to weigh the risks of that infection against the risk of rare clots caused by vaccination.
They will also likely have to rule out whether any of the cases had COVID. At least one study has reported CVST clots in three patients with confirmed COVID infections. In Europe, COVID infection did not seem to play a role in the formation of the clots with low platelets.
Hilda Bastian, PhD, a clinical trials expert who cofounded the Cochrane Collaboration, said it won’t be an easy task. Much will depend on how certain the committee members feel they know about all the events linked to the vaccine.
“That’s the really, really hard issue from my point of view for them right this moment. Have we missed any? Or how many are we likely to have missed?” asked Dr. Bastian, who lives in Australia.
“In a country that size with that fragmented [of] a health care system, how sure can you be that you know them all? That’s going to be a really difficult situation for them to grapple with, the quality of information that they’ve got,” she said.
A version of this article first appeared on.