What Is The Link Between COVID-19 Vaccines and Thrombosis Risk?

April 14, 2021
Kevin Kunzmann

A thrombosis expert details cerebral venous thrombosis, the side effect reported in a half-dozen women administered the Johnson & Johnson vaccine.

Just yesterday, the US Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) paused administration of the Johnson & Johnson COVID-19 vaccine at its federal vaccination sites, simultaneously launching an investigation cerebral venous thrombosis cases reported in 6 women aged 18-49 given the one-shot vaccine.

The decision, which advised state with available doses similarly withhold administration pending investigation, followed the reported death of 1 such affected woman, and the critical condition of another.

While the FDA and CDC convenes with the Advisory Committee on Immunization Practices (ACIP) will convene in an emergency meeting today, focus have shifted to what these adverse events—reported in approximately 1 in every 1 million vaccinated Americans—are associated with, and whether the benefit of the Johnson & Johnson vaccine outweighs any risks faced by its authorized patient population.

To understand cerebral venous thrombosis, HCPLive turned to a firsthand clinical and research expert.

Maja Zaric, MD, interventional cardiologist and assistant professor at Zucker Medical School, joined HCPLive to discuss the risk of thrombotic events now observed in a pair of adenovirus-based COVID-19 vaccines: that from Johnson & Johnson, as well as AstraZeneca.

As Zaric explained, the chimpanzee-based adenovirus vector is made inactive, made unable to replicate, and is therefore dosed in “large amounts” to vaccinated persons.

“Each vaccine carries somewhere over 50 billion particles,” Zaric explained. “This is a massive amount of adenovirus that has to be injected because it doesn’t replicate in order to achieve the effectiveness.”

What’s being observed in affected patients is the development of PF4 factor antibodies, which inhibits platelet development—a pathology similar to heparin-induced thrombosis (HIT).

“What happens is there is an abrupt cascade of clotting activated in the venous system that is not typically seen in COVID patients, but broader in the vaccine reaction,” Zaric explained.

This commonly occurs in the patient’s brain or cut systems. Symptoms have occurred up to 2 weeks following vaccination—far beyond the standard local site and symptomatic reactions anticipated with COVID-19 vaccines—and include headache, blurry vision, swelling, high temperatures, and shortness of breath.”

Mortality risk with cerebral venous thrombosis is up to 25%, Zaric said.

“The condition rapidly develops and consumes large amounts of coagulation factors and platelets, therefore causing simultaneous clotting,” she explained. “It can be treated early on, when diagnosed.”

“We’re just starting to pick and recognize these cases in the US,” Zaric added. “I would be alert on those folks who get the vaccination over the next 2-3 weeks.”


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