How Some COVID-19 Vaccines Could Cause Rare Blood Clots 

Brenda Goodman

April 14, 2021

An advisory committee to the Centers for Disease Control and Prevention (CDC) is addressing the safety of the Johnson & Johnson COVID-19 vaccine today after the CDC and FDA recommended that states hold off on using it pending a detailed review of six cases of the same kind of rare but serious event — a blood clot in the vessels that drain blood from the brain combined with a large drop in platelets, which increases the risk for bleeding.

This combination can lead to severe strokes that can lead to brain damage or death. Among the six cases reported, which came to light over the past 3 weeks, one person died, according to the CDC. All six were women and ranged in age from 18 to 48.

According to a report from the Vaccine Adverse Event Reporting System (VAERS), which is maintained by the US Department of Health and Human Services, the woman who died was 45. She developed a gradually worsening headache about a week after receiving the Johnson & Johnson vaccine.

On March 17, the day she came to the hospital, she was dry heaving. Her headache had suddenly gotten much worse, and the left side of her body was weak, which are signs of a stroke. A CT scan revealed both bleeding in her brain and a clot in her cortical vein. She died the following day.

In addition to VAERS, which accepts reports from anyone, the CDC and FDA are monitoring at least eight other safety systems maintained by hospitals, research centers, long-term care facilities, and insurance companies for signs of trouble with the vaccines. VAERS data is searchable and open to the public. Most of these systems are not publicly available to protect patient privacy. It's unclear which systems detected the six cases cited Tuesday by federal regulators.

"These are very serious and potentially fatal problems occurring in a healthy young adult. It's serious and we need to get to the bottom of it," said Ed Belongia, MD, director of the Center for Clinical Epidemiology and Population Health at the Marshfield Clinic Research Institute in Marshfield, Wisconsin. Belongia leads a research team that helps the CDC monitor vaccine safety and effectiveness. 

"Safety is always the highest priority, and I think what we've seen here in the past 24 hours is our vaccine safety monitoring system is working," he said.

Others agree. "I think what CDC and FDA have detected is a rare, but likely real adverse event associated with this vaccine," said Paul Offit, MD, director of vaccine education at Children's Hospital of Philadelphia, Philadelphia, Pennsylvania.

Although much is still unknown about these events, they follow a similar pattern of blood clots reported with the AstraZeneca vaccine in Europe. That vaccine is now sold under the brand name Vaxzevria

This has experts questioning whether all vaccines of this type may cause these rare clots.

"I think it's likely a class effect," said Offit, who was a member of the FDA advisory committee that reviewed clinical trial data on the J&J vaccine before it was authorized for use.

Adenovirus Vaccines Scrutinized

Both the Johnson & Johnson and Vaxzevria vaccines use an adenovirus to ferry genetic instructions for making the coronaviruses spike protein into our cells.

Adenoviruses are common, relatively simple viruses that normally cause mild cold or flu symptoms. The ones used in the vaccine are disabled so they can't make us sick. They're more like Trojan horses. 

Once inside our cells, they release the DNA instructions they carry to make the spike protein of the new coronavirus. Our cells then crank out copies of the spike protein, which then get displayed on the outer surface of the cell membrane where they are recognized by the immune system. 

The immune system then makes antibodies and other defenses against the spike so that when the real coronavirus comes along, our bodies are ready to fight the infection.

There's no question the vaccine works. In clinical trials, the Johnson & Johnson vaccine was 66% percent effective at preventing against moderate to severe COVID-19 infection, and none of the patients who got COVID-19 after vaccination had to be admitted to the hospital or died.

The idea behind using adenoviruses in vaccines isn't a new one. In a kind of fight-fire-with-fire approach, the idea is to use a virus, which is good at infecting us, to fight a different kind of virus.

Researchers have been working on the concept for about 10 years, but the COVID-19 vaccines that use this technology are some of the first adenovirus vector vaccines deployed in humans. 

Only one other adenovirus vaccine, for Ebola, has been approved for use in humans. It was approved in Europe last year. Before the Johnson & Johnson vaccine, no other adenovirus vector has been available for use in humans in the United States.

There are six adenovirus-vector vaccines for COVID-19. In addition to AstraZeneca and Johnson & Johnson, there's the Russian-developed vaccine Sputnik V, along with CanSino from China, and the Covishield vaccine in India.

Adenovirus vaccines are more stable than the mRNA vaccines. That makes them easier to store and transport. 

But they have a significant downside, too. Because adenoviruses infect humans out in the world, we already make antibodies against them. So there's always a danger that our immune systems might recognize and react to the vaccine, rendering it ineffective.  For that reason, scientists try to carefully select the adenovirus vectors, or carriers, they use.

The two vaccines under investigation for blood clots are slightly different. The Johnson & Johnson vaccine uses the vector AD26, because most of the population lacks preexisting immunity to it. Vaxzevria uses an adenovirus that infects chimpanzees, called ChAdOx1. 

Vaxzevria has been widely used in Europe but has not yet been authorized in the US.

Last week, the European Medicines Agency, Europe's counterpart to the FDA, ruled that unusual blood clots with low blood platelets should be listed as rare side effects on the Vaxzevria vaccine.

The decision came after reviewing 62 cases of cerebral venous sinus thrombosis (CVST) linked to the vaccine and 25 cases of another rare type of clot, called a splanchnic vein thrombosis. Splanchnic veins drain blood from the major organs in the digestive system, including the stomach, liver, and intestines. Eighteen of those events were fatal.

The reports were culled from reporting in Europe and the UK, where around 25 million people have received the Vaxzevria vaccine, making these clots exceptionally rare, but serious.

So far, six cases of CVST have been reported in the US, after more than 7 million doses of the Johnson & Johnson vaccines have been administered.

A key question for US regulators will be the background rate for these types of rare combinations of clots and deplenished platelets. The background rate is the number of events that would be expected to occur naturally in a population of unvaccinated people. On a press call Tuesday, Peter Marks, MD, PhD, director of the FDA's Center for Biologics Evaluation and Research, was asked about the frequency of this dangerous combination. He said the combination of low platelets and clots was so rare that it was hard to pinpoint, but might be somewhere between 2 to 14 cases per million people over the course of a year.

The first Johnson & Johnson doses were given in early March. That means the six cases came to light within the first few weeks of use of the vaccine in the US, a very short amount of time.

"These were six cases per million people for 2 weeks, which is the same thing as 25 million per year, so it's clearly above the background rate," Offit said.

Studies Suggest Possible Mechanism

Last Friday, 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 in New York City, 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 Tuesday alerted doctors to keep a high index of suspicion for VIIT 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 (ACIP) 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 US.

Last week, for example, the UK restricted the use of the AstraZeneca vaccine in people younger than age 30, saying the risks and benefits of vaccination are "more finely balanced" for this age group.

With cases of COVID-19 rising again in the US, 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 co-founded 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?" said Bastian, who lives in Australia.

"In a country that size with that fragmented [of] a healthcare 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.

For more news, follow Medscape on Facebook, Twitter, Instagram, YouTube, and LinkedIn

Comments

3090D553-9492-4563-8681-AD288FA52ACE
Comments on Medscape are moderated and should be professional in tone and on topic. You must declare any conflicts of interest related to your comments and responses. Please see our Commenting Guide for further information. We reserve the right to remove posts at our sole discretion.
Post as:

processing....