How some COVID-19 vaccines could cause rare blood clots


An advisory committee to the Centers for Disease Control and Prevention is addressing the safety of the Johnson & Johnson COVID-19 vaccine on April 14, 2021, after the CDC and Food and Drug Administration 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 years.

According to a report from the Vaccine Adverse Event Reporting System (VAERS), which is maintained by the Department of Health & 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 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 (Wis.) Clinic Research Institute. Dr. 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.

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 Dr. 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. Those 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 United States.

On April 7, 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; 18 of those events were fatal.

The reports were culled from reporting in Europe and the United Kingdom, 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 United States, after more than 7 million doses of the Johnson & Johnson vaccines have been administered.

A key question for U.S. 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 on April 13, 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 and 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 United States, 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,” Dr. Offit said.


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