Blood clots associated with COVID-19 vaccine are extremely rare

Several days before the Oct. 19 start of the National Basketball Association season, a widely circulated headline said a 26-year-old NBA player suffered blood clots shortly after receiving a COVID-19 vaccine.

The headline, shared in an Instagram post, referred to guard Brandon Goodwin, who played for the NBA’s Atlanta Hawks last season and before that with the Denver Nuggets and at Florida Gulf Coast University. 

Goodwin, who is not currently with an NBA team, said himself in an October video that he blamed the vaccine "a thousand percent" for clots that cut short his 2020-21 season with the Hawks. 

He seemed to walk back his claim when he later tweeted: "I got sick. Maybe it was the vaccine maybe it was covid (i don’t know) I’m not a expert." 

Details of Goodwin’s medical history aren’t public, so we weren’t able to fact-check his claim or the Instagram post. Nevertheless, his description of his experience has drawn new attention to the potential connection between the vaccine and blood clots. 

Researchers are still studying the connection to understand whether there’s enough clear evidence of a causal relationship. 

What they know so far suggests there is a plausible causal connection, but it involves only one of the vaccines — the least-used one — and an exceptionally rare condition.

The research on blood clots involves the Johnson & Johnson single-shot vaccine, which received emergency use authorization from the Food and Drug Administration in February, three months after the Pfizer and Moderna vaccines were authorized.

In April, the FDA and the federal Centers for Disease Control and Prevention recommended a pause on the Johnson & Johnson vaccine while they investigated six reports of a rare type of blood clot among the more than 6.8 million people who had received the vaccine.

The six people experienced a severe type of blood clot affecting the brain, along with low levels of blood platelets, which help to stop bleeding. Clots block the flow in blood vessels, and low platelets can lead to dangerous internal bleeding.

All six were women between the ages of 18 and 48, and each developed symptoms six to 13 days after vaccination, the two federal agencies said in a statement. One woman died, and another was in critical condition, officials from those agencies said at the time.

After a 10-day review, the two agencies recommended resuming use of the Johnson & Johnson vaccine. They determined that the benefits outweighed the potential risks, but warned that women under 50 years in particular "should be aware of the rare but increased risk" of what’s known as thrombosis (blood clots) with thrombocytopenia (low blood platelets count) syndrome, or TTS. 

There have since been a few dozen more reported cases of blood clots among people who received the Johnson & Johnson vaccine, which has now been administered to about 15 million people.

In evaluating the safety of a vaccine, scientists look at the risks it may pose and how they compare with the benefits — specifically, preventing infection or serious illness from the disease. They continue to monitor that risk over time to determine whether further action or warnings are needed.

Here are some of the factors they consider: 

Only one vaccine: Johnson & Johnson is the only vaccine among three in used in the U.S. that has been associated with TTS — and it’s the least administered. More than 174 million people are fully vaccinated with either the two-dose Moderna or the two-dose Pfizer vaccines.

"Plausible" causality: There is a "plausible causal relationship" between the Johnson & Johnson vaccine and TTS, according to the CDC, meaning that researchers suspect a connection and have not ruled the vaccine out as a cause. No increased risk of TTS has been detected with the Pfizer and Moderna vaccines.

TTS is serious: It has serious potential consequences, including death, but the virus poses a greater danger. "COVID-19 has a substantial risk of serious outcomes" and those risks "far outweigh the risk of very rare side effects such as TTS" associated with the vaccine, according to the American Society of Hematology.

It’s rare: As of Oct. 6, the CDC and FDA identified 47 confirmed reports of vaccinated people in the U.S. who later developed TTS. That’s 0.0003%. 

It’s been confirmed that three of those people died, though the CDC is continuing to investigate cases, a CDC spokesperson said.

While the earliest reports involved women, there have been cases among men. A CDC analysis done in July, when 12.5 million Johnson & Johnson doses had been given, found 38 confirmed TTS cases at that time, including 10 among men age 18 to 29. 

"I think the J&J vaccine can cause blood clots, but they are extremely rare and the risks are outweighed by the benefits in protection against COVID-19," said Dr. Walter Orenstein, professor and associate director of the Emory Vaccine Center at Emory University.

Goodwin said in the video that he began experiencing health problems after receiving the vaccine.

Basketball players and other high-performance athletes "have been well known to have higher risk for blood clots," said Dr. Shruti Gohil, associate medical director for epidemiology and infection prevention and assistant professor of infectious diseases, at the University of California, Irvine, School of Medicine. "Athletes have a low resting heart rate, dehydration, traveling long distances, trauma and a whole host of other risk factors."

COVID-19 infection itself "is very well known to cause blood clots," she added, "so your odds of getting COVID-induced blood clots if you catch COVID are higher than the vaccine induced blood clots."

A spokesperson for Janssen Americas, the Johnson & Johnson subsidiary that makes the vaccine, said the company is working with health officials around the world "to ensure healthcare professionals and individuals are fully informed, so that rare events can be identified early and treated effectively."