To date, more than 160 million Americans — about half of the U.S. population — have been fully vaccinated against Covid-19. Yet, some are still holding back due to their concerns surrounding the newness of the vaccines. Here, we take a deeper look into one potential reason for that hesitation: Covid-19 vaccines and the rare risk of blood clots.
While there is a rare risk of blood clots, doctors who’ve been on the frontlines of fighting Covid-19 say it’s safe to receive a vaccine, even for those who have had blood clots in the past.
Indeed, you’re more likely to get a blood clot — also called thrombosis — from contracting Covid-19 itself than from the vaccine.
As of May, the number of people who’ve gotten a blood clot after receiving the Johnson & Johnson vaccine was about 28 out of 8.7 million shots that had been administered. The other vaccines available in the U.S. — the Pfizer and Moderna vaccines — have not been associated with blood clots.
An April study out of Oxford University upholds the assertion that the Covid-19 illness poses far more danger than any rare risk from the vaccine. Covid-19 carries eight to 10 times the threat of blood clots in the brain than the coronavirus vaccines, the study says.
“There are tremendous amounts of inflammation with Covid-19 that essentially causes a thickening of the blood and thus clots,” says Mia Taormina, DO, an infectious disease physician with DuPage Medical Group.
Meanwhile, Alla Gimelfarb, MD, a hematologist with NorthShore University HealthSystem in Glenview, says neither she nor her colleagues have yet seen a blood clot in a patient who received the Pfizer or Moderna vaccine.
“I think one of the important things for people to know is that even for people with a history of clotting there is no increase in risk of clotting from the vaccines,” Gimelfarb says.
The risk is greater from the disease itself. A research letter published in JAMA in July 2020 — before vaccines were available — showed that Covid-19 itself raises the risk of blood clots. Researchers found that 16% of adults with Covid-19 treated in a hospital in New York City had at least one blood clot. And of the patients admitted to the intensive care unit, 29.4% had a blood clot.
“An important thing to know is that having Covid-19 is a huge clotting risk,” says Gimelfarb, who adds that it is common to treat patients with Covid-19 with blood thinners to try to prevent them from getting a blood clot.
The clotting happens because Covid-19 can cause severe inflammation, which then spurs a person’s immune system to activate their clotting system. In addition, when a person is sick they often are immobile, which is another risk factor for clots.
The rare blood clots that have been associated with the Johnson & Johnson vaccine are different from other types of blood clots, says Gimelfarb. These rare clots are thought to be from an immune response that produces an antibody in a person’s clotting system. This causes a decrease in platelets, which are components that help the blood to clot. At the same time, the body’s response causes an unusual clotting specifically in the brain’s blood vessels or abdomen.
Given the extreme rarity of the clotting problem associated with the Johnson & Johnson vaccine, and the much greater risk of getting a blood clot from Covid-19, both Taormina and Gimelfarb advise people to receive one of the Covid-19 vaccines — even those who’ve previously had a blood clot.
“We’ve gotten questions about the vaccines from people who are on blood thinners or who’ve had a history of clotting,” Gimelfarb says. “They ask ‘Is it safe for me?’ and ‘What would you recommend?’ Our advice has generally been to go ahead with the vaccine because the data does not support an increase in clotting risk.”
Taormina says one appeal of the Johnson & Johnson vaccine is that it requires only one shot. The Pfizer and Moderna shots, which are messenger RNA vaccines, require two shots but may be more attractive to people with concerns about the clotting risk.
“My go-to advice is to reiterate that the messenger RNA vaccines don’t appear to have this rare side effect of clots associated with them,” Taormina says.
Anyone who gets the Johnson & Johnson vaccine can keep an eye out for symptoms of a blood clot, such as a headache that doesn’t go away or trouble breathing. These issues will emerge about two weeks after an individual gets a vaccine.
To avoid the risk of blood clots, start by protecting yourself against Covid-19, because the disease itself carries a risk of blood clots, Gimelfarb and Taormina advise.
Other ways to keep blood clots at bay include controlling the conditions that increase the risk of a clot: obesity, smoking, pregnancy, birth control pills, and immobility.
Both Gimelfarb and Taormina say the best ways to avoid blood clots in general are to live a healthy lifestyle that includes maintaining a healthy weight, staying active, and not smoking.
But their most important advice for avoiding clots is to get a Covid-19 vaccine.
“The biggest risk of clots right now is Covid,” Gimelfarb says. “That is number one.”
Annemarie Mannion is an experienced journalist who has covered health and wellness for major newspapers, magazines and associations.
