Vaccine-induced immune thrombocytopenia and thrombosis (VITT) is a rare condition in which the body’s immune response to a vaccine triggers the production of an antibody called anti-platelet factor 4 (APF4). These antibodies send the body’s clotting mechanisms into overdrive and cause large or widespread blood clots. The condition occurs only with adenovirus vaccines, including the Johnson & Johnson and AstraZeneca COVID-19 vaccines, and does not occur with the mRNA vaccines, though researchers don’t yet understand why this is the case. It’s also very rare — the most recent Centers for Disease Control and Prevention (CDC) report recorded just 28 cases of VITT by May 2021, when nine million doses of the Johnson & Johnson Janssen COVID-19 single-dose vaccine had been administered. The estimated risk for VITT is about seven in one million, the CDC says. As of October 5, 2021, four people in the United States have died of VITT related to the COVID-19 vaccine. Studies have shown that COVID-19 illness carries a much higher risk of blood clots than do either of the adenovirus vaccines. A study published in July 2020 in Thrombosis Research included nearly 400 patients hospitalized with COVID-19 in Italy. The researchers found that about 20 percent of patients developed venous thromboembolism (VTE), or blood clots in the deep veins, after being hospitalized. “The thing to remember is that [blood clots are] very rare, and it’s much more important to be immune to COVID-19, because COVID-19 can be very serious,” says Sue Pavord, MBChB, a fellow of the Royal College of Physicians, a consultant hematologist at Oxford University Hospitals, and an associate senior lecturer in medicine at Oxford University in the United Kingdom.
Risk Factors for Thrombosis
According to Beverley Hunt, MD, OBE, the chair of the World Thrombosis Day Steering Committee in London, though VITT is still very rare, younger people are at a higher risk than those over 50 — the risk for people under age 50 is about 1 in 50,000, twice as high as for people older than 50. But among that population, it’s difficult to determine who’s at higher risk. “There are a lot of people who believe that they are at a higher risk for VITT because they’ve had blood clots before, but it isn’t related,” says Dr. Hunt. “Having a history of clots doesn’t put you at higher risk.” According to the European Medicines Agency, an organization of the European Union, 25 people experienced blood clots after receiving the AstraZeneca vaccine, at a time when 11 million had received the vaccine. It was unclear if these cases were related to the vaccine or not, but almost all occurred in people younger than 55, and the majority were women. A separate report, published on July 18, 2021, in StatPearls, noted that 80 percent of VITT cases were in females ages 20 to 55 years. For this reason, some countries have recommended against androgen COVID-19 vaccines for younger women. Women of childbearing age are already at a higher risk for blood clots because estrogen increases clotting risk. Although the risk is still very low, clotting risk increases fivefold in pregnant women, according to the CDC. But that doesn’t necessarily mean these factors make women more susceptible to VITT. According to Dr. Pavord, clotting risk can be managed with medications and lifestyle habits. And, she added, VITT risk appears to lower with booster shots.
Warning Signs of Thrombosis
It’s normal to have a sore arm, headache, and generally feel sick after getting vaccinated against COVID-19, but those symptoms clear up in a few days. Symptoms of VITT take about a week to start. “When you have an immune response you have to have time to make antibodies, so we aren’t seeing anybody with any clots until at least five days after being vaccinated,” says Hunt, adding that symptoms usually appear within 5 to 30 days of vaccination. “Very occasionally we see them up to day 42, and those are people who have had a pulmonary embolism or deep vein thrombosis (DVT). We know that DVTs take a while to show themselves, so they’re probably forming in that time frame but not showing clinically until later,” says Hunt. As with all blood clots, time is critically important. Immediate treatment drastically increases your chances of survival. Symptoms from blood clots can vary depending on the location of the blood clot. Warning signs can include:
Leg swelling limited to one legShortness of breathChest painDizzinessAbdominal painHeadacheVisual disturbance
According to Hunt, VITT clots don’t appear in the usual clot sites, such as legs or groin. Around half occur in the main vein that runs down the middle of the head, called a cerebral sinus thrombosis. These can be particularly life-threatening, says Pavord. “These people present with headache, potentially vomiting, and with neurological signs,” says Hunt, adding that around 30 percent of VITT occurs in the neck. The rest typically occur in veins in the legs or abdomen. “The good thing is that the international community of doctors that look after people with clots have been working together through the pandemic, so signs of VITT are very widely known by both patients and medical professionals,” says Hunt. “That’s a good thing, because patients present early and the doctors know about it, and the earlier you treat VITT, the better the outcome.”
