A new blood-clotting syndrome seen in a small minority of COVID-19 vaccine recipients continues to draw significant attention, but experts maintain the event is exceedingly rare — and treatable, in most cases.
Vaccine-induced thrombotic thrombocytopenia, or VITT, has been identified in a handful of Oxford-AstraZeneca recipients in Canada, causing death in one patient.
Dr. Jeff Weitz, the executive director of the Thrombosis and Atherosclerosis Research Institute (TAARI) at McMaster University, says doctors are learning more about VITT every day.
And that's good news for those worried about potentially developing the disorder, which is so new a definitive link to the AstraZeneca shot hasn't been confirmed.
"We now know how to diagnose this and we know how to treat it," Weitz said. "So even if you are one of those unfortunate, rare individuals — maybe four out of a million — who get it, we know what to do."
Experts maintain the benefits of vaccinations far outweigh the small risk of the clots, with Weitz noting that clots have been detected far more often in COVID-19 patients.
Here's what we know about VITT:
HOW OFTEN DOES IT HAPPEN?
The risk of VITT has been estimated as anywhere from one case in 50,000 doses given, to one in 250,000.
Thrombosis expert Dr. Menaka Pai said that range is due to different reporting standards around the world.
Pai said one in 50,000 to one in 100,000 seems to be an appropriate range, but those numbers can change as more vaccines are administered globally.
"Every day we're giving more shots, and of course, doctors are surveilling for cases, so it's literally changing every hour," she said.
Seven cases were reported in Canada as of Tuesday, out of some 1.7 million doses of AstraZeneca's viral vector product given as of April 24.
Meanwhile, 17 cases of VITT had been confirmed as of April 23 in the United States — out of more than eight-million doses of Johnson & Johnson's viral vector vaccine. AstraZeneca is not approved in the United States, and Johnson & Johnson, though authorized in Canada, has not yet been used here.
Dr. Michael Hill, a stroke research expert and clinical neurosciences professor at the University of Calgary, said the worry is understandable.
He suggested people look at it the other way around — a recipient of an AstraZeneca or Johnson & Johnson shot has 99,999 out of 100,000 odds of being completely fine, with the added benefit of excellent protection from severe COVID-19.
"The risk (of getting VITT) is real, it's not zero," Hill added. "But on the other hand, nothing we do has zero risk."
WHAT'S CAUSING VITT?
Pai said the disorder appears to be an immune response to viral vector vaccines. As antibodies are produced within four to 28 days of the jab, platelets somehow get "switched on" in a small number of people.
"You get this immune phenomenon that causes low platelets, and it gives you very aggressive blood clots," she said.
Pai said it's not clear why a minority of recipients are affected and others are not.
Previous blood clots or family history of clots don't appear to hold significance, she said, nor does being on the birth control pill — which carries its own clotting risk of about 5 in 10,000.
Gender also doesn't seem to indicate who's more susceptible to VITT, Pai added.
"At first they thought women were much more likely to get it, but now we've learned that there's likely only a slight predisposition," she said.
WHAT ARE THE SYMPTOMS TO WATCH FOR?
A number of wide-ranging symptoms could be signs of VITT if they occur four to 28 days after receiving a viral vector vaccine, Pai said.
They include: persistent and severe headache, difficulty moving parts of your body, seizures, blurred or double vision, shortness of breath, or back, chest or abdominal pain. Significant changes in a limb — swelling, redness, a pale appearance or cold feeling in an arm or leg — could also indicate clotting.
Hill said clots typically appear in the head, lungs, legs or stomach, so symptoms associated with those body parts should not be ignored. Weitz added that some VITT clots have been seen in parts of the body not usually associated with clots, including the splanchnic veins that drain the liver, spleen and bowel.
The rare VITT clots have been seen almost exclusively after the first dose of a viral vector vaccine, but Pai noted one case was reported following a second dose in the United Kingdom. That person did not experience a clot after their first dose.
"What that tells me is that it's possible (to see VITT after the second dose), but it's very likely that it's even rarer (than after the first dose)," she said.
WHAT DO YOU DO IF YOU THINK YOU HAVE VITT?
Pai said people should seek medical care if symptoms emerge. A primary care physician — a family doctor, walk-in clinic or telehealth practitioner — can offer guidance "if symptoms are manageable."
But if they appear more severe, head to an emergency room.
Hill said that like many other conditions, VITT appears to be easier to treat when caught early.
HOW IS IT DIAGNOSED?
A doctor would order a complete blood count (CBC) to determine whether a patient's platelets are low, followed by additional blood tests or imaging — an MRI, for example — to detect whether or not a clot has formed.
HOW IS IT TREATED?
Effective treatments are available, Pai said, "if started in a timely fashion," including special types of blood thinners and drugs that modulate the immune system.
She stressed that while rare, the clotting disorder has been serious and treatment may be intensive for some.
"It's not like you get a shot of something and go home," she said. "Generally it will involve hospitalization and some really thoughtful care by a number of different doctors."
Hill said doctors are staying away from using the blood-thinner heparin on VITT cases, which seem to closely resemble another disorder called HIT, or heparin-induced thrombocytopenia.
Weitz said in some cases where platelets have been exceptionally low, steroids have been used.
While the mortality rate among the rare VITT cases worldwide seems to be around 20 per cent, Weitz said that number is expected to decrease.
"We're getting better at knowing what it is and how to treat it, diagnosing it earlier," he said. "But it's still not something you want to get."
This report by The Canadian Press was first published May 4, 2021.
Melissa Couto Zuber, The Canadian Press