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A look at what we know about VITT, the rare blood clotting disorder

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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 at least 18 Oxford-AstraZeneca recipients in Canada, with 10 more under review. There have been three deaths associated with the condition.

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The disorder, characterized by low platelet counts, has been associated with but not definitively linked to the viral vector shots from AstraZeneca and Johnson & Johnson.

Some provinces have started to move away from using AstraZeneca for first doses. Canada’s increased supply of mRNA vaccines from Pfizer-BioNTech and Moderna means more Canadians are getting access to those shots.

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Here’s what we know about VITT:

How often does it happen?

The National Advisory Committee on Immunization (NACI) says the rate of VITT in Canada was closer to one per 100,000 doses given as of April 28.

More than two million doses of AstraZeneca have been administered in Canada. Last week, Ontario said the risk in that province was closer to one in 60,000.

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The Ontario Science Table says there have been few reported cases of VITT with the Johnson & Johnson vaccine used in the United States, where the incidence appears to be approximately one case per 500,000 doses administered.

AstraZeneca is not approved in the United States, and Johnson & Johnson, though authorized in Canada, has not yet been used here.

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What’s causing VITT?

Experts say 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.

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, experts say, 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.

What are the symptoms to watch for?

A number of wide-ranging symptoms could indicate VITT. Initially, NACI said to watch for signs between four and 20 days after receiving a viral vector vaccine, but recently extended that timeline to four to 28 days.

Read more: AstraZeneca vaccine 1st dose recipients in limbo after some provinces press pause

Symptoms 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.

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Clots typically appear in the head, lungs, legs or stomach, so symptoms associated with those body parts should not be ignored. 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.

What do you do if you think you have it?

Experts say 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.

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, sometimes followed by additional blood tests or imaging — an MRI, for example — to detect whether or not a clot has formed.

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How is it treated?

Effective treatments are available, including special types of blood thinners and drugs that modulate the immune system.

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While rare, experts say the clotting disorder has been serious and treatment may be intensive for some.

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.

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