McMaster lab to screen for rare antibody that causes blood clots after AstraZeneca vaccination

A team of doctors at Hamilton's McMaster University is preparing to test blood samples from across Canada in search of a rare type of clot linked to the AstraZeneca-Oxford COVID-19 vaccine.

There have been no cases of vaccine-induced prothrombotic immune thrombocytopenia (VIPIT) in Canada, but there have been cases in Europe, prompting Canada's National Advisory Committee on Immunization (NACI) to recommend pausing administration of the AstraZeneca-Oxford vaccine to those under age 55.

The McMaster team is preparing to screen blood samples of Canadian patients who may have had VIPIT.

In the meantime, clinical hematologist Meneka Pai, among doctors involved in the McMaster testing, says she likes the idea of pausing the vaccine for "more study" into its safety.

"Ten days ago, we didn't know that this condition existed, so things are moving really fast," Pai told CBC News. 

"I think when things are moving so fast, just taking this brief pause [allows us to] figure out what's going on … gather more data, tighten up those estimations and continue doing surveillance in Canada to make sure we're not seeing these cases."

When a blood clot occurs, the flow of blood stops, said Pai. 

"In many blood clots, it's platelets, little sticky cells — they basically turn on and they form a jam. If you get a blood clot, it stops healthy blood from flowing to the area. And it actually stops deoxygenated blood from draining, so there's a problem with flowing in and flowing out," said Pai.

"In VIPIT, what happens is that patients get the vaccine, and then it seems that four to 20 days later, their body makes a molecule called an antibody and that antibody actually attacks the person's own platelets. These platelets switch on and they get sticky. Now you have these sticky cells and they start to form clots.

"So the real key with VIPIT is there's this predisposing factor of the vaccine, and then the immune response revs up, and then four to 20 days later, we see this very dangerous antibody forming," the hematologist said.

More women than men, but why?

Pai, who is also a member of Ontario's COVID-19 Science Advisory Table, said while there have been no vaccine-induced VIPIT cases in Canada, the majority of them elsewhere involved people under 55, and more often women.

But Pai said it's not known if this is a predisposition, because countries in Europe preferentially gave the AstraZeneca-Oxford vaccine to younger people and health-care workers. There are more women in the latter group.

"We don't know if it's true that it's women under age 55 who are very at-risk, or is it just because this is who got the AstraZeneca vaccine in the countries that are reporting? So, we're waiting on data," she said.

"Is the U.K. going to tell us something different, because they vaccinated older people? We're sort of keeping our ears open but right now."

Dr. Donald Arnold, medical director of the Platelet Immunology Laboratory at McMaster, said preliminary reports from Europe show people who develop VIPIT had "very positive tests in what's called platelet activation assays."

He said the McMaster lab is "very equipped" to do those tests. The lab runs specialized tests for different types of platelet disorders, including screening for Heparin-induced thrombocytopenia (HIT).

Know the risks

People with HIT sometimes develop a low platelet count and blood clots.

"As it turned out, this vaccine-induced syndrome called VIPIT is very similar to Heparin-induced thrombocytopenia and the testing is quite similar as well," Arnold told CBC News.

"We're the only reference lab in Canada that does this whole battery of testing. It has, up until now, been a fairly niche area and not a common test that's been required although it had its own place in medicine, looking for this type of disorder called HIT."

Arnold said the lab is offering "confirmatory testing" for anyone in Canada suspected of developing vaccine-induced VIPIT.

"I think what really has to happen with this vaccine is people who get it, and doctors who are giving it or overseeing it, have to be aware that this is a potential risk and know what signs and symptoms to look out for."

Symptoms include headaches and seizures

Arnold added: "If a person does end up presenting with some of those signs and symptoms, they should be investigated with routine tests first and then a consultation with a hematologist should happen.

"If there is still a suspicion that this could be a real case of vaccine-induced VIPIT, then they should send the sample to us to do the confirmation." The lab will be able to detect VIPIT in two or three days. 

An advisory issued by the Ontario COVID-19 Science Advisory Table says VIPIT symptoms include:

• Persistent and severe headaches.
• Seizures.
• Blurred vision.
• Shortness of breath.
• Chest or abdominal pain.
• Redness in a limb.

"We just want to give clinicians and Canadians the best information on how to detect this and treat this — that's the goal of the science table," said Pai.

"We're not making vaccine policy. We're here to tell people we know what this is, we know how to treat it, if it happens you're going to be ready to give the patient really good care."