Rate of COVID-19 vaccine-related blood clots may be as high as 1 in 55,000: NACI

The rate of serious blood clots after getting vaccinated for COVID-19 may be as high as one in 55,000 cases, Canada’s vaccine advisory committee says — nearly twice the rate previously thought.

In updated guidance published Friday, the National Advisory Committee on Immunization (NACI) said the current rate of vaccine-induced immune thrombotic thrombocytopenia, or VITT, as of May 12 is close to one in 83,000 doses administered.

However, the committee said the rate could be higher “as investigations continue” into the condition, which has been linked to the AstraZeneca vaccine.

The NACI has previously said that the rate of VITT is closer to one in 100,000.

The committee also notes that blood clots related to VITT “can be very aggressive and challenging to treat.” Doctors have previously assured that VITT is treatable and that serious effects can be prevented, particularly if symptoms are caught early.

The updated guidance comes on the heels of Canada’s fourth death related to VITT, which was reported Friday.

The New Brunswick woman is the second in that province to die from the condition after receiving the AstraZeneca vaccine. Quebec and Alberta have each reported one death as well.

Federal data updated Friday said as of May 14, 22 cases of serious blood clots have been reported across Canada, all of whom have received the AstraZeneca vaccine. Thirteen of those cases have been classified as VITT due to showing the presence of certain platelet antibodies.

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The NACI’s latest guidance stands behind its recent recommendation that the Pfizer-BioNTech and Moderna vaccines are “preferred” over the AstraZeneca and Johnson & Johnson shots.

The latter two are both viral vector vaccines and have both been linked to VITT, although J&J’s shot has yet to be administered in Canada. Pfizer and Moderna’s shots, which are mRNA vaccines, have seen no instances of blood clotting.

The messaging has prompted criticism from health experts who warn against ranking the vaccines. Instead, they still recommend that Canadians take the first vaccine offered to them.

“It’s really difficult to say that this one class of vaccine is better than the other class when all the vaccines that have been approved in Canada show a very, very high efficacy rate and very good safety records,” Anita Ho, an associate professor of bioethics at the University of British Columbia, previously told Global News.

The NACI appears to justify its recommendation of mRNA vaccines by highlighting the increased supply of Pfizer and Moderna, the “minimal” supply of AstraZeneca, and safety concerns related to VITT.

Provinces have stopped administering AstraZeneca as a first dose, due to concerns over both VITT and supply. Officials say AstraZeneca doses will still be available as second doses for those who have already received it.

Rates of VITT drop as far as one instance in 600,000 after a second dose of AstraZeneca is administered, the NACI noted Friday.

The NACI’s guidance still recommends that two-dose regimens use the same vaccine for both doses. However, it noted that updated guidance on mixing one dose of AstraZeneca with one of either Pfizer or Moderna will come in June, based on new scientific research.

For now, the guidance says a second dose should be of a similar type as the first dose — meaning if AstraZeneca isn’t available, another viral vector vaccine should be used instead.

A Spanish study on mixing COVID-19 vaccines has found that giving a dose of Pfizer’s drug to people who already received a first shot of AstraZeneca vaccine is highly safe and effective

— With files from Saba Aziz, the Canadian Press and Reuters