More than a million Canadians mixed COVID-19 vaccines

Health Canada's latest weekly report on vaccinations shows at least 1.3 million Canadians opted for a mixed-dose finish to their COVID-19 vaccination schedule in June.

The report, published Monday, shows of 6.5 million people who got their second shot between May 31 and June 26, one in five got a different vaccine than their first.

Some provinces began mixing the two mRNA vaccines from Pfizer-BioNTech and Moderna as early as April, depending on what supplies were on hand. The practice became more common in the third week of June, when a delayed shipment from Pfizer pushed many provinces to turn to Moderna only for a few days.

Mixing the two types of vaccines — an mRNA with the viral vector vaccine from Oxford-AstraZeneca — only started in early June. That was after the National Advisory Committee on Immunization said people who got AstraZeneca for their first dose could safely get an mRNA vaccine as their second.

NACI went further on June 17, when it said getting an mRNA vaccine after AstraZeneca was the preferred option. The committee cited the rare but serious risk of blood clots potentially linked to getting the AstraZeneca vaccine and evidence that mixing vaccine types produces a stronger immune response.

Health Canada, which has reported weekly on how many people get each type of vaccine Canada has authorized, added a "combination" category for people whose doses weren't the same, on June 5.

An analysis of that data by The Canadian Press shows at least 627,000 people got AstraZeneca first and then opted for either Pfizer or Moderna.

It appears that during the week when the Pfizer shipment was delayed, nearly 300,000 people who got Pfizer for their first dose were fully vaccinated with a different vaccine. The data does not specify which one.

Dr. Zain Chagla, an infectious diseases doctor at St. Joseph's Healthcare in Hamilton, said many people seemed to take to heart the encouragement to get whatever vaccine they're being offered.

"I think people followed the evidence, they took the NACI suggestions, and it's good," he said. "That means there's more fully vaccinated people out there."

As of June 26, only 189,907 people are listed as being fully vaccinated with two doses of AstraZeneca. That data may be incomplete because Quebec is not providing Health Canada with a breakdown by first and second doses, and is only reporting data on people with at least one dose.

Dr. Christos Karatzios, a pediatric infectious diseases doctor at Montreal Children's Hospital, said if one thing is clear during COVID-19, it is that the vaccines are working and people are vulnerable until they get both doses of any two vaccines.

"Mixing and matching, who cares," he said. "Just get two vaccines into people."

He said it is more important than ever to get both doses as well, because the delta variant of the virus that causes COVID-19 is proving to be a little less affected by only one dose.

Dr. Kieran Moore, Ontario's chief medical officer of health, said Tuesday that between May 15 and June 12, only 1.2 per cent of more than 29,000 confirmed cases of COVID-19 in his province were found in people with two doses of vaccine.

Conversely 83.2 per cent of cases were in people with not even one dose, and 15.6 per cent were in people with only a single dose.

Nationally, Canada has partially vaccinated at least 78 per cent of people over the age of 12, and fully vaccinated more than 42 per cent. Kids under 12 aren't yet eligible to get a vaccine.

This report by The Canadian Press was first published July 6, 2021.