Leanne Watson spent parts of the last few weeks wondering if she'd made a mistake getting the Oxford-AstraZeneca COVID-19 vaccine in early May — and if she'd have the option to take a different jab the second time around.
While the Halifax resident trusted the science, she said news of extremely rare but serious blood clots associated with AstraZeneca made for an anxious four weeks. Watson said her worries subsided as time went on, but she added that she'll probably choose another vaccine for her second shot.
Watson and other AstraZeneca recipients will be able to opt for either the Pfizer-BioNTech or Moderna product as their second dose, Canada's National Advisory Committee on Immunization said Tuesday.
Dr. Theresa Tam, Canada's chief medical officer, said pairing AstraZeneca's viral vector technology with either of the two mRNA products is safe and effective, citing the "strength of emerging evidence and ... decades of experience with vaccines."
Data from recent studies in Germany, the United Kingdom and Spain suggested mixed-vaccine schedules were safe, with the Spanish trial particularly suggesting a "really robust immune response," Tam said.
But data from a larger study from the United Kingdom is not expected until later this month, according to the University of Oxford.
"I wish it was more clear, but I do trust the scientists and I believe there's people working really hard to figure it out," Watson said. "So if ... they say it's OK to mix, I'm probably going to go with that."
NACI says those who received Pfizer or Moderna can also swap for the other mRNA jab if necessary, but encourages people to stick with the original brand.
Dr. Allison McGeer, an infectious disease consultant at Mount Sinai Hospital in Toronto, said all of the authorized COVID-19 vaccines serve the same purpose on a microbiological level — by showing the immune system how to recognize the coronavirus's spike protein, which latches onto cells when people become infected.
The mRNA and viral vector technologies each use different methods of getting to that endpoint, she said, but both get there.
"So there's no reason to think that there would be an issue with mixing vaccines," McGeer added.
"It's possible it might be (more effective), it's possible it might not be quite as good. But it probably isn't going to make any difference."
Preliminary data from the U.K. study suggested more "reactogenicity" — common side effects including headaches and arm soreness after vaccination — arose when an AstraZeneca dose was followed by Pfizer.
McGeer said that's not cause for alarm, however, because it means the immune system is "being stimulated."
"There's no evidence that says if you have side effects you get better antibodies ... but it is common and it's what happens with many vaccines."
McGeer said certain people would benefit from mixing vaccine doses, including those who got AstraZeneca the first time around but worry about receiving it again.
Tam said the likelihood of a blood clot appearing after the second dose of AstraZeneca is currently estimated at 1 in 600,000. But, she added, that number could change as more people are inoculated.
Watson, who got her first AstraZeneca dose on May 4, was frustrated over the last month to hear guidance changing on the vaccine, with several provinces limiting its use shortly after her appointment.
The 41-year-old teacher still considers AstraZeneca a safe and effective vaccine, noting its role in slowing spread of COVID-19 in the U.K., but she was "slightly panicked" to hear of the rare clotting condition that led to five deaths in Canada.
More than two million Canadians received the AstraZeneca vaccine.
Dr. Zain Chagla, an infectious disease expert and associate professor at McMaster University, said there's likely "a good chunk of people" who won't come back for a second dose of AstraZeneca, regardless of how rare the clotting disorder is.
With emerging variants that could dampen the effectiveness of a single dose, Chagla said it's more important now to get people fully immunized. Offering them a Moderna or Pfizer jab could ensure that.
"We do need to get second doses into people and biologically it makes sense (to mix vaccines)," he said. "They all make spike protein, they just do it in a slightly different way ... and I think getting a second dose of something is probably better than hanging out with just one dose of one vaccine."
This report by The Canadian Press was first published June 1, 2021.
Melissa Couto Zuber, The Canadian Press