Initial results from a new Canadian study are reinforcing the importance of getting that second COVID-19 vaccine shot — particularly if the first dose was AstraZeneca.
The study, which was supported by the federal government, found a single dose of either the Pfizer-BioNTech or Moderna vaccine produced short-term antibody levels more than one-and-a-half times greater than those produced by one dose of AstraZeneca.
“We are certainly trying to encourage individuals, particularly if they’ve gotten the AstraZeneca vaccine, to get that second dose,” said Dr. Philip Awadalla, the national scientific director of the Canadian Partnership for Tomorrow’s Health (CanPath), which conducted the study.
The early results come from over 5,600 dried blood spot samples collected from CanPath’s volunteer pool of more than 330,000 Canadian adults between early February and mid-May. Participants also filled out an online questionnaire answering when they were vaccinated and with what type of shot.
So far, the results have shown that about 10 per cent of those vaccinated with one dose of either Pfizer or Moderna — both mRNA shots — did not develop antibody levels against COVID-19 that were any higher from the unvaccinated population.
Yet that number jumps to 30 per cent of those who have received their first shot from AstraZeneca, which is a viral vector vaccine.
Recipients of all three vaccines didn’t see antibodies develop until two to three weeks after getting their shot, the study found, which is consistent with clinical trials from the vaccine manufacturers.
“The difference (is that) over time, what we see in the AstraZeneca vaccines is they’re not producing as much antibodies as those individuals who were vaccinated with the Pfizer or Moderna vaccine,” Awadalla said.
Based on these results, the researchers behind the study are encouraging Canadians who have received AstraZeneca as their first dose to get either Pfizer or Moderna as their second.
Last week, Canada’s National Advisory Committee on Immunization (NACI) made the same recommendation, saying that current evidence pointed at a “better immune response” from mixing a viral vector and mRNA vaccine.
That news was announced just minutes after Frances Maas, a 55-year-old child care worker who lives in Burlington, Ont., booked her second AstraZeneca dose at a local pharmacy.
While she ultimately went through with the appointment due to fear of potential side effects from mixing vaccines, she says the constantly changing advice about AstraZeneca has her feeling like “a troubled kid that nobody knows what to do with.”
“For me, it was about getting vaccinated as soon as I could,” she said when explaining why she chose AstraZeneca for both doses.
“I’m happy to have the information out there, because it allows people to make their own choice. But I feel like for some people, it’s been hard to make that choice because the guidance around (AstraZeneca) changes almost weekly.”
The study’s authors say the initial results do not include people who have received two doses of AstraZeneca, but they say evidence has shown two viral vector doses still offers good protection against COVID-19.
“The data is extremely promising for those individuals,” Awadalla said, teasing further results from the ongoing study that won’t be released until later this summer.
Maas says she feels safe after getting her second shot. But even if she’s not as protected as those with at least one mRNA dose, she’s willing to take any extra steps deemed necessary to boost her immunity.
“If, down the line of six months, they discovered that I need a booster shot just to top it up or whatever, I’m okay with that too,” she said.
By the summer, researchers are hoping to have collected and tested up to 20,000 dried blood samples that will further shed light on the effectiveness of two doses.
Awadalla says CanPath, partnered with the government’s COVID-19 Immunity Task Force and the Public Health Agency of Canada, is also aiming to collect data from the same participants up to a year after getting vaccinated in order to study the long-term antibody response.
Health officials have been urging Canadians to get their second doses as soon as they are eligible for them, and provinces have accelerated the rollout of second doses to get more people fully inoculated.
Anxiety has been building over the spread of the highly transmissible Delta variant, which has proven to be more resilient to a single vaccine dose than the standard strain of COVID-19, yet is effectively blocked by two doses.
More than 23.5 per cent of eligible Canadians aged 12 and over have gotten both vaccine doses, according to the COVID-19 Vaccine Tracker, while leaving 53 per cent that have only received their first shot.
While Awadalla says Canada’s strategy to rush out single doses and hold off on boosters has proven effective in driving down infections and hospitalizations, he says Canadians should be encouraged to get their second dose “as fast as possible.”
“We shouldn’t let individuals feel that they are protected (with just one dose),” he said. “They are not fully protected.”
— With files from Jamie Mauracher
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