Mixing vaccines could make the best of ‘bad situation’
In the midst of a variant-driven third wave, Canada’s beleaguered vaccine advisory panel is considering whether to endorse mixing vaccine doses.
With supplies of Astrazeneca mostly exhausted, and future delivery dates uncertain, the National Advisory Committee on Immunization is weighing up options for swapping second doses.
There is no reason why people who have had one dose of Astrazeneca shouldn’t receive a second dose of the same vaccine, the panel said Monday, despite reiterating its earlier stance that MRNA vaccines — Pfizer-biontech and Moderna — are “preferred” over the Astrazeneca and Johnson & Johnson shots, due to a remote risk of an unusual type of blood clotting complication.
But with delays in Astrazeneca supplies, and 36 million doses of Pfizer and Moderna scheduled to be delivered over the next two months, mixing for some people, potentially hundreds of thousands, may become unavoidable.
The panel’s co-chair has already said that Pfizer and Moderna shots are similar enough that people who receive the Pfizer shot could receive Moderna for their second dose, and vice versa. “Let’s say you run out of Pfizer’s vaccine because you’ve administered all your doses, or you don’t know what your patient had at first … this eases up the complexity,” Dr. Caroline Quach-thanh told the Canadian Medical Association Journal in December. Quebec has already announced it would substitute the second Moderna dose with a Pfizer dose, if necessary, to fully vaccinate long-term care residents as soon as possible.
But the vaccine advisory group is also considering combining Pfizer or Moderna with Astrazeneca. The panel said it will make recommendations about “next schedules” once it receives more data.
All three vaccines require two doses. Johnson & Johnson is a one-shot jab.
Pfizer and Moderna, both mrna-based vaccines, deliver genetic instructions that tell the body’s cells to start producing and pushing out harmless pieces of the spike protein found on the surface of the virus that causes COVID-19. The cells make the protein piece, triggering an immune response and priming the immune system to attack the virus should the person later becomes infected with the virus. Tiny droplets of fat called lipid nanoparticles deliver the MRNA into human cells.
Astrazeneca and the J & J vaccine use a different approach. The Astrazeneca shot, developed with the University of Oxford, uses a weakened version of a common cold virus to introduce coronavirus proteins into cells, triggering the body to generate an immune response.
The safety and efficacy of swapping out vaccines is still unproven, though it is being studied. “In an ideal world, we obviously would have kept to the original study protocols,” Montreal cardiologist Dr. Christopher Labos wrote in the Montreal Gazette. But, like the recommendation by NACI to stretch doses out by up to four months, instead of the officially authorized three or four weeks, in order to get at least one dose into more people faster, “the practical realities of the pandemic have forced us to change course and alter our position in an effort to make the best of a bad situation,” Labos said.
The first-dose-fast strategy was bold, “and the right thing to do,” said internal medicine physician and University of Toronto assistant professor Dr. Fahad Razak. “It’s clearly been effective in the United Kingdom.” In January, COVID-19 rates in the U.K. were three times higher than Canada’s. “With smart public health measures” and a delayed second dose vaccine rollout, “their rate is now one-sixth our current rate.”
“But what’s happening now is that many people (in Canada) will be starting to get close to the timing of that second dose. And they’re going to start to wonder in their mind what they should do,” said Razak, a member of Ontario’s COVID19 science advisory table.