Halving and delaying doses a big gamble on limited science
Getting as many vaccinated as quickly as possible provides some protection
TORONTO — Barely weeks after the approval of COVID19 vaccines, officials are toying with the idea of adjusting dosing schedules to eke out supplies, a gamble that could leave people only partially immune and wobble confidence in an already shaky vaccine rollout.
With provincial premiers warning of dwindling supplies and being forced to work with a “squirt gun” rather than a fire hose, as Manitoba’s Brian Pallister said this week, Canada’s Chief Public Health Officer Dr. Theresa Tam this week revealed the topic of “making adjustments in the face of constrained supplies” is actively being discussed.
Both vaccines approved so far — Pfizer-biontech and Moderna shots — require two doses, 21 days (Pfizer) and 28 days (Moderna) apart. Tam is seeking advice from the country’s immunization advisory committee about what is known, and what isn’t, about stretching out the interval between doses.
It’s not about not ultimately giving people two doses, Tam said, stressing Canada is committed to a two-dose regimen. While available data on how effective vaccines are after that first dose look promising, they’re limited, she said.
Already, Ontario, Quebec and other provinces are no longer holding back half of the doses required for the second jab with each delivery, counting instead on the supply chain to produce the second doses in time. British Columbia this week announced second doses would be administered approximately 35 days after the first in order to “maximize the number of priority populations to receive the first dose.” Quebec residents who have already received the first shot have also been told they’ll have to wait longer than expected for the second.
In the United States, officials are exploring administering half doses of Moderna’s vaccine to people aged 18 to 55. Britain said it will stretch out the time between shots to as long as three months, “effectively turning that country into a living laboratory,” according to STAT news.
Already, Ontario, Quebec and other provinces are no longer holding back half of the doses required for the second jab with each delivery, counting instead on the supply chain to produce the second doses in time.
Canada’s vaccine advisory group has said Moderna and Pfizer vaccines, which share similar technology, can, if authorities are hard pressed, be interchanged. “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,” the group’s chair, Dr. Caroline Quach, told the Canadian Medical Association Journal.
Mixing, halving, delaying. With infections surging and a new super-infectious strain sweeping the U.K., all may be reasonable options to consider, some experts say: getting more people vaccinated as quickly as possible provides some level of protection from severe disease across a wider swath of the population.
But not only is it a gamble — will those second doses come in as expected, and how much space can you safely leave between shots? — there’s no science yet to support a change.
“The available data continue to support the use of two specified doses of each authorized vaccine at specified intervals,” the U.S. Food and Drug Administration said in a statement this week. Deviate, without supporting data, and “we run a significant risk of placing public health at risk, undermining the historical vaccination efforts to protect the population from COVID-19,” the FDA commissioners said.
Federal officials this week said Canada is on track to have more than 1.2 million doses of Pfizer and Moderna delivered by end of January, and that the government is working “diligently” to secure a predictable flow of vaccines. But Ontario Premier Doug Ford said the province will run out of its next shipment of Pfizer shots by next week and that the province is getting only a day or two’s notice before shipments arrive.
Pfizer Canada hasn’t changed its recommendation that half the doses sent to provinces be held back to be used as second doses. The safety and efficacy trials were based on a particular schedule and a particular dose, and the majority of volunteers got the second jab “within the window specified in the study design,” Pfizer’s Christina Antoniou told the Montreal Gazette.
Messaging is key here, said Mcgill University bioethicist Jonathan Kimmelman.
“People may not appreciate that a vaccine proven to be 90 to 95 per cent effective may be considerably less effective if it’s not deployed according to the regimen that was validated in the clinical trial,” he said.
By sending a message that it’s safe to wait longer between doses, people may walk out of clinics after the first jab and think they’re completely immune.
It’s always good to scrutinize the evidence, Kimmelman said. “It’s possible that if you look deeply at the clinical trial evidence ... that you may find some evidence to support a deviation from those dose schedules.” But when deploying a massive intervention, “you want to go with the best evidence you have available,” Kimmelman said. “And we don’t have very firm or solid evidence that deviating will be as protective.”