Vaccine Dose Interval in Canada

"I think having the single dose strategy, which was put forth by our Canadian public health agencies, has really saved a lot of lives and has been really instrumental and will be important for us getting back to a more normal life." 

Alyson Kelvin, vaccinologistm University of Saskatchewan, Vaccine and Infectious Disease Organization

"With such a highly protective first dose, the benefits derived from a scarce supply of vaccine could be maximized by deferring second doses until all priority group members are offered at least one dose."

"Given the current vaccine shortage, postponement of the second dose is a matter of national security."

Researchers Danuta M.Skowronski, Gaston De Serres, paper, New England Journal of Medicine

According to multiple vaccine experts, delaying or mixing vaccine doses may seem unorthodox on the surface, but the approaches are rooted in decades of science, backed up by emerging research, and could be preventing COVID-19 deaths. (Ben Nelms/CBC)

 

Canada -- in a carefully considered move advanced by a special committee of medical experts assigned to give critical advice on the coronavirus pathogen in Canada -- decided to take on a move to preserve vaccine doses by lengthily spacing out second doses so that the second dose intended to complete the vaccine manufacturer's recommended time of 21 days between doses, was instead diverted to be used as a first dose for other, unvaccinated people. The theory being that fully vaccinating as many people as possible with two doses is desirable, but vaccinating twice as many people with a single dose to be followed up much later (four months on) with a second dose, would be even more desirable, to give a larger proportion of the population protection against the SARS-CoV-2 virus that causes COVID-19.

Initially, Canada was facing a threatening situation where the onset of a third wave of COVID seemed inevitable, but worse, that third wave would be comprised largely of more infectious variants, mostly the British variant, but followed by the South African one, and more latterly the mutation that took place in Brazil and India. There were concerns over whether the vaccines would be as protective with the variants. And there were larger concerns that with a third wave imminent, not enough vaccines were available to inoculate enough people in high-risk categories.

During the third wave, the variants turned out to be more communicable, but vaccinations proceeded apace, even though Canada lagged well behind other countries in its capacity to vaccinate given its shortage of vaccines. The country began its vaccination program later than many others who were already in possession of a considerable number of vaccine doses; countries who had the manufacturing capacity to produce their own vaccines like the U.K. and the U.S. and India, for example. When Pfizer and Moderna and then AstraZeneca came on line and by contract began delivering to Canada, the vaccinations picked up.

And when the National Advisory Committee on Immunization (NACI) advised the federal government of its conclusion that withholding second doses to those already having been once vaccinated would enable greater numbers of Canadians to obtain at least their first dose, the federal government concurred and so did the provinces and territories. This was in essence, a bold move, a recommendation that came about through desperation to speedily vaccinate as many people as possible, to stay ahead of increasing cases of COVID-19. Canada became the only country to initiate such a program, a controversial program criticized by some within the scientific community as not adhering to manufacturers' guidelines.

Canada's experiment appears to have emboldened other countries to do the same thing, and for the very same reasons, and the U.K. and India have both seen fit to follow suit. Simple math as much as an observed goal succeeding in inoculating a greater percentage of the population with a single dose was the persuasive element.  The first persuasive bit of intelligence that guided the NACI was the  amount of gain realized between the first and second doses, where the Pfizer vaccine first dose is up to 85 percent effective in preventing COVID-19 infection at 15 to 28 days following the shot.

 

A woman walks by a sign advertising for COVID-19 vaccines in Montreal on May 14. (CBC / Radio-Canada)

 

With the second dose that number can rise to 95 percent effectiveness, so all that much is not being gained by giving two doses in the recommended time-frame as opposed to inoculating twice as many people, with a single shot, to be followed at a later date of approximately four months with the second dose. Two people given 85 percent immunity against COVID-19, is an improvement and decided advantage over one person inoculated, given 95 percent immunity.

That scarce supply of vaccine situation that gave birth to the hypothesis of immunizing a greater proportion of the people for greater immunity protection overall, is more latterly being relieved, with vaccine manufacturers like Pfizer, Moderna and AstraZeneca now fulfilling their contractual obligations in providing the numbers of vaccine doses they were contracted for. Which has led to a situation where the latest update on numbers vaccinated now stands at 48.5 percent of Canadians having received at least one dose of the COVID vaccine.

None of the vaccine manufacturers has publicly endorsed the decision taken to stretch out the time interval between doses. There is however, the argument that taking examples from other vaccines, immunologists generally recommend a longer gap between doses; where for example, whooping couch gets a booster after two months. It appears to be generally understood that a longer dosage interval (between six to 12 months) usually increases immunity, while serving to reduce the potential for side effects to occur. It is pointed out as well, that COVID-19 vaccines earned such short dosing intervals resulting from the emergency situation of a devastating world pandemic, and tight timelines.

Among people 80 and over, antibody response was 3.5 times greater when the second dose of the Pfizer vaccine was administered at 12 weeks instead of three weeks, according to a May14 study out of the United Kingdom. Similarly, studies of AstraZeneca effectiveness have seen that immunity is raised when the gap is lengthened. 

 

Healthcare workers with Humber River Hospital administer doses of the Moderna COVID-19 vaccine at a temporary clinic for member’s of Toronto’s Spanish-speaking community at the Glen Long Community Centre on May 14, 2021. (Evan Mitsui/CBC)