Amid limited supplies of coronavirus vaccines and growing concerns around the spread of more transmissible variants, some Canadian provinces are taking a rationing approach for their rollout by delaying the second dose and putting more needles in arms more quickly with a first shot.
Last week, New Brunswick announced that for low-risk individuals there would be a longer delay between the first and second doses — beyond the recommended time frame for the Pfizer-BioNTech and Moderna vaccines.
In mid-January, Quebec announced that it was pushing the time between the two doses to a maximum of 90 days in an attempt to vaccinate more seniors and health-care workers faster with a first injection. Vaccine manufacturers Pfizer and Moderna propose intervals of 21 and 28 days, respectively.
Now, preliminary data from Quebec shows their strategy is paying off. The National Institute of Public Health of Quebec (INSPQ) said on Feb. 18 that both vaccines from Pfizer-BioNTech and Moderna are said to be approximately 80 per cent effective in preventing disease 14 to 28 days after the administration of a first dose.
Some experts say delaying the second shot will bode well for the entire country.
“In the short term, you can potentially vaccinate twice as many people knowing that you do have to give them their second doses in the future,” said Ashleigh Tuite, an infectious disease epidemiologist at the University of Toronto.
This could “potentially save more lives and prevent more hospitalizations and more severe outcomes,” she said.
“The immediate reduction in sickness, hospitalization, mortality in those people who are receiving a single dose will be great,” said Daniel Coombs, a mathematics professor and epidemiologist at the University of British Columbia.
Reserving the second jab could also allow health authorities to get to the younger population, who typically have more social and work contacts, more quickly, he said.
“There are a lot of benefits to delaying the second dose.”
Due to delays in vaccine supplies from Pfizer, both Ontario and British Columbia decided last month to prolong the wait between doses to 42 days, which is in line with the recommendations from Canada’s National Advisory Committee on Immunization (NACI) and the World Health Organization (WHO).
So far, at least four provinces in Canada have used this strategy, but Coombs, who has been developing COVID-19 models for the B.C. Centre for Disease Control, said it is “inevitable” that more will follow.
Meanwhile, there is emerging early evidence in support of protection and reduced transmission after the first dose.
A new study on Friday by Cambridge University in the U.K. suggests that a single dose of the Pfizer-BioNTech vaccine can reduce fourfold the number of asymptomatic COVID-19 infections. The results of the study are yet to be peer-reviewed.
Another Israeli study published in the Lancet medical journal last week found an 85 per cent reduction in symptomatic COVID-19 infections within 15 to 28 days of receiving a first dose of the Pfizer vaccine.
In the United Kingdom, where the highly contagious B.1.1.7 variant was first detected in mid-December 2020, both the Pfizer and AstraZeneca vaccines are being given 12 weeks apart.
The protocol for giving the booster shot set and used in the clinical trials for AstraZeneca was one month after the first dose. The WHO has said the AstraZeneca vaccine can be given with an interval of eight to 12 weeks.
The U.K.’s move has drawn criticism from some experts, but data suggests that the longer time interval could offer more protection.
A peer-reviewed study in The Lancet on Feb. 19 showed that the 12-week gap between two doses of the AstraZeneca-Oxford vaccine resulted in 81 per cent vaccine efficacy compared to 55 per cent for less than six weeks.
“We know that for other vaccines, oftentimes extending the time between doses, first of all, isn’t detrimental and oftentimes is beneficial,” said Tuite.
“So you get a stronger boosting response when you increase the time between doses.”
A new report in the United States that was published this week recommends that in the face of a resurgence in infections due to the B.1.1.7 variant, deferring the second doses of vaccines to after the surge and for those with confirmed infection should be considered.
“If the B.1.1.7 surge overlaps with low vaccine coverage, it will cause immense strain on an already burdened healthcare system, threatening the level and quality of care available to all patients,” the authors from the University of Minnesota’s Center for Infectious Disease Research and Policy said.
What are the risks?
While there is a buffer and wiggle room of a few days and weeks between vaccine doses, experts have cautioned against significant delays that deviate from the prescribed time period used in the clinical trials.
“The longer you spaced that interval, the higher risk (of) … maybe not getting the full immune response possible,” Dr. Zain Chagla, an infectious diseases physician at St. Joseph’s Healthcare in Hamilton, said in a previous interview with Global News.
Currently, there is no data on the maximum interval between doses or on medium- or long-term efficacy of COVID-19 vaccines, according to the NACI.
In an emailed statement to Global News, Christina Antoniou, director of corporate affairs at Pfizer Canada, said: “It is critical for health authorities to conduct surveillance on implemented alternative dosing schedules to ensure that vaccines provide the maximum possible protection.”
When there is an unstable supply chain, most experts agree that the priority should be to give elderly people the two doses on time.
“We know (for) older people, sometimes the vaccines are not as effective as they are in younger people, so it definitely does make sense to continue to vaccinate some of the older groups on a reasonably tight schedule,” said Coombs.
— With files from Global News’ Heather Yourex-West and Linda Boyle.
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