Ontario’s government announced on Saturday that long-term care residents, staff and essential caregivers who have received their first doses of the Pfizer-BioNTech vaccine will now get the second jab within 21 to 27 days. Everyone else will receive their second dose after three weeks but before 42 days.
The time span between doses specified by Pfizer and BioNTech is 21 days.
This comes in the wake of a temporary delay in shipments of the Pfizer vaccine to Canada as the company is scaling up its European manufacturing capacity.
Last month, British Columbia said it was changing its second-dose schedule to 35 days to allow giving the first doses to as many people as possible right away.
Meanwhile, Quebec is pushing the time between the two doses to a maximum of three months in an attempt to vaccinate more seniors faster with a first injection.
These timeline changes have raised concerns and questions about the impact this may have on the effectiveness of the vaccines.
“There is a risk in this approach,” said Dr. Alberto Martin, professor of immunology at the University of Toronto.
“It’s possible that a large fraction of these individuals will not develop full immunity and thereby waste many of these doses,” he told Global News.
While there is a buffer and wiggle room for 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,” said Dr. Zain Chagla, an infectious diseases physician at St. Joseph’s in Hamilton, in an interview with Global News.
However, since this is an experiment with no sufficient data, it still remains unclear what the exact impact of the delay will be, both Chagla and Martin said.
The World Health Organization (WHO) has advised that in case of shortages in vaccine supplies, the second Pfizer dose can be extended to up to six weeks or 42 days after the first, which is what Canada’s National Advisory Committee on Immunization (NACI) has also recommended.
Currently, there is no data on the maximum interval between doses or on medium- or long-term efficacy of COVID-19 vaccines, according to NACI.
NACI experts say delaying the second dose up to six weeks, instead of three or four, could more quickly get at least some protection against COVID-19 to more people.
But anything beyond that would wade into “tricky territory” with the risk of not getting the appropriate response and long-term immunity from the vaccine, Chagla said.
Since the start of the nationwide rollout in mid-December, provinces have been weighing different options around distribution, and whether to reserve the second dose as vaccine supplies from Pfizer and Moderna slowly trickle into the country.
The United Kingdom, where a new, more contagious variant of COVID-19 began spreading last month, has gone against the advice of the drugmakers, as well as WHO, and delayed the second dose of Pfizer and AstraZeneca’s vaccine by up to 12 weeks.
The United States, meanwhile, is taking a different approach and sticking with the originally specified timeframe of 21 days between Pfizer’s shots and 28 days for Moderna.
In a statement on its website, the U.S. Food & Drug Administration says changes in the schedules of vaccine administration without appropriate supporting data can pose a “significant risk of placing public health at risk” and “undermining the historic vaccination efforts.”
Prioritizing the elderly
When there is an unstable supply chain, most experts agree that the priority should be to give the elderly and long-term care residents the two doses on time.
Dr. Isaac Bogoch, an infectious disease physician at Toronto General Hospital, said the focus in Ontario is to put all the resources into vaccinating those who are most vulnerable and at greatest risk of death.
“We know that people in long-term care are typically older or have other health problems, (yet) are just less likely to mount the same immune responses as otherwise healthier, younger individuals. So, the goal is really to give those two vaccine doses as per that schedule to those who live in long-term care. I think that’s very reasonable,” he told Global News.
As for the rest of the population, provinces should not purposely delay the second dose beyond the 42-day mark, he added.
“That 42 days is the maximum, it’s not the goal, … and if people can be closer to that day 21 goal, the better,” Bogoch said.
According to recent modelling data from researchers at the University of Toronto, administering most of Canada’s coronavirus vaccines now, as opposed to reserving half of them to be used later as second doses for the first recipients, could prevent a significant amount of new symptomatic COVID-19 infections.
Dr. Anna Banerji, an infectious disease specialist at the University of Toronto, said it is actually better to spread the vaccine to as many people as quickly as possible with a first dose rather than reserving the second shot in case of shortages.
“If the end goal is to save lives and prevent hospitalization and vulnerable populations, then it’s better to get the vaccine out to all the people who are at high risk, like long-term care facilities, first rather than getting two to half the amount of people,” she told Global News.
How protected are you after the first dose?
According to the data from the clinical trials, Pfizer’s vaccine, which is 95 per cent effective, can offer partial protection as early as 12 days after the first dose.
That protection can last for at least two months, according to Dr. Donald Vinh, an infectious disease specialist and a medical microbiologist at the McGill University Health Centre. A second dose is then required to achieve the vaccine’s full potential.
The data also showed that a second dose given 21 days later boosted the immune response, offering protection starting one week after the second jab.
Moderna’s vaccine, which also requires a second shot and has been shown to be 94 per cent effective, can mount protective antibody levels within two weeks of the first dose and last for at least three months, Vinh said.
Analyzing the clinical data for Moderna, Dr. Mark Siedner, infectious disease clinician at Massachusetts General Hospital, said that people who got the first dose and not the second had about a 50 per cent reduced chance of getting a symptomatic COVID-19 infection.
“So there is protection. It’s certainly better than not getting vaccinated,” he told Global News.
However, it is still possible to get infected with the virus after either dose one or two, Bogoch said.
“The whole benefit of these vaccines are that they should hopefully significantly reduce your likelihood of getting this infection and, if you are unlucky enough to get infected after having two doses of the vaccine, it will likely reduce the severity of illness that you have.”
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