Test results showing promising levels of immunity after a single dose of COVID-19 vaccines have Manitoba leaning towards a focus on getting first shots into as many arms as possible before moving on to the second.
The incoming third wave of COVID-19 cases in Manitoba has left the province juggling questions of how best to protect as many residents as possible with a limited supply of vaccines, while experts across the country are working to better understand the efficacy of single vaccine doses.
Winnipeg-based epidemiologist Cynthia Carr said while the choice to delay a second dose may be risky, single doses have begun to prove effective in real-world research.
"The Pfizer and Moderna are highly effective after one dose," Carr said Sunday. "The question of course is how long does that effectiveness last in reducing the risk of symptomatic disease and how much impact is there on disease transmission."
While the first shot teaches your immune system how to respond to a COVID-19 infection, the second dose is a booster, reminding your cells of what they were taught the first time around, Carr explained.
Experts are still working to determine how long people can wait before their bodies need the reminder, but in the mean time Carr said provinces must decide what priorities will keep the most people safe.
"You’re trying to look at what is the optimal use of resources with the optimal harm reduction — harm reduction of course being deaths and hospitalizations from COVID," Carr said.
"Do we take the limited resources that we have right now and try to stop that transmission, stop the worst impact of the virus and the variant by getting as many people with at least some level of protection, even if it doesn’t last as long."
A provincial technical briefing last month indicated that while mRNA vaccines like the Pfizer and Moderna formulas are more than 94 per cent effective after two doses, "significant protection started about 12-14 days after the first dose."
"Real world evidence" from the U.K., Scotland and Israel suggested one dose of an mRNA vaccine is around 70 to 80 per cent effective in "preventing lab confirmed COVID-19 infection and also significantly reduced hospitalization and death," the province added.
Dr. Joss Reimer, medical lead for the province's vaccine implementation task force, said the province is looking to get at least some vaccine into as many arms as possible before focusing on booster shots.
"When we decided to delay the second dose we did so because we didn’t have enough supply to be able to provide doses to all Manitobans as quickly as we wanted," Reimer said in a press conference Friday.
"Our goal has been and continues to be getting as many Manitobans protected as quickly as we can, especially as we’re seeing these third waves in other provinces and the third wave starting here now as well."
Reimer said studies from B.C., Manitoba, Quebec, the U.K. and Israel showed immunity from the first doses lasted for the full length of the studies, with no change in immunity at the two-month mark (the longest study).
"If we at any point in Manitoba or in any other jurisdiction, start to see signs that that immunity is going away, then we can move towards starting second doses for the most vulnerable Manitobans," Reimer said.
"For today most jurisdictions are still seeing good protection after one dose. The best way for us to prevent this third wave, or at least prevent it from being larger than it needs to be, is to have as many people immune as possible."
Reimer said the province is confident the vaccine will remain effective for up to two months, but the province believes protection from a first dose could last as long as six months based on the performance of other vaccines.
"Even for those folks who have to wait for their second dose, they will be better protected if all the people around them have also received a first dose."
A spokeswoman for the province said Sunday that Manitobans can only book first-dose appointments for now, adding that more information about planning for second doses will be available in the weeks to come.
Julia-Simone Rutgers is a general-assignment reporter.