WINNIPEG -- The time between first and second COVID-19 vaccine doses in Manitoba could be as long as four months, but the province is willing to pivot if there is good evidence to support a change.

Currently, the province is focused on first doses and is not booking new second dose appointments based on recommendations from the National Advisory Committee on Immunization (NACI).

But if new research comes out showing certain populations will not be protected with just one shot, the province is willing to shift.

“We can always pivot again and start to offer the second dose to folks who we start to see may not be protected enough from dose one,” said Dr. Joss Reimer, the medical lead for Manitoba’s Vaccination Implementation Task Force, on Wednesday.

“So we will work with NACI, and we'll work with our Medical Advisory Committee, our epidemiologists, our medical officers of health, to make sure that we are keeping on top of all of the studies as they come out, and we are open to shifting towards whatever the best science is at the time.”

Canada’s chief science adviser Dr. Mona Nemer told CTV News Channel’s Power Play on Monday that studies have shown that extending the interval between doses has had a negative effect on their efficacy, particularly in seniors.

“I’m not aware of data showing that there is efficacy beyond two months of the first dose,” she said. “In the past few weeks, we’ve seen different studies come out showing that the response to the first dose of the vaccine in the people who are elderly, in the people who are immunocompromised is actually not that good and it wanes quite rapidly.”

Health Canada’s authorization for the vaccines initially indicated a 21-day interval between doses of the Pfizer-BioNTech vaccine, 28 days between doses of the Moderna version, and up to 12 weeks between doses of AstraZeneca.

The NACI said it decided to extend the intervals after considering evidence from scientific research and “real-world effectiveness” that shows high levels of protection following a single dose.

When CTV News asked Dr. Reimer Wednesday about this issue, she said when the vaccine campaign first started, Manitoba followed those initial guidelines because of the concern for seniors.

“We do know that as people age, their immune response can weaken and either become less overall or be slower to reach that peak,” she explained. “And so until we had data to rely on, we decided in Manitoba to ensure that for our personal care home residents, top of mind, that we had that second dose available until we had some evidence showing us that the first dose would be sufficient.”

Since then, the focus has shifted to just the first dose and Dr. Reimer said the data supporting that decision came from BC, Manitoba, and Quebec. She said the data was heavily weighted towards long-term care residents, because that's who was receiving vaccines in all three of those data sets.

Dr. Reimer said there was a very effective response, in the 70 per cent or 80 per cent range for vaccine effectiveness in the health-care workers, as well as the residents of long-term care facilities.

“And that's really where we had been worried and so it was very reassuring to see that in those settings that elderly Canadians did experience good protection from one dose of the vaccine.”

Winnipegger David Magrel wants more investigation to happen into vaccine efficacy in seniors and delaying second doses.

“I have contacted everybody except God on this one, I’ll tell you,” the 79-year-old told CTV News.

Magrel, who got his first dose on March 23, said he was very concerned to read Dr. Nemer’s concerns.

“As a senior with pre-existing conditions, and I’m 79 years old, it is of concern to me that they want to extend this out,” he said.

He said if protection wanes in seniors, delaying the second dose defeats the purpose of protecting the age group that is most at-risk for hospitalization or death. He is also concerned about the potential stress a spike in cases would have on the medical system if more people 65 and older get sick even after one dose.

“They should be given the priority for getting the second dose as soon as they can,” Magrel said. “I realize logistically they’re trying to get as many single doses into as many people as possible, but the group that could be hurt the most is the elderly.”

Reimer said Manitoba is collecting data on COVID-19 infections and immunizations through the same public health information management system.

“We are monitoring all the time, for every single case that gets diagnosed in Manitoba, to determine whether or not it happened in someone who was immunized, or not immunized, and that allows us to compare the general population who has not received a vaccine, comparing it to those who have, to determine how protective that vaccine truly is.”

She said what Manitoba public health is seeing so far, particularly in personal care homes, is a massive drop in the number of cases that occur after vaccines have had a chance to lead to that immune response.

“Broadly speaking, it's been very encouraging,” Reimer said. “Where we're seeing a little bit of concern after one dose is in some of our First Nations communities where we know due to being the same reasons that they were at higher risk in the first place.”

Reimer said factors possibly in play in First Nations communities are poverty, housing, or other effects of colonization.

“It's possible that (the) first dose is not as effective because there's so much more exposure to the virus in those congested settings,” she said. “That first dose may not be as protective as we had hoped, but we're still early on, even with the data on that, and are working with the First Nations partners and if they tell us that we need to shift, an approach in those populations, we can do that.”

-with files from CTV’s Ben Cousins