OTTAWA -- As the precise order of who will follow seniors, health care workers and high-risk populations in line to get COVID-19 vaccines is still being sorted out, Prime Minister Justin Trudeau says the federal and provincial governments agree that there should be a cross-Canada "consensus" on the matter.

With Health Canada now beginning its assessment of a fourth potential vaccine candidate — Johnson & Johnson’s — the prime minister said talks are ongoing with the provinces and territories about the “challenging ethical and societal” aspect of the country’s vaccine rollout.

Logistics aside, governments and health care experts are having to weigh and decide who will be prioritized and what the eventual order of precedence will be for Canadians to line up and be vaccinated.

According to the preliminary guidance issued by the National Advisory Committee on Immunization, prioritization will be based on three factors: the state of the pandemic when the vaccine is available; the supply available and number of doses required; and the risk-benefit analysis of key populations such as those who are at higher risk for adverse outcomes if they contract the novel coronavirus.

Based on that advisory group’s preliminary guidance, the recommendation is that essential workers and others who face increased risks related to COVID-19 should be vaccinated against the disease before everyone else. Examples of those at higher risk include providers of essential services, or those whose living or working conditions put them at higher risk.

The subsequent order of who gets vaccinated next remains a largely open question, however, in the race to see 70 per cent of Canadians vaccinated by September.

“We talked about it with the provinces last week on our 22nd first minister's call, and there was a number of perspectives, but there seemed to be a consensus that we should all agree across the country on what that list looks like and make sure that it is applied fairly right across the country,” Trudeau told reporters on Tuesday.

“There are more conversations to come and we will keep Canadians informed as we determine what that right order of priority is. Other elements of it is, certain vaccines might be more effective with certain populations versus others, and that's why the experts are going to be so important in making determinations around, what is the best path to move forward for our country,” said the prime minister.

Though, Health Minister Patty Hajdu said later that provinces will be able to refine the prioritizations based on their own regional demographics.

“At the end of the day it is the provinces who deliver health care and it is the provinces who will decide on the priority populations and of course we're working closely to make sure that we have coordination across the country, and that we agree on the principles, which in fact we have, we have a shared set of principles,” Hajdu said.

“There are also some federal populations that we will obviously have to take care of ourselves as the federal government,” Hajdu said. Examples of these groups would presumably be Indigenous communities and federal inmates.

Manitoba Premier Brian Pallister said on Tuesday that he and other premiers still have outstanding questions that need to be answered.

“Clearly we need our most vulnerable folks, our seniors… our front-line care workers to get the vaccine earlier, we can all agree on that. But the devil’s in the details, when you get beyond that. Should it be done on the basis of age? Or how do you determine vulnerability? Should it be done on the basis of ethnicity? Should it be done on the basis of race in some way? These questions have to be addressed,” Pallister said.

“We’re not saying the federal government has to do it all but we’re saying that we need to have the criteria established and the priority should be common, not different in one side of Saskatchewan’s border with Alberta than it is on the other, or not different than it is in Ottawa from Gatineau, but rather that we have a co-ordinated strategy.”

In an interview on CTV’s Power Play, New Brunswick Premier Blaine Higgs said in his province he doesn’t anticipate there will be a huge line up of people who want to get vaccinated early on, but communicating as clearly as possible in advance of who will be eligible first will help avoid a “panic situation.”

So far, just over $284 million has been spent on distributing vaccines to Canadians, with overall more than $1 billion allocated to Canada’s vaccine procurement effort, as part of a more than $14-billion commitment over the next several years on research into and development of vaccines and therapeutics.

AGE MAY BE KEY FACTOR: TAM

Chief Public Health Officer Dr. Theresa Tam said Tuesday that work is underway right now on getting more “granular” in planning who among the highest risk groups will be first.

“That detail work is, you know, being taken very seriously by the provinces and territories as they begin to plan their immunization clinics.”

Then, once the priority groups are immunized, it’s possible the next easiest way to break down the order would be by age, said Tam.

“The age group, based on our analysis is actually the easiest and the most scientifically-sound way, I think, of increasing the population coverage,” she said.

“We know that underlying medical conditions put people at high risk but when we actually analyze all the different underlying medical conditions, and their age, it still comes out that the age is in fact the most important where you look at severe illness and mortality.”

There will also be groups who won’t be able to get a vaccine early on, due to the lack of research into the potential impacts on them, such as children and people who are pregnant.

“Kids haven’t really been engaged in a lot of the clinical trials, so that would be another age group for which data is needed, and we'll be looking towards more data on pregnant women as well,” Dr. Tam said.

Asked whether he anticipates being among the earliest groups to get vaccinated, Trudeau said that he’s “going to trust the experts to make the right determination of what the priority populations are.”

With files from CTV News’ Ryan Flanagan