MISSISSAUGA, ONT. -- A re-elected Liberal government would certify provincial vaccine passports for use in international travel as an interim measure until a federal version is put in place, Liberal Leader Justin Trudeau said Friday.

Asked to lay out a timeline for the federal vaccine passport his party has promised to deliver, Trudeau said any national program would first have to involve work at the provincial level, since health information is the purview of provinces.

"Before getting to a federal proof of vaccination to allow for international travel in something that could be aligned with, and perhaps inserted in, your Canadian passport, we have to work with the provinces anyway to get that information," he said.

Several provinces have already created their own system, or plan to do so, and Trudeau said his party is working with them to "add a federal element of certification so you can show it at international airports" for travel purposes.

"It is an interim measure that will be very good for the next year or so, easily, while we prioritize the things that are going to keep Canadians safe," he said.

"We will be bringing in that more formalized version in the coming months or a year perhaps, but the priority is giving people a solid document that will allow them to do both things (travelling and accessing non-essential services domestically)."

Last week, Trudeau announced a re-elected Liberal government would set aside $1 billion to help provinces seeking to create their own vaccine passports for domestic use.

The Liberal leader spoke Friday from a hangar at Canada's busiest airport where he again touted his plan to make vaccination mandatory for travellers on planes, trains and cruise ships.

Mandatory immunization has been a hot topic in the election -- and one that the leaders addressed in Thursday night's French-language debate.

Trudeau said he won't force anyone to get a COVID-19 shot, but plans to restrict the privileges of those who refuse to get one without a medical reason.

Meanwhile, the Liberals' vetting process for candidates came under scrutiny Friday after an Alberta candidate issued an apology for comments regarding Green Leader Annamie Paul.

Irene Walker tweeted in June that Paul had "done a great disservice to women, people who are not white and Jews."

On Friday, Walker said: "Earlier this year, I made comments online that I shouldn't have made and I regret. I have removed those comments and unreservedly apologize."

Speaking in Toronto, Paul said she had not received a direct apology, but hoped to receive one.

Paul said she has faced daily attacks for being Jewish and a woman of colour.

"It's incredibly painful, it's incredibly damaging, and it will continue to drive good people seeking to serve Canada out of politics," Paul said.

"I'm alarmed to know that someone who was vetted by the Liberal party was able to tweet something like this and was still approved as a candidate without having first apologized. And it seems that the apology came today, and therefore they were vetted and approved even after having said those incredibly hateful and hurtful things in a public forum."

She said the incident points to a pattern with the Liberals of green-lighting problematic candidates, referencing the allegations of inappropriate conduct against Raj Saini.

Saini, a Liberal candidate in Ontario, has faced allegations that he harassed a former member of his staff, which he denies. The allegations, reported by CBC, have not been independently verified by The Canadian Press.

Trudeau has repeatedly said the allegations against Saini were taken seriously and rigorously investigated, and on Friday, he ruled out an independent inquiry into the matter.

The Liberal party declined to comment on Paul's remarks about its vetting process.

Also Friday, Trudeau said he expects Canadian Blood Services to soon overturn a discriminatory policy that prevents many gay and bisexual men from donating blood.

The Liberals first made the promise to overturn the ban during their successful 2015 campaign, but it isn't mentioned in the party's re-election platform this year.

Canada introduced a lifetime ban for gay men in 1992 and in 2013 changed it so blood would be accepted from a man who abstained from sex with another man for at least five years.

The waiting period then dropped to one year, and became three months in 2019.

Trudeau blamed cuts by the previous Conservative government for the delay by the independent blood-donation agency, which he says didn't have the data needed to make a final decision.

Recently, Canadian Blood Services said that by the end of 2021, it would recommend to Health Canada that it remove the waiting period and bring in screening for all donors based on sexual behaviour.

It reiterated this goal in a statement on Friday and said it would take time to go through Health Canada's approval process and then implement the change.

"We believe we now have the evidence we need to make a strong submission and are hard at work on this important step toward a more inclusive system," the agency said.

Trudeau would only say that he expects an announcement soon.

"We have said consistently right back since 2015 that the ban on blood donation, on giving blood, for men who have sex with men is unacceptable, discriminatory and wrong," Trudeau said.

"We are very, very hopeful and we expect Canadian Blood Services to announce soon that the ban ΓǪ will be lifted shortly. It is something that we are working on, continue to work on. It is something we will ensure happens."

Canadian Blood Services is a not-for-profit organization that operates at arm's length from the government, and is largely funded by the provinces.