TORONTO -- Federal officials are mounting a renewed push to instill Canadian confidence in Oxford-AstraZeneca's COVID-19 vaccine, but observers predict they're in for a tough battle.

Members of the federal body tasked with advising the country on vaccine use took the unusual step of addressing public fears in a televised press conference Tuesday that stressed the vaccine's safety for those over the age of 65.

The update from the National Advisory Committee on Immunization came roughly two weeks after it suggested seniors avoid the AstraZeneca vaccine because of insufficient trial data, and committee chair Dr. Caroline Quach explained that evolving data will shift advice.

"It's not (that) we're flip-flopping, it's just that we try to monitor the evidence," Quach said Tuesday, noting the committee now had enough "real-world evidence" to show it is both safe and effective for seniors.

Infectious disease doctor Zain Chagla said it's an important message, but confusing for Canadians as AstraZeneca also faced concerns over safety in Europe where officials are investigating reports of blood clots.

Health Canada said Tuesday it was monitoring the European reports but stressed there was no indication of an increased risk of blood clots linked to the vaccine.

Chagla said daily updates will be crucial to reassuring the public as well as doctors administering the vaccine, but noted the barrage of negative stories surrounding AstraZeneca's product has undermined its image.

"Even if all of the dust settles on all of this stuff and it's (proven) effective in 65-year-olds and it's actually 80 per cent effective and there's no clot risk, you've already introduced three strikes that are hard to wash away from people who are already hesitant to take this vaccine over Moderna and Pfizer," said Chagla, a specialist at St. Joseph's Healthcare Hamilton.

Germany, Italy, France, Sweden and Spain are among the countries that have suspended use of AstraZeneca's vaccine, even though the European Medicines Agency, AstraZeneca and the WHO have said there is no evidence it carries an increased risk of blood clots.

The company says there have been 37 reports of blood clots among the more than 17 million people who received the vaccine across the European Union and Britain.

Infectious disease specialist Dr. Sumon Chakrabarti said such facts will hold little sway for some people whose hesitancy is rooted elsewhere. He guards against the temptation to "beat people over the head with facts."

"Sometimes data and facts is not what people want," says Chakrabarti, a Mississauga, Ont. physician.

"They want the truth, of course, but they also just want the reassurance.... Sometimes it is just fear, sometimes it's mistrust of the government, or some people in general are mistrustful of any kind of medical therapy or the medical field in general."

Chagla agrees that unfounded doubts about efficacy and safety have been deeply sown and some people will have a hard time accepting updated advice.

But he said that can be addressed by clear and consistent information from officials including NACI, Health Canada, and the various provincial health ministries.

"This needs to be aggressively put out and people need to be really, really, really transparent, open and honest about this."

NACI said it would hold information briefings with every new release going forward, but that the public should understand that contrary evidence might arrive after a decision is made, and that changes are inherent to the way the committee operates.

"There's always inevitably going to be a bit of a lag between when the committee deliberates and when the advice is made public," said Matthew Tunis, executive secretary to the committee.

"We're going to continue to see this throughout the pandemic as new evidence emerges."

This report by The Canadian Press was first published March 16, 2021.