Canadian public health officials are endorsing plans by some local governments to delay the second dose of COVID-19 vaccines beyond the timeline recommended by pharmaceutical companies.

An advisory group overseeing the vaccine rollout said Thursday that provinces can likely wait as long as six weeks to give a second dose without impacting the vaccine’s effectiveness -- double the timeline recommended.

The report from the National Advisory Committee on Immunization provides some backing to efforts by at least one province, Quebec, to ration second doses so it can inoculate more people amid a spike in cases. Canada’s top medical health officers, at a separate press conference, also appeared to back extending the time between the two doses.

The advice, however, runs counter to guidance from Health Canada, the federal department that approved the vaccines, which is to follow the vaccine manufacturer’s recommendations.

Pfizer Inc. and BioNTech SE say their shot should be administered in two rounds within 21 days of each other. Moderna Inc. recommends its two doses be given within 28 days.

“Although partial protection from the vaccine appears to begin as early as 12 days after the first dose, two doses of the vaccine are required to provide the maximum protection against the disease,” Christina Antoniou, Pfizer’s director of corporate affairs in Canada, said by email. “There are no data to demonstrate that protection after the first dose is sustained after 21 days.”

Dr. Howard Njoo, Canada’s deputy public health chief, told reporters any province that chooses to provide the second dose beyond the recommended period should share its results to better understand the impact of the delay on the vaccine’s effectiveness.