Ottawa CNN  — 

Canada has changed its guidelines on mixing doses of Covid-19 vaccines, the country’s health agency said Tuesday, and Canadians may get a different type of vaccine for their second shot.

The Public Health Agency of Canada said people who received an AstraZeneca vaccine for the first dose may get either a Pfizer or Moderna vaccine for the second dose. People who received one of the two mRNA vaccines – Pfizer’s or Moderna’s – may get either of the two brands for the second dose.

It is still optimal, however, to use the same brand of vaccine for both doses, the country’s top doctor said.

“Try and find the same vaccine, the same mRNA vaccine – but if you can’t, for some reason, then consider them interchangeable,” Dr. Theresa Tam, Canada’s chief public health officer, told a news briefing. “This advice provides provinces and territories with safe and effective options to manage the vaccine programs.”

After a slow start, Canada has been vaccinating nearly 1% of its population each day, on average, for the past few weeks.

Supply remains an issue, however, with the Moderna and AstraZeneca vaccines.

Canada’s National Advisory Committee on Immunization (NACI) now says people who received a first dose of the AstraZeneca vaccine may receive a second dose of either AstraZeneca or an mRNA vaccine. It does not recommend that people who got either the Pfizer or Moderna vaccine for the first dose get the AstraZeneca vaccine, which uses a different technology, for the second dose.

The advisory committee says Canadians who received a first dose of an mRNA vaccine, either Pfizer or Moderna, can receive the other mRNA vaccine for their second dose if the same mRNA vaccine “is not readily available.”

“NACI has worked to quickly adapt this guidance on the use of Covid-19 vaccines in Canada to ensure optimal protection of Canadians at pace with the ever-changing circumstances during this pandemic,” said Tam.

According to Canadian data, as of the last week of May, 13 million Canadians had received at least one dose of the Pfizer vaccine, while 3.5 million had received the Moderna vaccine and nearly 2 million had received the AstraZeneca vaccine.

CNN’s Jacqueline Howard contributed to this report.