Ottawa, Jun 2 (IANS): Canada's National Advisory Committee on Immunization (NACI) has announced its updated guidance to allow mixing and matching approved Covid-19 vaccines in most scenarios.

Under the updated guidance, people who received a first dose of the AstraZeneca vaccine may receive Pfizer-BioNTech or Moderna for their second dose, unless contraindicated. Four vaccines, developed by Pfizer, Moderna, AstraZeneca and Johnson & Johnson, have been authorized for use in Canada to date, Xinhua reported.

The new guidance also advises that the Pfizer and Moderna vaccines can be mixed for first and second doses.

But it is not recommending AstraZeneca after a first shot of Pfizer or Moderna because of safety concerns and limited data on the use of this combination.

"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," Canada's Chief Public Health Officer Theresa Tam said at a press conference on Tuesday.

While NACI is advising that the vaccines can be safely used in combination, Tam said Canadians should try to get the same dose in a series if possible, specifically when receiving Pfizer or Moderna.

"NACI is not recommending just use them interchangeably, but mostly try and use the same vaccine for the second dose," Tam said. "If you can't find it, or you don't know what someone had, whether it's Pfizer or Moderna for the first dose, another vaccine can be considered."

The updated NACI guidance is based on emerging research from Spain and the UK that found mixing and matching AstraZeneca and Pfizer vaccines was both safe and effective at preventing COVID-19.

"This is not a new concept, so having a multi-dose series in terms of vaccines given by manufacturers is something that public health have used over time for many other vaccines," Tam said. "When vaccines programs and supplies change this is not an unusual thing to do."

More than 13 million people have received at least one dose of the Pfizer vaccine in Canada, in contrast to over 3.5 million of the Moderna vaccine and more than 2.1 million of the AstraZeneca vaccine as of May 22, according to the Public Health Agency of Canada.