Canada’s National Advisory Committee on Immunization (NACI) will now allow mixing of mRNA and conventional COVID-19 vaccines for first and second doses, citing safety data gathered in other countries.

The federal panel will green light giving someone who received a first dose of the AstraZeneca COVID-19 vaccine a second dose of either the Pfizer-BioNTech or Moderna vaccines, and will also allow mixing of Pfizer and Moderna from dose 1 to dose 2 to allow vaccinations to proceed faster.

But the panel is not suggesting anyone who received an mRNA vaccine be offered AstraZeneca for their second dose.

“NACI’s latest guidance released today considered a range of factors from safety concerns to vaccine supply to provide recommendations on the safety and effectiveness of the interchangeability of authorized COVID-19 vaccines or mixed vaccine schedules for first and second doses of COVID-19 vaccines,” Canada’s Chief Medical Officer Dr. Theresa Tam said on Tuesday.

Concerns about the safety of AstraZeneca as well as limits on its supply in Canada prompted questions about whether it could be mixed with other vaccines.

“Those who received the first dose of an mRNA vaccine so that's Pfizer-BioNTech or Moderna vaccine should be offered should be offered the same mRNA vaccine for the second dose. But if the same mRNA vaccine is not readily available, another mRNA vaccine can be considered interchangeable and should be offered to complete the vaccine series,” Tam told reporters on Tuesday.

A recent study from Spain found no safety issues with following a first dose of the AstraZeneca coronavirus vaccine with a second dose of Pfizer.

A second study from the UK found a higher likelihood of mostly minor side effects from mixing doses.

“There is a possibility of increased short term side effects when using this mix schedule, such as headaches, fatigue, feeling generally ill, but they are temporary and without complications,” Tam said.

Other studies have found a mix-and-match approach with Pfizer and Moderna jabs still produces a robust immune response.

Canada had earlier adopted a 16-week gap between doses, far exceeding recommendations from vaccine manufacturers, in a bid to maximize the impact of vaccine doses in the context of limited supply.

NACI now is urging provinces to speed up second doses, especially for those 60+ and others with complex health needs and weak immune systems.