Canada’s National Advisory Council on Immunization (NACI) has recommended that Canadians wait to receive the messenger RNA (mRNA) vaccines from Moderna (NSDQ:MRNA) and Pfizer (NYSE:PFE) and BioNTech (NSDQ:BNTX) when feasible. The advice stands in contrast to prior guidance that Canadians seek the first-available vaccines.

“What we’re saying and what we’ve said all along is that mRNA vaccines are the preferred vaccine,” said NACI’s co-chair, Dr. Shelley Deeks.

Canada had recently put on hold plans to distribute 300,000 doses of the COVID-19 vaccine from Johnson & Johnson (NYSE:JNJ). The reason for the move lies in J&J’s contract manufacturing partner, Emergent BioSolutions (NYSE:EBS), which inadvertently ruined 15 million doses of the vaccine.

NACI has recommended the J&J vaccine for those over the age of 30 where mRNA vaccine supplies are limited.

In late March, AstraZeneca had also paused the use of the AstraZeneca vaccine for people under 55 over potential blood clot risks. “From what is known at this time, there is substantial uncertainty about the benefit of providing AstraZeneca COVID-19 vaccine to adults under 55 years of age,” NACI wrote in prepared remarks.