VANCOUVER -- The premier of British Columbia received a second dose of AstraZeneca's COVID-19 vaccine on Friday, following updated national immunization recommendations that have left some Canadians scratching their heads.

John Horgan shared a picture of his blurred out vaccination card on social media, writing that "the best vaccine is the first vaccine you're offered."

The premier received his first shot of AstraZeneca in the middle of April, as health officials were in the process of reviewing the vaccine's link to very rare blood clots, and offered a similar message encouraging people to join B.C.'s immunization effort at the earliest opportunity.

"The advice I’ve been given today is that you should take the first vaccine that’s offered to you because it's in the interest of not just yourself, but your neighbours, your family, and your community," Horgan said at the time.

On Thursday, the National Advisory Committee on Immunization presented updated guidance for Canadians, recommending that those who received AstraZeneca for their first dose get one of the mRNA vaccines for their second.

NACI cited growing evidence that a mix-and-match approach for AstraZeneca recipients provides a stronger antibody response.

The announcement caused some confusion among vaccine recipients, including those who had recently received their second dose of AstraZeneca, though officials stressed that the vaccine remains safe and effective.

Still, some B.C. pharmacies that are currently administering second doses of AstraZeneca vaccine said they have received a wave of cancellations since the announcement.

Provincial health officer Dr. Bonnie Henry said the updated recommendation was not unexpected, given the preliminary data emerging in other parts of the world, but that B.C. was not changing its approach to the vaccine.

"It's important to remember that this is new information, that we are continuing to learn as we use more of these vaccines around the world and as more studies are done," Henry said.

"Mixing an mRNA after a dose of AstraZeneca may give some boost to the immune system, but we don't know whether that translates into whether you're better protected or not. We don't know that definitively, and we may not know that for some time."

NACI also cited the rare risk of vaccine-induced immune thrombotic thrombocytopenic, or VITT, associated with AstraZeneca. As of May 31, only three people in B.C. had developed VITT after receiving the vaccine, and none of the incidents were fatal.