Canadian Prime Minister Justin Trudeau just got a second Moderna shot after getting his first COVID-19 vaccine from AstraZeneca

• Canadian Prime Minister Justin Trudeau got his second dose of the COVID-19 vaccine.
• He got a shot of the Moderna vaccine to follow his first AstraZeneca shot.
• Mixing vaccine doses is currently allowed in Canada, but not approved in the US.

Canadian Prime Minister Justin Trudeau got his second dose of the COVID-19 vaccine this morning. He joins a growing number of people who have mixed and matched their doses.

Trudeau received his first dose of the AstraZeneca vaccine in April, and previously said he planned to get a second shot from the same manufacturer when it's available. But today, he followed up his AstraZeneca shot with a dose of Moderna.

Canada's National Advisory Committee on Immunization updated its guidance at the beginning of June to recommend that people who got one dose of the AstraZeneca vaccine follow up with a second dose of either Moderna or Pfizer. Canadians are also allowed to mix and match Pfizer and Moderna.

The agency noted that, due to the risk of people developing rare blood clots from the AstraZeneca vaccine, offering alternative vaccines was "ethically justifiable."

Still, the US Food and Drug Administration and Centers for Disease Control and Prevention have stood firmly against mixing vaccines.

Early research suggests mixing vaccines triggers a robust immune response

Studies have shown that combining different vaccines may trigger a stronger reaction than sticking to a single kind.

One study, led by the University of Oxford, found that people who received a dose of AstraZeneca and a dose of Pfizer were more likely to have mild to moderate side effects compared to other vaccine recipients. Side effects tend to indicate a robust immune response, but the researchers have not yet released immunogenecity data for the mixed group.

Preliminary results from another study in Spain found that mixing doses successfully produced a boosted immune response. However, the study has not yet been peer-reviewed, and those initial results were announced in an online presentation.

"It appears that the Pfizer vaccine boosted antibody responses remarkably in one-dose AstraZeneca vaccinees. This is all around wonderful news," Zhou Xing, an immunologist at McMaster University in Hamilton, Canada, who was not affiliated with either study, said in a press release.

Those promising results, along with Trudeau's second vaccination, may mark a shift towards allowing mix-and-match vaccines, which could accelerate the global vaccine rollout.