CALGARY -- Now that Alberta's COVID-19 vaccine supply is running low, the province says it has "no choice" but to focus on second doses for residents who need them.

However, Alberta's top doctor admitted there are still a lot of things we don't know about the vaccine and its efficacy.

"There are so many unknowns with this specific Pfizer and Moderna vaccine," Alberta's Chief Medical Officer Dr. Deena Hinshaw said Thursday.

"We know with other vaccines that when someone has their first dose, there is no end date at which time they're no longer eligible for a second dose. And we know sometimes with some other vaccines that if there is a little bit of a longer interval between first and second dose, that the overall long lasting immune response can sometimes be better with a little bit of a longer interval."

While there is evidence that the vaccine does bring on an immune response, one of the unknowns Hinshaw spoke of is how long a single dose of COVID-19 vaccine can protect someone from infection.

"Studies on the vaccine show that fourteen days after the first dose, on average, 92 per cent of people are protected from getting sick with COVID-19 for the next several weeks," said Tom McMillan, assistant director of communications with Alberta Health in an email to CTV News.

He said that the most recent data from the Pfizer-BioNTech trials included participants that received the second dose within 19 to 42 days of the first. In Moderna's research, patients were given second doses between 21 and 42 days later.

"There is limited clinical trial data for doses given beyond 42 days," McMillan says, adding that those trials are what Alberta based its own guidelines on.

"This aligns with the (World Health Organization's) and (National Advisory Committee on Immunization's) recommendations," he says.

Hinshaw said Thursday that health officials are working hard to ensure all patients who need second doses will get them within that 42-day time period.

Because of the delay in new shipment of vaccines, McMillan says Alberta has few options when it comes to immunizing residents.

"We have no choice but to focus on delivering second doses for those who have already been vaccinated," he says. "These doses have started rolling out to residents at (long-term care) or (designated supportive living) sites who have received their first dose. Eligible residents who did not receive a dose in the first round are also receiving a first dose at the same time."

COVID-19, vaccine, Alberta

According to the latest data from the Government of Canada, more than 120,000 doses of COVID-19 vaccines have been distributed in Alberta.

Of those, there have been 88,725 Pfizer/BioNTech and 34,000 Moderna doses delivered to the province.