Canadians are being urged by the country's federal, provincial and territorial governments to get one of two approved bivalent COVID-19 vaccines as soon as they're eligible.

Because vaccine-conferred immunity wanes over time, boosters are important for keeping the population protected against COVID-19, health officials say, especially as the cooler weather threatens to bring another surge of cases.

The new shots – one by Moderna and one by Pfizer – were approved by Health Canada on Sept. 4 and Oct. 7, and provide broader protection against COVID-19 and its variants than previous versions of COVID-19 vaccines, the federal government says.

Bivalent vaccines target two different viruses or two strains of the same virus. In this case, the updated Pfizer-BioNTech Comirnaty vaccine targets the original SARS-CoV-2 virus as well as the Omicron BA.4 and BA.5 subvariants, which were known to be resistant to previous versions of the vaccines. Meanwhile, the updated Moderna Spikevax vaccine targets the original strain of the COVID-19 virus as well as the Omicron BA.1 subvariant.

The vaccines are produced using the same methods as previous COVID-19 vaccines, except that they contain two messenger RNA (mRNA) components instead of one, which is what allows them to target more than one strain of the virus.

WHO IS ELIGIBLE FOR BIVALENT BOOSTERS?

Health Canada has approved Moderna's bivalent vaccine for people 18 and older, and Pfizer-BioNTech's vaccine for people 12 and older, but provinces and territories may choose to further restrict eligibility rules based on supply or priority of population needs.

For example, residents of Ontario are eligible to receive their bivalent booster at least three months after their last dose. However, provincial health officials recommend that residents receive their bivalent booster six months after their last dose.

People who are at high risk of developing severe COVID-19 illness – including seniors age 65 and older; First Nations, Inuit and Metis people; those with underlying medical conditions; pregnant people and those living in long-term care or congregate care settings – are eligible at three-month intervals.

Both vaccines are only offered as a booster dose, so only people who have already completed a primary series of a COVID-19 vaccine are eligible to receive them.

WHAT IS THE DIFFERENCE BETWEEN THE MODERNA AND PFIZER BIVALENT BOOSTERS?

There are a few slight differences between the two vaccines, explains Kelly Grindrod, who is a researcher and professor of pharmacy innovation at the Ontario College of Pharmacists.

"Moderna and Pfizer are fairly similar," Grindrod told CTVNews.ca in an email on Friday.

"The Moderna vaccine has a slightly higher dose and is based on the original virus and the Omicron BA1 variant. Pfizer is a slightly lower dose and is based on the original virus and the Omicron BA4 and BA5 variants."

According to Health Canada, the Moderna vaccine is administered in 50-microgam doses, while the Pfizer-BioNTech vaccine is delivered in 30-microgram doses.

CAN PEOPLE MIX AND MATCH THE BIVALENT BOOSTERS?

Grindrod, who also teaches at the University of Waterloo, said that just as it was fine to have mixed dose schedules containing the previous Pfizer and Moderna vaccines, it's fine to mix and match doses of the bivalent boosters.

"You do not need to continue to get the same brand for your vaccine. This would be similar to how you get other vaccines, where the brand used each year is mostly related to availability," she said. "The best approach is to pick the booster that is easiest for you to access, such as at your nearest clinic or pharmacy."

This is consistent with guidance by Health Canada on mixed dose schedules for mRNA vaccines.

In its updated guidance on bivalent COVID-19 vaccines, released on Oct. 7, the National Advisory Committee on Immunization wrote it is "likely that the immunological benefits and safety profile will be similar in individuals receiving a bivalent Omicron-containing mRNA COVID-19 vaccine as a booster dose, regardless of number or type of doses previously received."

ARE SIDE EFFECTS DIFFERENT FOR EACH ONE?

Grindrod said side effects should be similar for both bivalent vaccines, and Health Canada reports they should be similar to those associated with previous versions.

People react differently to being vaccinated, but redness, soreness and swelling are common symptoms felt at the injection site, while chills, fatigue, joint pain, headache, mild fever and muscle aches are common general symptoms.

These symptoms usually last between a few hours and a few days after vaccination, and are a sign that the body is responding to the vaccine and building immunity against the disease, according to the Public Health Agency of Canada.

"Even if you experience temporary side effects, keep up to date on the vaccinations recommended for you, including booster doses," the agency's "COVID-19: Vaccine safety and side effects" webpage reads.

"This will help protect you from serious outcomes from COVID-19."