You may soon be offered a fourth COVID-19 shot. What’s known about second boosters?

More Canadians may soon be offered a fourth dose of the COVID-19 vaccine, but the extent of the protection it provides remains to be seen.

Canada’s advisory panel on vaccines is expected to update its guidance in coming days as concern mounts that the country could be on the brink of a sixth wave.

U.S. regulators approved additional boosters this week for Americans aged 50 and older if it’s been at least four months since their last dose, as well as certain younger people with severely weakened immune systems.

Here’s what scientists say about what could be ahead for Canada’s next stage in the COVID-19 fight.

WHO IS ELIGIBLE FOR A SECOND BOOSTER?

The National Advisory Committee on Immunization suggests anyone “moderately to severely immunocompromised’’ get a fourth dose six months after their third shot.

Many provinces have already prioritized this population and set their own guidelines on booster timing and eligibility.

As a number of provinces move to expand access to second boosters, a spokeswoman for the Public Health Agency of Canada said Thursday that it plans to publish NACI’s guidance in early April.

“NACI has been asked for advice on the potential use of second booster doses in elderly populations at higher risk of severe disease,” Anna Maddison said in an email.

In addition to immunocompromised people, Ontario offers fourth doses to residents of long-term care homes and other congregate assisted-living settings, while Quebec seniors aged 80 and older became eligible this week.

Meanwhile, B.C. Health Minister Adrian Dix signalled Thursday that the province is looking to extend more boosters to vulnerable groups such as long-term care residents, with details expected Tuesday.

WHAT’S THE POINT OF ANOTHER VACCINE?

Experts warn that current vaccines appear to be a poor match for the more infectious Omicron variant.

The goal of a booster shot is to restore protection that naturally fades over time, says Matthew Miller, an associate professor with McMaster University’s Immunology Research Centre.

A primary vaccine series trains the body to identify a virus and defend itself. Eventually, the immune system’s front-line fighters — antibodies — retreat, but it retains instructions on how to quickly deploy its defences if it encounters the virus, says Miller.

Boosters reactivate this immunological army, he says.

The problem, says Miller, is that these vaccine drills were designed to recognize the original version of the COVID-19 virus andOmicron’s heavy mutations make it better at evading detection.

“We know current vaccines are not a great match for Omicron,” Miller says of infection protection, underscoring that shots still guard well against severe illness and death.