VANCOUVER -- The B.C. government has announced that booster shots of COVID-19 vaccine will be offered to several vulnerable groups before the end of the year, and to the broader population beginning in January.

The province has already started distributing third doses in long-term care homes and assisted living facilities, where some elderly residents are said to have developed weaker antibody responses following vaccination, resulting in another wave of deadly outbreaks.

Extra shots have also been given to some B.C. residents considered extremely clinically vulnerable due to serious health conditions, and to some Indigenous communities that have seen high rates of transmission.

But provincial health officer Dr. Bonnie Henry and Health Minister Adrian Dix said emerging data suggests a booster shot could provide much longer-lasting protection to the general population as well, prompting the decision to expand eligibility to everyone age 12 and up next year.

"The immunity that we get from these vaccines wanes a little bit over time," Henry said. "But it doesn't fall off a cliff – it's not like you're no longer protected overall, it slowly goes down."

Henry stressed that two doses of vaccine, in any combination, will be enough to protect most people through the winter respiratory season, so the government is first focusing on those most at risk of decreasing immunity and serious infection.

"Most of us have good, strong protection and we don't need a booster dose right now, but come next spring it is something we should consider for longer-term protection," she said.

Through the end of December, additional doses are being offered to seniors age 70 and up, Indigenous residents age 12 and up, clients of long-term home supports, seniors in independent living, and health-care workers who received their two doses in quick intervals back when B.C. began its immunization program last year.

Growing research has found that shorter intervals have led to weaker immunity over time, with the strongest protection forming in those who received their second dose more than six weeks after their first.

While the United States opted for an interval of three to four weeks, B.C. was an early adopter of extending the waiting period between shots. Henry said that's shown a positive impact on cell-mediated immunity, a bodily response that's independent of antibodies.

"What we have seen is the cell mediated immunity – so those memory cells that we have as part of our immune system – those mature and give good strong protection over time and that interval made a difference in how that cell mediated immunity develops in people," she said.

The province said only the Moderna and Pfizer-BioNTech mRNA vaccines will be used for boosters, and can be used in any combination.

Going forward, health officials said only first doses will be offered through drop-in appointments. Second doses, third doses and boosters will be by appointment only for logistical purposes and to avoid waste.

Once again, the government is asking pharmacies across the province to help distribute boosters along with local health authorities.

The announcement of widely offered third doses led some B.C. residents to immediately question whether there would be an eventual impact on proof-of-vaccination requirements for non-discretionary activities like attending sporting events and going to the movies. Health officials said third doses will not be required under the B.C. vaccine card system.