Ontario will move to give third doses of COVID-19 vaccines to immunocompromised people and the elderly in the coming weeks and formally allow some 11-year-olds to take the Pfizer vaccine ahead of the start of the school year.

As soon as this week in some areas, organ transplant recipients, those diagnosed with hematological cancers undergoing targeted therapy, rituximab, ocrelizumab or ofatumumab drug recipients, long-term care, retirement home and First Nation elder care lodge residents will begin receiving third doses of mRNA vaccines.

“Locations and timing for third doses will vary by public health unit and high-risk population based on local planning and considerations, with some beginning as early as this week where opportunities exist,” officials said Tuesday afternoon.

Provincial officials say the third doses are being offered to counter the impact of the now-dominant Delta B.1.617.2 coronavirus variant, which can cause breakthrough symptomatic infection in some fully vaccinated people.

Several recent studies also indicate antibodies created through vaccination can begin to wane several months after doses one and two, in the elderly and other vulnerable populations.

Ontario will begin offering third shots to transplant recipients and cancer patients a minimum of eight weeks after receipt of their second dose.

For long-term care, retirement and First Nation elder care lodge residents, they will be eligible for boosters five months after receipt of their second dose.

Based on deployment of vaccines in the winter months, many long-term care residents would now be at or near five months since their last COVID-19 vaccine shot.

American officials are also preparing deployment of third doses of vaccine for everyone regardless of age or associated health conditions, eight months after their second dose.

The move is supported in part by the province’s massive increase in vaccine inventory.

At the current consumption of about 40,000 doses per day, Ontario has enough mRNA vaccine doses for the next 144 days, even accounting for considerable spillage.

But global health experts including those at the World Health Organization have repeatedly warned wealthy countries to donate unused vaccines to poorer nations abroad before embarking on a program to administer third doses or “booster” shots.

Meanwhile, starting on Wednesday, anyone born in 2009 who will turn 12 by the end of the year will be allowed to book a Pfizer vaccine appointment through Ontario’s central online booking portal.

“Ontario has closely monitored data from Alberta and British Columbia in making this decision, and these provinces have offered the Pfizer vaccine to youth born in 2009 for several months with no risks identified,” officials said.

The 12-17 age group currently has the lowest rate of vaccination of any eligible age bracket in Ontario, with only 56 per cent of youths fully vaccinated against COVID-19.