Starting on Saturday, the province will begin offering COVID-19 booster doses to more Ontarians if at least six months have passed since their last dose.
The province announced Tuesday that it is expanding third dose eligiblilty to more vulnerable populations, including individuals who are 70 and over, health care workers and designated essential care givers, individuals who received two doses of the Astra Zeneca vaccine, and First Nation, Inuit, Metis adults and their household members.
Ontario Chief Medical Officer of Health Dr. Kieran Moore indicated booster doses are being offered to these individuals based on evidence of gradual waning immunity six months after receiving their second dose.
"There is no sudden drop off that we've seen in any of our data. We do see a slow and gradual fading of immunity in those specific populations that we've already addressed," said Moore.
He said while people who received two doses of Astra Zeneca are still well protected, evidence has shown an mRNA vaccine following the viral vector vaccine can boost immunity.
"Two doses of AstraZeneca still has a significant protection against hospitalization and severe outcomes but it is less than the mRNA by about 10-15 percentage points on levels of protection," said Moore. "Now knowing that we get profound second boost from a subsequent mRNA after two Astra Zenecas it is our advice to provide the mRNA vaccine after those two to boost you back to a higher level."
Each public health unit has been advised to prepare for the rollout of the booster doses. While the provincial booking system will start taking appointments on Saturday, the Windsor Essex County Health Unit's booking system allowed eligible individuals to book as of 3 p.m. Wednesday.
The province expects to expand eligibility for booster doses to all Ontarians over time. It will be based on age and risk with an interval of six to eight months from the second dose.
Moore also indicated he expects Health Canada will vaccination of children aged 5-11 by the end of the month. According to the provincial government the Pfizer COVID-19 vaccine for children aged five - 11 is a different formula at a lower dose which will be rolled out in parallel to booster doses.