Ontario is offering fourth doses of COVID-19 vaccines to all residents aged 60 and up beginning Thursday morning.

The province says it will also open up eligibility for fourth doses to all Indigenous residents of the province 18 and up at the same time.

"Expanding booster eligibility will provide an extra layer of protection against the Omicron and BA.2 variants and, in addition to antivirals, are another tool the province is using to live with and manage COVID-19," Health Minister Christine Elliott said in a statement issued Wednesday.

People can receive a fourth dose of vaccine at an interval of at least five months since their last COVID-19 vaccine shot.

The provincial vaccination appointment booking portal will begin accepting fourth dose appointments at 8 a.m. Thursday.

People can also call the provincial vaccine contact centre at 1-833-943-3900.

UHN infectious diseases specialist Dr. Isaac Bogoch told CP24 this is a targeted measures to prevent severe outcomes in the absolute most vulnerable groups in the population.

“I think it is fair to say that a fourth dose will be beneficial for people who have the greatest risk of hospitalization,” he said. “(National Advisory Committee on Immunization) says those are people over the age of 80 and of course people who are severely immunocompromised or in long-term care facilities and you can consider people a bit younger than that.”

Previously only residents in congregate care settings and other immunocompromised people were able to get a fourth COVID-19 vaccine shot.

But the benefits of a fourth vaccine dose appear to be short-lived, with a study from Israel published in the New England Journal of Medicine on Tuesday finding the protective benefits against symptomatic infection for elderly groups begin to wane at four weeks after injection and almost completely disappear after eight weeks.

The new vaccine eligibility comes as the province's limited testing and wastewater signal data indicates Ontario is entering a sixth wave of COVID-19.

The number of people admitted to hospital testing positive for COVID-19 was up 40 per cent Tuesday compared with a week earlier.