Ontario will allow all adults under 60 to access fourth COVID-19 vaccine doses starting Thursday, Chief Medical Officer Dr. Kieran Moore says.

Anyone can receive a fourth dose provided five months have passed from their last shot and their last COVID-19 infection was at least three months ago.

But Moore says his “real call is for anyone aged 18-59 who wasn’t in the older (+60) group with underlying illnesses” such as diabetes or heart disease.

Everyone else who is generally healthy should wait for Omicron-specific bivalent vaccines, expected to arrive in Canada in the fall.

“There is not an obligation to get a booster dose if you are young and healthy,” Moore said.

Eligibility previously included residents of congregate care, people diagnosed with a select grouping of immune-suppressive conditions, Indigenous people and anyone 60+.

Ontario opened up third dose eligibility to all adults nearly seven months ago, but the U.S. and Quebec opened up fourth dose eligibility much sooner, leading some Ontarians to travel to those areas to get a fourth dose.

Moore said there “nuances” to his recommendations for a fourth dose, saying that even though healthy people who have “persistent and powerful immunity” should wait until fall, certain personal circumstances could change the calculus.

“If your parent is undergoing chemotherapy, then absolutely you should get a second booster dose,” he said, adding healthcare workers, people who work in busy crowded settings should get a fourth dose even though they themselves have no underlying health condition.

Ontario’s epidemiologists are split about the value of offering a fourth dose now versus waiting until the fall, when federal health officials expect the country will face another wave of COVID-19 transmission.

But Ontario and the rest of the nation is already facing a wave this summer driven by the BA.5 subvariant of coronavirus.

Moore says this wave will likely peak in the next “two weeks.”

Vaccine appointments can be made through the provincial vaccine booking portal.

Moore said the province and local public health units, pharmacies and doctors’ offices have the capacity to vaccinate 100,000 per day.

On July 7, the province administered about 16,000 shots.

Toronto Chief Medical Officer Dr. Eileen de Villa strongly encouraged all Toronto residents who are eligible to get a fourth COVID-19 shot as soon as possible.

"Keeping up to date with your immunization is the best way to ensure you, your loved ones and your community are protected from the virus and its variants," she said, adding people with a third dose had the lowest rate of hospitalization during previous waves.

"While the third dose provides good protection of COVID-19, the fourth dose provides even better protection."

She said that 230,000 fourth doses have already been administered, and another 1.84 million other people are eligible for fourth doses in the city as of Wednesday.

Ottawa doctor Nili Kaplan-Myrth, who threatened to sue the province over its refusal to offer 4th doses to all adults, told CP24 on Wednesday she is ending her legal effort and refunding her 191 backers on crowdfunding website GoFundMe.

“I had three lawyers helping with this,” she said. “They had collected affidavits they were ready to go – but we don’t have to proceed with that.”

She said she understood Moore’s caution and nuance that not everyone has to rush out to get a fourth dose, but for healthcare workers and anyone who works in a busy indoor setting, it’s kind of a no-brainer.

“If it gives you some modicum of additional protection and knowing you’re not getting your (bivalent) vaccine until November, why wouldn’t we want to have people have their boosters instead of waiting as this wave is ramping up.”

She said the suggestion Moore made for people to contact their healthcare provider is cold comfort for the hundreds of thousands of Ontario residents who have no family doctor.

But she said she was satisfied that the rules were changing.

“It was ridiculous for us to be told by Ontario that we couldn’t give the vaccine. (Adults under 60) should be eligible and now they are.”

Ontario NDP interim leader Peter Tabuns said the party was happy fourth doses will now be available to all adults.

"While we’d like to have seen fourth doses sooner, we’re glad Ford has finally listened to our calls and to the advice of Ontario’s medical community, and offered Ontarians another shot," he said. "We’re grateful to Ontario’s medical community for their relentless advocacy, even when Ford doesn’t want to listen. They deserve much of the credit for moving the Ford government to finally provide fourth doses."

He urged that doses be available in family doctors' offices and brought directly to home care recipients.

In light of the BA.5 summer wave, Moore says the province will continue to distribute free packs of rapid antigen tests to pharmacies and grocery stores until at least the end of 2022.

He said that the later Omicron variants have impacted the sensitivity of a standard rapid antigen test, reducing sensitivity to 50 to 60 per cent.

Moore said you can assume you have COVID-19 if you test positive, but one negative test isn’t enough to conclude you are COVID-free.

You should take two tests, 24 hours apart, and make sure you swab your nasal passage, buccal area inside the mouth and throat before processing the specimen.