Ontario is expanding eligibility for fourth COVID-19 vaccine doses to include all adults, but the province's top doctor says healthy residents under the age of 60 can choose to wait until the fall for a new shot that's expected to better target Omicron variants.

Chief Medical Officer of Health Dr. Kieran Moore said Wednesday that anyone aged 18 and older who had their first booster five months ago will be able to book a fourth shot – or second booster – starting on Thursday.

"While my call for arms remains the loudest for the most vulnerable in our communities, we've made the decision to expand the eligibility for second boosters," Moore said.

A new COVID-19 vaccine better targeting Omicron variants is expected to be approved this fall, said Moore, encouraging Ontarians to speak with their doctors about whether getting a second booster now is right for them.

Most residents aged 18 to 59 have strong protection against the virus more than six months after their first booster, he said, but expanding fourth-dose eligibility will ensure they can make an "informed decision" based on their personal circumstances, pointing to risk factors like smoking or diabetes.

"It's not a 'should,' it's absolutely a 'may' depending on your personal circumstances," Moore said.

People who choose to take the second booster this summer may have to wait five months for their next shots, or three if they are immune suppressed, though Moore noted that the exact timeline is unclear.

He said province will likely start vaccinating high-risk individuals with targeted vaccines earlier in the fall, so most Ontarians likely wouldn't be eligible until later anyway.

"That timing will not interfere or slow down if you get your second booster dose in July," he said.

Ontario had recently been under pressure to expand eligibility for fourth doses beyond people aged 60 and older, immunocompromised individuals and Indigenous adults.

Moore said the province wanted to encourage the most vulnerable to get boosters and for people to catch up on earlier COVID-19 vaccines before opening up eligibility.

The province is currently experiencing a summer wave of virus infections driven by the BA.5 Omicron subvariant that Moore said could peak within the next few weeks.

"We know it's the summertime and people want to move on past the pandemic, but it's a clear reality now that we have to deal with this BA.5 variant," he said.

Ontario is also extending a program providing free rapid antigen tests for COVID-19 through venues that include grocery stories and pharmacies until Dec. 31.