TORONTO -- Many Ontarians aged 80 and older became eligible to book their second COVID-19 vaccine doses on Monday, though some regions said a lack of supply meant they couldn't offer second shots to the oldest seniors right away.

The government announced last week that it was shortening the intervals between doses, starting with the 80 and older cohort this week, but left it up to individuals to schedule earlier shots.

Some said they were able to quickly move up appointments for their loved ones on Monday morning.

Karthik Kanagas said he was able to move his father's second-dose up to Wednesday -- far earlier than the original appointment booked for July at a Toronto clinic.

"He's incredibly relieved," Kanagas said. "He's looking forward to having a little bit of freedom of mobility again, to go grocery shopping and do those little things."

Mo Saeed was able to reschedule a second dose for his 80-year-old father-in-law, moving up the shot by six weeks.

"He's excited to be getting it sooner," said Saeed, adding that he would have preferred if his Mississauga, Ont.-based father-in-law had received an email or call from about rescheduling.

The government has been criticized for not having a plan to contact recipients directly to move up second doses. Residents will keep their original second-dose appointments -- four months from the first -- if they don't book an earlier shot.

Some health units said, however, that they wouldn't be able to immediately offer earlier second doses to the 80 and older cohort due to vaccine supply challenges.

The Middlesex-London Health Unit reported it would offer earlier second shots to that group starting June 7, or possibly sooner, as it continues to prioritize first shots.

"The province indicated those 80 years of age and older could begin rescheduling their second dose appointments as early as this morning, given that some health units have more vaccine supply than needed for those seeking first doses in their areas," it said.

"That is not the case in Middlesex and London, where most vaccine appointments continue to be first doses."

The health unit covering Sudbury, Ont., said second shots for the oldest seniors would likely become available the week of June 28, based on known supply and plans to prioritize youth for first shots in the coming weeks.

"I thank everyone for your ongoing patience and understanding," Dr. Penny Sutcliffe, the region's top doctor, said in a statement. "Some may have to wait a little bit longer for their second dose as public health ensures first doses are in arms and second doses are provided to those in previously announced groups."

The shortened dose interval is part of the government's plan to fully vaccinate all willing adults by the end of August.

Those 70 and older will see their interval shortened in mid-June, and after that, residents will become eligible for second doses based on when they received their first shot.

The expanded vaccine effort came as Ontario reported 916 cases of COVID-19 on Monday -- its lowest daily total since Feb. 17 -- and 13 more deaths from the virus. The new cases were based on 18,200 completed tests.

Later Monday, the government was set to table a motion today that would replace the province's top public health doctor.

Health Minister Christine Elliott said the province is moving to replace Dr. David Williams with Dr. Kieran Moore.

Moore currently serves as the top doctor at the Kingston-area public health unit, and is expected to take over as Ontario's chief medical officer of health on June 26.

Williams had been slated to retire in September, but his last day has been pushed up by several months. Elliott said Moore will start working with Williams on June 7 to ensure a smooth transition.

Meanwhile, Premier Doug Ford continued to mull on Monday whether to reopen schools for the last few weeks of the academic year and said an announcement would come "in the next day or two."

He had asked experts, public health officials, teachers' unions and numerous stakeholders for advice on the matter late last week.

- With files from Denise Paglinawan.

This report by The Canadian Press was first published May 31, 2021.