TORONTO -- The province is now expecting to receive 250,000 more doses of the AstraZeneca vaccine next week, a shipment that will be reserved for second shots as the province has suspended use of the vaccine until further notice.

Officials confirmed on Wednesday morning that the province recently received information that Ontario will receive 254,500 more doses of the AstraZeneca vaccine during the week of May 17.

But those doses will be shelved for future use following the province’s announcement Tuesday that it has paused administering first doses of the AstraZeneca COVID-19 vaccine amid concerns over rare but serious blood clotting incidents.

The province’s Chief Medical Officer of Health Dr. David Williams said Tuesday that new data suggests the risk of vaccine-induced thrombotic thrombocytopenia (VITT) may be higher than initially thought – about one in 60,000 people rather than one in 125,000.

In Ontario, there have been eight cases of vaccine-induced blood clotting reported in those who received the AstraZeneca shot and while the condition remains extremely rare, it can be fatal.

Increased supply of the Pfizer vaccine in the province over the next couple of months means Ontario is in a position where it can suspend the use of AstraZeneca for now “out of an abundance of caution,” Williams said.

Declining case numbers in Ontario was also a factor in the decision, he added.

Officials said data from the United Kingdom suggests that there is a significantly reduced risk of clots in those who receive their second dose of AstraZeneca and the province is still reviewing the data.

The province is also requesting that Canada’s National Advisory Committee on Immunization (NACI) provide direction on the interchangeability of COVID-19 vaccines, including using mRNA vaccines for second doses for those who received AstraZeneca as the first dose. It is not yet clear if those who received AstraZeneca for their first dose will be able to choose what they receive for their second.

Ontario currently has about 50,000 doses of the AstraZeneca vaccine that have not been used.

Much of the remaining supply is set to expire at the end of May or early June and while the timeline for administering second doses of the AstraZeneca vaccine is not clear, the province is trying to work out a plan to use the leftover doses before they spoil.

"For the moment with a small number of doses that we already have, we're just holding on to them, pending the final review by NACI and Health Canada," Ontario Health Minister Christine Elliott said at a news conference on Wednesday.

"As far as the doses that will be available during the week of the 17th, that is something that no final determination has been made as yet as to whether they will be received in Ontario. So it's all pending the review by NACI and Health Canada."

The province has not said whether it plans to bump up second doses for those who received AstraZeneca for their first shot.

"We know that the second doses aren't coming up until the end of June so we certainly have time to consider all of this data and await the results and the recommendations of both NACI, Health Canada, and of course our chief medical officer of health," Elliott said.

Officials say they intend to provide more guidance on second doses to those who received the AstraZeneca shot “shortly.”

Ontario hopes to begin vaccinating those aged 12 to 17 in June

During a briefing on Wednesday morning, the province also confirmed that it aims to begin vaccinating children between the ages of 12 and 17 starting in June but a detailed timeline was not provided.

Earlier this month, Health Canada approved the Pfizer COVID-19 vaccine for use in children as young as 12.

"We are actively working on the vaccine rollout plan... with the Ministry of Education, and working with our public health units as well. This is extremely important. We want to make sure that our young people can receive the doses as soon as possible so we will have further information very shortly on that when the details are finalized," Elliott said on Wednesday.

The province previously said it is on track to administer first doses to 65 per cent of adults in Ontario by the end of the month and on Wednesday, officials said they could actually exceed that target based on the current rollout.

According to the latest data, about 54 per cent of adults in hot spot postal codes have received their first shot. Over the past two weeks, the province has diverted 50 per cent of all incoming doses to hot spot postal codes in an effort to vaccinate those most at risk of infection. Despite calls to continue doing this, the province has said it will return to per capita distribution starting next week.

“We’re in this long, deep and frustrating lockdown because Doug Ford allowed hot spot fires to rage and spread out of control,” Ontario NDP Leader Andrea Horwath said in a written statement released Wednesday.

“Now that we have a steady supply of vaccines, we have a chance to put out the hot spot fires. Instead, the Ford government is going to leave them smouldering. It could set the entire province back again.”

The province's own Science Advisory Table recommended diverting more doses to hot spots for at least four weeks.

Elliott defended her government's decision on Wednesday, saying that hot spot communities will still be receiving a significant number of vaccine doses going forward.

"We are receiving almost double the number of Pfizer doses for the month of May and into June and so when we go back to 100 per cent distribution across the 34 public health units based on population and based on risk, that we will still see a very large number of vaccines going into those at-risk communities," Elliott said.

"We are going to continue with the progress we have already made."

Starting this week, Elliott said the province is also working to book and administer second doses earlier than the four-month interval for the highest-risk health care workers and urban Indigenous communities.

The province is also continuing to roll out the Pfizer and Moderna vaccines in more pharmacies, with a total of 1,630 sites across the province as of next week. Officials say during the week of May 24, 2,490 pharmacies in the province are expected to offer shots.