Ontario says thousands of Oxford-AstraZeneca vaccines expiring Monday are still undergoing quality checks before being sent to pharmacies and doctors' offices.
It says 12,000 doses are expected to be cleared tomorrow, and others are still being reviewed.
A spokeswoman for the health minister says approximately 26,600 doses had been sent out as of Thursday morning to be administered as second shots.
Ontario stopped giving out first doses of the AstraZeneca COVID-19 vaccine earlier this month after saying it was linked to an increased risk of a rare but serious blood clotting disorder.
The province is now offering second shots of the vaccine at a 10-week interval to the first batch of AstraZeneca recipients as it works through roughly 45,000 doses set to expire on May 31.
Ontarians who received the vaccine between March 10 and March 19 at participating sites in Kingston, Windsor and Toronto are eligible to book those second doses.