Japan is poised to buy more doses of the COVID-19 vaccine developed by Pfizer Inc. and BioNTech SE, a move that would pave the way for the country to immunize its adult population without using any of the shots it purchased from AstraZeneca PLC, which have been linked to rare blood clots.

Pfizer is expected to provide an additional 50 million doses by September in a contract that could be agreed on this month, government sources said Wednesday.

Together with existing agreements with Pfizer for 144 million doses and Moderna Inc. for 50 million doses, Japan is now set to procure enough vaccines for its roughly 110 million people age 16 or older.