The company said the delivery of the doses was expected in the first quarter of 2022 and more details about the deal would be announced in the coming weeks.
The additional doses will add to previous purchase of 700,000 doses of the antibody therapy, which in December showed it retained neutralising activity against the highly transmissible Omicron coronavirus variant in a lab study.
The U.S. Food and Drug Administration last month authorized Evusheld to prevent COVID-19 infections in individuals with weak immune systems or a history of severe side effects from coronavirus vaccines.
It is the only antibody therapy authorised in the United States to prevent COVID-19 symptoms before virus exposure, the company said.