The efficacy of the two-vaccine mixture is still being studied, but Sri Lankan health officials approved it due to a severe shortage of AstraZeneca.
Sri Lanka received 26,000 doses from Pfizer on Monday as the first batch of 5 million injections to be received this year. It began distributing them on Wednesday to residents 55 and older in the capital Colombo.
Approximately 384,000 people were fully vaccinated before Sri Lanka ran out of AstraZeneca doses with 540,000 people only partially vaccinated.
Channa Jayasumana, the minister of state overseeing pharmaceuticals, said obtaining Pfizer as a second dose was optional and that a shipment of the AstraZeneca vaccine was expected this month.
Sri Lanka has also used Sinopharm and the Russian Sputnik V to vaccinate people against Covid-19.
Sri Lanka has seen a sharp increase in coronavirus infections and deaths since April. It has registered 266,499 cases with 3,268 deaths overall.