Pharmaceutical company AstraZeneca says it is buying U.S. drug developer Alexion in a deal worth $39 billion.

Cambridge, England-based AstraZeneca PLC, which is involved in one of the efforts to develop a COVID-19 vaccine, said Saturday it's using a combination of cash and shares for the acquisition of Boston-based Alexion Pharmaceuticals Inc.

The boards of both companies unanimously approved the transaction, which still needs regulatory and shareholder approval. The deal is expected to close in the third quarter of 2021.

"This acquisition allows us to enhance our presence in immunology," AstraZeneca CEO Pascal Soriot said in a statement.

AstraZeneca and Oxford University have jointly developed a coronavirus vaccine that British and Canadian regulators are assessing, alongside a rival effort by U.S. drugmaker Pfizer and Germany’s BioNTech that has already earned some emergency approvals, and another by U.S. biotechnology company Moderna.