British pharmaceuticals group AstraZeneca, which is finalising a vaccine against Covid-19, said Saturday it was buying US biotech firm Alexion for $39 billion to boost its work on immunology.

"Alexion has established itself as a leader in complement biology, bringing life-changing benefits to patients with rare diseases," AstraZeneca chief executive Pascal Soriot said in a statement, announcing the year's biggest deal in the pharma sector.