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.

"We look forward to welcoming our new colleagues at Alexion so that we can together build on our combined expertise in immunology and precision medicines to drive innovation that delivers life-changing medicines for more patients," he said.

Both boards have agreed to the cash-and-stock deal, which values Alexion at $175 a share, a premium of 45 per cent to the Boston-based company's closing price on Friday.

Since taking the helm at AstraZeneca in 2012, Soriot has pushed the group into lucrative treatments such as cancer therapies, and the Alexion takeover will give it more heft in areas such as treating blood disorders.

Along with researchers from the University of Oxford, AstraZeneca has developed an effective Covid-19 vaccine which the UK government plans to use as the lynchpin of its inoculation drive, after first rolling out another vaccine by Pfizer/BioNTech.

But approval of the AstraZeneca/Oxford drug has been held up after queries over its initial trial results.

On Tuesday, the UK partners became the first Covid-19 vaccine makers to publish final-stage clinical trial data in a scientific journal, clearing a key hurdle.

The new acquisition is expected to close in the third quarter of 2021, and Alexion shareholders will own 15 percent of the combined company.

"This transaction marks the start of an exciting new chapter for Alexion," Alexion chief executive Ludwig Hantson said.

"We bring to AstraZeneca a strong portfolio, innovative rare disease pipeline, a talented global workforce and strong manufacturing capabilities in biologics," he said.