Leading Covid-19 vaccine developer AstraZeneca agreed on Saturday to buy Boston pharmaceutical firm Alexion\n \n (ALXN) for $39 billion. The acquisition is one of the largest deals of the year for the drug industry and would expand AstraZeneca’s\n \n (AZN) reach in rare diseases. Alexion has an immunology unit in Boston that serves patients with rare disorders. AstraZeneca, a British-Swedish drugmaker headquartered in the United Kingdom, has partnered with the University of Oxford to produce a Covid-19 vaccine. It’s in the final stages of development and is awaiting authorization from European and British governments. Last month, the company found its vaccine was 70% effective on average, but scientific experts raised concerns over a lack of transparency in reporting the vaccine trial data, potentially slowing down the timeline for approval. AstraZeneca agreed to pay for the purchase with a mix of cash and stock, and said it took out a short-term loan of $17.5 billion from Morgan Stanley, J.P. Morgan and Goldman Sachs to help pay for it. Both companies put out a press release Saturday stating their boards of directors unanimously approved the deal. The acquisition is still pending government regulations and shareholder approval. It’s expected to close mid-2021, according to the press release. In a statement, Alexion told CNN Business that by combining its expertise in rare diseases with AstraZeneca’s comprehensive global reach and broad portfolio, “we will substantially expand our impact to the patients we can serve around the world.” After the acquisition, AstraZeneca expects to increase its annual revenue by double digits through 2025. Alexion will help with cash-flow and debt reduction, the company said.