British-Swedish pharmaceuticals firm AstraZeneca approached one of its US rivals, Gilead Sciences, last month over a potential merger, the Bloomberg news agency has reported. A transatlantic tie-up would be the biggest healthcare merger yet, forging a company worth around £200bn, and bringing together firms leading the pharmaceutical industry’s efforts to develop a vaccine and treatments for Covid-19. Cambridge-headquartered AstraZeneca, which is valued at £110bn, recently overtook Royal Dutch Shell to become the UK’s largest company by market value. Gilead was valued at about $96bn (£74bn) at Friday night’s closing price. AstraZeneca contacted Gilead in May, but did not provide the terms of any transaction, according to Bloomberg. A spokeswoman for AstraZeneca said the company did not comment on rumours or speculation. Gilead did not immediately respond to a request for comment. However, sources familiar with AstraZeneca said that there appeared to be little rationale for such a proposal right now and questioned the report. AstraZeneca has been at the forefront of efforts to develop a coronavirus vaccine, in collaboration with researchers at Oxford University, which could potentially lift its value significantly if successful. Face-to-face meetings to push through a colossal merger would also be extremely difficult given the travel constraints during the pandemic, another said. Gilead’s antiviral drug Remdesivir has been approved for treatment of Covid-19 patients by governments around the world in recent weeks after clinical trials showed that it could improve their condition. The two companies are among a host of firms, including Eli Lilly, Pfizer and Merck, racing to develop a vaccine and other treatments for Covid-19. Bloomberg said Gilead had discussed the merger idea with advisers, but that no decision had been made on how to proceed and the companies were not in formal talks. Gilead was not interested in selling to or merging with another big pharmaceutical company, and preferred to focus on partnerships and smaller acquisitions, the report said. AstraZeneca said last week that it had doubled manufacturing capacity for its potential coronavirus vaccine to 2bn doses in two deals involving the Microsoft billionaire Bill Gates that guaranteed early supply to lower-income countries. Trials of the vaccine are to start in Brazil. AstraZeneca’s partnership with Oxford University to develop one is among a handful of initiatives backed by Donald Trump’s Covid taskforce. AstraZeneca has also said that it was close to a breakthrough on a potential antibody treatment that could save the lives of people who become infected.