Anglo-Swedish drugmaker AstraZeneca is weighing a merger with US biotech Gilead Sciences, the Bloomberg news agency has reported.

Surpassing Bristol Myers Squibb’s $74 billion takeover of Celgene, it would be the biggest pharma deal in history, with a value of just under $240 billion, but analysts who commented on the report said they think it may be too big a risk.

The UK-based drugmaker is worth $140 billion, Gilead $96 billion.

According to Bloomberg, Astra Zeneca approached Gilead about such a deal last month but no formal talks are believed to be under way, and the news agency’s sources say Gilead isn’t interested in selling to a larger rival at present.

Both companies are at the forefront of responding to the Covid-19 outbreak, with AstraZeneca – through its deal with Oxford University – deeply into the vaccine side, and Gilead moving forward with a therapeutic to treat the novel coronavirus.

While there could be some synergies, from a purely strategic perspective Bloomberg Intelligence analyst Sam Fazel said the attraction of the mooted deal to AstraZeneca is not obvious.

With US emergency approval as a Covid-19 treatment, Gilead’s remdesivir drug has caught investors’ attention; however, its commercial prospects remain uncertain, Fazel said. The company’s portfolio also is weaker than that of the UK-headquartered drugmaker, which has several vaccine deals in the works alongside a full pipeline of drugs in oncology, cardiovascular and respiratory disease.

On the plus side, the analyst noted, Gilead has steady cash flow from its leading HIV drug franchise in addition to a strong balance sheet that would help AstraZeneca, which is carrying net debt roughly twice its EBITDA and having to sell stakes in promising drugs and bring in partners to co-fund research and development.

For Reuters analysts, AstraZeneca’s proposal is a “strange reversal in corporate logic.” In 2014, they recalled, CEO Pascal Soriot fended off a bid from Pfizer, arguing that the company would do better on its own. If a deal with Pfizer was unappealing, “a combination with Gilead looks doubly so,” they added.

AstraZeneca in more vaccine deals

In other news, AstraZeneca has clinched a $750 million deal with the Coalition for Epidemic Preparedness Innovations (CEPI) and Gavi, the Vaccine Alliance to manufacture and distribute 300 million doses of Oxford's adenovirus-based Covid-19 vaccine by the end of 2020.

Under the agreement, CEPI will manufacture the vaccine, while Gavi will handle procurement. The Access to COVID-19 Tools Accelerator backed by the Bill and Melinda Gates Foundation and the World Health Organization will handle marketing and distribution of the vaccine across the world.

AstraZeneca also has agreed a licensing deal with the Serum Institute of India to provide 1 billion doses of the vaccine to low- and middle-income countries.