Dec 14 (Reuters) - Shares of AstraZeneca fell 8% early on Monday, hit by doubts about the British firm’s strategy and speculation of a rival bid after it announced the $39 billion purchase of Alexion Pharmaceuticals over the weekend. Chief Executive Officer Pascal Soriot had told reporters on Saturday that the deal, a bet on rare-disease and immunology drugs, was the result of exclusive talks and no competitive bidder was involved. The move also helps AstraZeneca, which is also working on a COVID-19 vaccine, diversify from its fast-growing cancer business. (Reporting by Pushkala Aripaka in Bengaluru; Editing by Arun Koyyur)