While admitting he's had concerns over the business model for the past 18 months, Oppenheimer analyst Hartaj Singh says the 11% selloff in shares of Alexion Pharmaceuticals over the past two days is "irrational." The valuation is now 50% below the company's nine-year average and consensus estimates have decreased dramatically over the past two years, Singh tells investors in a research note. He believes any positive news, such as reiterating Q1 sales guidance and/or a cost rationalization, could change Alexion's profile "fairly dramatically." The analyst is "intrigued" at current share levels but keeps a Perform rating on Alexion pending further visibility.