SAN FRANCISCO ( TheStreet) -- Biogen Idec ( BIIB - Get Report) said Tuesday that sales of its multiple sclerosis drug Tysabri surpassed $1 billion in 2009, but the number of new patients starting treatment on the drug in the fourth quarter appears on the low end of Street expectations. In advance of an afternoon presentation at the J.P. Morgan Healthcare Conference here, Biogen said that 48,800 patients were being treated with Tysabri worldwide as of the end of December. That works out to 2,600 new patients added to Tysabri's treatment role in the fourth quarter, or 200 new net patients per week. In a research note Monday, Deutsche Bank biotech analyst Mark Schoenebaum estimated that there would be 200 to 250 new Tysabri patients added per week in the fourth quarter. In a note issued last week previewing his bank's healthcare conference, J.P. Morgan analyst Geoffrey Meacham said he was looking for 250 new Tysabri patients per week in the fourth quarter. Biogen shares Tysabri revenue with Irish drugmaker Elan ( ELN). The $1 billion in Tysabri global sales for 2009 were in-line with Street expectations. Biogen shares were down 15 cents to $53.75 in early Tuesday trading. In other news, Biogen and Acorda Therapeutics ( ACOR - Get Report) announced the approval filing in Europe for fampridine, a drug to improve the walking ability of multiple sclerosis patients. The U.S. Food and Drug Administration is currently reviewing fampridine for approval. -- Reported by Adam Feuerstein in San Francisco. Follow Adam Feuerstein on Twitter.