The Cambridge, Mass., biotech company reported no additional deaths in patients that have developed progressive multifocal leukoencephalopathy, or PML, keeping the total at 11.

Tysabri is considered a highly effective therapy for MS, and its sales growth is important to the future of both Biogen and Elan. But sales have been slower than first anticipated because of concerns about the risk of PML, which led to the drug's 18-month market withdrawal beginning in 2005.

The overall global rate of PML infection is about 0.77 per 1,000 patients, Biogen said, which falls within the 1-in-1,000 rate previously seen in clinical trials and implied on the drug's label.

Shares of Biogen added 23 cents Wednesday to $48.35, while the American Depositary Shares of Elan slid 11 cents to $5.04.

Of the 55 cases Biogen reported, 20 were in the U.S., 32 in the European Union and three in other areas.

Biogen provides monthly updates on the number of PML cases in patients taking Tysabri.

A patient's risk of getting PML increases with the number of monthly infusions that he or she receives, something the Food and Drug Administration highlighted in a January safety update. The agency concluded that the benefits of the medicine continue to outweigh the risks.

The most recent monthly update translates to a rate of 1.24 cases per 1,000 for patients on the drug for a year or longer, but rises to 1.76 per 1,000 for those on the drug at least two years.

The rate of incidence is about 0.35 cases per 1,000 patients on the drug for between one and two years and 1.47 per 1,000 for those taking the drug between two and three years.