BOSTON - Biogen Idec Inc said on Tuesday it would be premature to conclude that the risk of developing a potentially deadly brain infection increases the longer patients take its multiple sclerosis drug Tysabri.

The company told investors at the Morgan Stanley healthcare conference that despite two new reports of patients who have developed progressive multifocal leukoencephalopathy, or PML, it is too early to draw any specific conclusion.

Cambridge, Massachusetts-based Biogen, which sells Tysabri with Irish drugmaker Elan Corp Plc ceased updating investors with the number of patients who developed PML at the end of July, as part of a broader strategy to focus more on the positive aspects of the drug.

At that time, Biogen had confirmed 11 cases of PML since the drug was relaunched in July 2006. It had been temporarily withdrawn in 2005 because of its links to PML.

Recently, however, two new cases have emerged, bringing the total to 13. One case was reported in the New England Journal of Medicine. The other was reported by Ralf Gold, of the Ruhr University Bochum in Germany, who presented the data recently at the European Committee for Treatment and Research in Multiple Sclerosis.

Both cases occurred in Europe.

As of June 30, about 43,300 patients were taking Tysabri. More than 30,000 had taken it for more than a year and about 10,000 for more than two years. Most cases of PML so far have developed in patients taking the drug for more than a year.

Biogen has said it is looking into all possible risk factors associated with PML, including duration of treatment, but that it would be premature to conclude that there is a definitive correlation between duration and risk.

The company said it would notify investors if the number of patients who develop PML exceed the rate of one in 1,000 patients, which is the rate listed in the drug's prescribing information.

Biogen's shares were up 6 cents at $50.89 in late morning trading on Nasdaq. (Reporting by Toni Clarke; editing by Gunna Dickson)