UPDATE 1-Study shows new data on MS drug Tysabri--analyst

(Adds comment from Biogen)

BOSTON, March 4 (Reuters) - A new study of the multiple sclerosis drug Tysabri shows some patients developed the virus associated with a potentially deadly brain infection, although not the infection itself, according to an analyst report.

Citing a summary, or abstract, of a presentation to be presented in April at the American Academy of Neurology, Corey Davis, an analyst at Natixis Bleichroeder, said an examination of 175 patients treated at one center found two cases of JC virus -- one in the cerebrospinal fluid and one in the plasma.

"It's the one in the CSF that a priori might be concerning," said Davis in a research report.

But he noted none of the cases showed any signs of progression to progressive multifocal leukoencephalopathy, or PML, the potentially deadly brain infection.

Tysabri, which is made by Biogen Idec Inc (BIIB.O: Quote, Profile, Research, Stock Buzz) and Elan Corp Plc (ELN.N: Quote, Profile, Research, Stock Buzz)(ELN.I: Quote, Profile, Research, Stock Buzz), was temporarily suspended from the market in 2005 after several patients developed the disease but was allowed back in 2006 with certain restrictions after U.S. regulators decided MS patients were willing to accept the risks in return for the potential benefits.

"We do not know the reliability, sensitivity, nor specificity of the assay being used by this one group to detect the presence of the virus, and hence are hesitant to draw too many conclusions," Davis said.

Shannon Altimari, a spokeswoman for Biogen said that the company has seen the abstract but the key point is that at the end of December more than 21,000 patients were taking Tysabri and there have been no new cases of PML.

Biogen's shares were up 21 cents at $59.05 in late afternoon trading on Nasdaq. (Reporting by Toni Clarke; Editing by Andre Grenon and Gerald E. McCormick)