* Total PML cases at 28 as of Nov. 30, 2009 - Biogen
* Total at end of October was 24
* December figure to be released in week or two
BOSTON, Jan. 6 (Reuters) - Biogen Idec Inc (BIIB.O) said on Wednesday that four more patients taking its multiple sclerosis drug Tysabri had developed a potentially deadly brain infection known as PML in November, bringing the total to 28.
The biotech company said it will release the number for December within a week or two.
Cambridge, Massachusetts-based Biogen makes Tysabri, its biggest driver of growth, in partnership with Elan Corp Plc ELN.I of Ireland.
Tysabri was temporarily withdrawn from the market in 2005 after being linked with progressive multifocal leukoencephalopathy, or PML. It was reintroduced in July 2006 with stricter safety warnings.
The risk rate is still within the rate of 1/1000 listed in the drug's package insert, Naomi Aoki, a spokeswoman for Biogen, said .
"While I understand this interest in counting the cases, one case does not make any impact on the rate at all," she said. (Reporting by Toni Clarke; Editing by Gary Hill)