The patients died of a deadly brain infection known as PML, and 24 cases of the disease have been diagnosed since Tysabri was reintroduced to the market in 2006, the European Medicines Agency told Bloomberg News. The FDA confirmed the number of PML cases as 24, but declined further comment.

“We’re not commenting on individual cases of PML,” Jennifer Neiman, a Biogen spokeswoman, tells Bloomberg. “The incidence of PML is rare and the rate remains consistent with what was seen in clinical trials and what is described in our current label.”

Just last week, the number of reported PML cases was pegged at 23, up from the 13 previously disclosed. That 10 new cases emerged in the last six weeks is “of particular concern,” UBS analysts Guillaume van Renterghem and Martin Wales wrote in an investor note.

The FDA temporarily suspended sales of Tysabri in 2005 after three cases of PML, or progressive multifocal leukoencephalopathy, were reported. Patients pleaded for the product’s return and since then investors and doctors have closely monitored reported side effects, Bloomberg reminds us. Since Tysabri returned in 2006, 14 of the cases were found in Europe, eight in the US and two in Switzerland.