In a rare move, the Food and Drug Administration allowed Tysabri back on the market in 2006 with strict protocols. Patients are tested for the virus, must come in for the IV treatment every four weeks and must check in with their neurologist every three months.
The companies said in the fall about 71,000 patients were being treated with Tysabri in 2012. That was up 13 percent from a year ago.
Biogen shares rose $3.62, or 2.3 percent, to $160.98 in trading Wednesday after rising as high as $167.35 earlier in the session. That was an all-time high, according to FactSet.