The observed clinical trial rate was in PwMS who had received a mean of 17.9 monthly doses of natalizumab. In comparison, the post-marketing rate is calculated as the number of PML cases since reintroduction in PwMS who have had at least 1 dose of natalizumab. Incidence estimates by treatment epoch are calculated based on natalizumab exposure through 31st May 2011 and and 133 confirmed cases as of 1st June 2011. The incidence for each epoch is calculated as the number of PML cases divided by the number of patients exposed to natalizumab (e.g. for 25 to 36 infusions all PML cases diagnosed during this period is divided by the total number of patients ever exposed to at least 25 infusions and therefore having risk of developing PML during this time). This data has been provided by Biogen-Idec.
"The data is becoming much clearer in relation to the risk of PML on natalizumab. Please see earlier post on new data in relation to risk profiling in relation to duration of exposure, JCV seropositivity and previous exposure to chemotherapy; click here for post on risk stratification."