The risk for a disease or adverse event represents the probability for the event to occur among a defined susceptible (or exposed) population. For example, the risk of developing a skin rash in the first week of treatment in patients receiving an antimicrobial. When risk is presented as a proportion (per 100 patients), we assume that all patients were exposed for the same length of time. In the examples above, all subjects were observed for 1 week. However, it is expected that the longer a person is exposed, the higher is the probability for the event to be observed.3

Incidence rate (or incidence density rate or person-time incidence rate)

Incidence rates can be used to account for different lengths of times exposed when measuring risk.  It is a statistic that represents a summary measure of an event occurrence per population, taking into account the length of time that each member of the population was susceptible or exposed (person-time at risk). In technical terms, incidence rate of an event in a population is the ratio of new events in a specified time period (numerator) divided by the number of patients at risk during that period (denominator). For example, 1 case per 1,000 patient-years exposed, expresses 1 new case observed among 1,000 patients during one year of exposure.3


Refers to the time after a drug or biological product has been approved by FDA.4

Carry-over PML

PML that develops a few months after stopping one disease modifying therapy (DMT) and starting a different DMT. In these cases, PML could have developed without causing symptoms while the patient was still on the previous DMT, or shortly after stopping the previous DMT.5




Porta M (Ed.) (2014) A dictionary of epidemiology (6th ed.). New York, NY: Oxford University Press. http://irea.ir/files/site1/pages/dictionary.pdf. Accessed June 2018


US Food and Drug Administration. Postmarketing Requirements and Commitments: Reports. https://www.fda.gov/Drugs/GuidanceComplianceRegulatoryInformation/Post-marketingPhaseIVCommitments/ucm064436.htm. Accessed January 2018


Giovannoni G, et al. Pract Neurol. 2016; 16:389–393.