Diagnosis of measles by clinical case definition in dengue-endemic areas: implications for measles surveillance and control.

In many countries, measles surveillance relies heavily on the use of a standard clinical case definition; however, the clinical signs and symptoms of measles are similar to those of dengue. For example, during 1985, in Puerto Rico, 22 (23%) of 94 cases of illnesses with rashes that met the measles clinical case definition were serologically confirmed as measles, but 32 (34%) others were serologically confirmed as dengue. Retrospective analysis at the San Juan Laboratories of the Centers for Disease Control showed also that at least 28% of all laboratory-confirmed cases of dengue in Puerto Rico in 1985 met the measles clinical case definition. If the true measles vaccine efficacy (VE) is assumed to be 90%, the occurrence of laboratory-confirmed dengue cases that meet the measles clinical case definition results in a reduction of the apparent measles VE to only 64% (a 29% relative reduction from the true VE). The results of the study demonstrate the importance of a laboratory-based surveillance system in measles control or elimination efforts in dengue-endemic areas.