The effect of aggregating data has been studied in several situations, including some related to health surveillance, from which univariate count data are frequently observed. However, this effect has not been studied in more complex situations such as the annual aggregations of cases of several diseases, discriminated by the age of patients. In this article we study the effect of aggregation of multivariate count data that depend on a single covariate, in the context of Statistical Quality Control, using a methodology based on Poisson profile monitoring. More specifically, the relative performance of two monitoring schemes is evaluated in terms of their average time to signal (ATS) when aggregated and disaggregated information is used. We also study whether or not the aggregation effect is the same for the two schemes.