The masking effect is a statistical issue associated with commonly applied signal detection methodologies in which signals for a product of interest are hidden by the presence of other reported products.

Due to vaccine novelty, and an unprecedented dynamic of reporting, statistical signals of adverse events related to coronavirus disease 2019 (COVID-19) vaccines are more prone to masking and, therefore, to being undetected or delayed.

A more advanced class of signal detection methodologies, based on regression, can address masking and expose strong statistical associations that would otherwise be deemed uninteresting.

The extent, direction, impact, and root causes of masking change in accordance with the changing nature of data.