Unmasking in an observational vaccine safety study: Using type 2 diabetes mellitus as an example

Highlights • Establishing the timing of adverse events is essential to vaccine safety studies. • Pre-existing conditions are usually excluded from safety analysis. • Exclusion of T2DM on or prior to Day 0 does not reliably exclude all pre-existing T2DM. • Work up performed at the time of vaccination may unmask pre-existing conditions. • Chart review may be required to assess whether symptom onset preceded vaccination.