Figure 1. Illustration of the self-controlled case series design.

The observation period for each patient begins with pediatric vaccination date (leftmost upward arrow) and continues for a total of 28 days. In the primary analyses, each day post vaccination is considered a risk interval, and consecutive days with a statistically significant t elevation in relative incidence were pooled to create a combined risk interval. Days 20–28 comprise the control interval. The intervening days represent the wash-out period.