Figure 2.

Directed acyclic graph illustrating the time-varying effects of prior exposure to the influenza vaccine and influenza virus across 2 seasons. The current season is depicted with a subscript 2, while the prior season is depicted with a subscript 1. The effect of interest is that of vaccination status in the current season V₂ on current season influenza status I₂. Influenza status in the previous season may influence a person's decision to be vaccinated in the current season (I₁→V₂) and will affect his/her susceptibility to infection in the current season, if the strains are similar or confer some cross-protection (I₁→I₂). Vaccination in the previous season influences vaccination in the current season (V₁→V₂), and protection may linger until the current season (V₁→I₂). Confounder status in the previous season C₁, particularly age and high-risk status, is an ancestor of, but may differ from, confounder status in the current season C₂ (C₁→C₂) and will influence both vaccination status (C₁→V₁) and influenza status (C₁→I₁).