Figure 3.

Directed acyclic graph illustrating confounding by health-care-seeking behavior. A) Health-care-seeking HS confounds the relationship between vaccination V and influenza infection I. The parentheses indicate that HS is unmeasured. Additional confounders C may be parents of HS (C→HS). B) By including only patients with positive health-care-seeking behaviors (shown by the rectangle around HS = 1), the test-negative design implicitly removes this source of confounding (biasing paths removed).