Figure 1.

A, Simplified directed acyclic graph illustrating the relationship between coronavirus disease 2019 (COVID-19) and influenza vaccination probabilities. Vaccination motivation $M$is a common ancestor of influenza vaccine uptake $V_{flu}$ and COVID-19 vaccine uptake $V_{covid}$. The parentheses indicate that $M$is unmeasured. Through $M$a forked, confounding pathway exists linking $V_{covid}$ to medically attended influenza acute respiratory illness (ARI) $I_{flu}$ ($V_{covid}\leftarrow (M)\to I_{flu})$, 
and $V_{flu}$ to medically attended severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) ARI $I_{SARS-CoV-2}$ ($V_{flu}\leftarrow (M)\to I_{SARS-CoV-2})$. B, Adjustment in the statistical model for $V_{flu}$ closes the confounding pathway from $V_{covid}\leftarrow (M)\to I_{flu}$ in COVID-19 vaccine effectiveness (VE) test-negative studies that include influenza controls. C, Similarly, adjustment for $V_{covid}$ in an influenza VE test-negative study that includes SARS-CoV-2 controls closes the confounding pathway from $V_{flu}\leftarrow (M)\to I_{SARS-CoV-2}$.