Vignette effect to prevent motion sickness

to prevent motion sickness in my FPV VR game, I’d put a vignette effect (like in Google Earth VR) to the screen (do you think, actually would it help?), which’s Opacity is controlled by a Boolean “Is Moving?”.

I created this Graph in my Widget Blueprint to control the Boolean value, but it does not change during any movement.
How should I modify this Graph?