Additional Study Information: The main goal of this study is to look at the effect and safety of efgartigimod PH20 SC in people with chronic inflammatory demyelinating polyneuropathy (CIDP). Another goal of the study is to collect information on how well efgartigimod works, how the body processes efgartigimod, what efgartigimod does to the body, and how your immune system reacts to efgartigimod. This trial will be conducted in 2 stages: An open-label Stage A and a randomized-withdrawal, double-blind, placebo-controlled Stage B. Approximately 360 patients aged 18 years and older male and non-pregnant female are planned to be enrolled in Stage A with a maximum of 180 randomized patients in Stage B.