A Study to Evaluate the Efficacy and Safety of Intravenous (IV) Prasinezumab in Participants With Early-Stage Parkinson's Disease

Inclusion Criteria:

• Body weight within 40-110 kilograms (kg) (88-242 pounds [lbs]) and a body mass indexwithin the range 18-34 kg/m2

• Diagnosis of idiopathic PD based on Movement Disorder Society (MDS) criteria

• Has received monotherapy treatment

• An MDS-UPDRS Part IV score of 0 at screening and prior to randomization

• Hoehn and Yahr (H&Y) Stage 1 or 2 off medication at screening and prior torandomization

• Agreement to adhere to the contraception requirements

Exclusion Criteria:

• Pregnant or breastfeeding, or intention of becoming pregnant during the study orwithin the time frame in which contraception is required

• Medical history indicating a parkinsonian syndrome other than idiopathic PD

• Diagnosis of a significant neurologic disease other than PD

• Chronic uncontrolled hypertension