A Study to Evaluate the Efficacy and Safety of Subcutaneous MK-3222, Followed by an Optional Long-Term Safety Extension Study, in Participants With Moderate-to-Severe Chronic Plaque Psoriasis (MK-3222-010)

Participants are initially randomized to receive tildrakizumab 200 or 100 mg once weekly at Weeks 0, 4, and every 12 weeks thereafter; or placebo at Weeks 0 and 4. At Week 12, participants initially randomized to placebo will be re-randomized to receive either tildrakizumab 200 or 100 mg at Weeks 12 and 16. At Week 28, all participants enrolled will be assessed for their improvement in PASI score from baseline. RESPONDERS: Participants initially randomized to tildrakizumab who achieve at least a 75% improvement from baseline PASI will be re-randomized to either continue on their initial treatment or to receive placebo at Week 28. * Participants who are re-randomized to continue on their initial treatment will continue tildrakizumab 200 or 100 mg every 12 weeks through Week 64. * Participants who are re-randomized to placebo will receive placebo every 4 weeks until relapse (reduction in maximum PASI response by 50%). If relapse occurs, the tildrakizumab dose that the participants was originally randomized to at baseline will be re-initiated (tildrakizumab 200 or 100 mg). Participants will be dosed tildrakizumab at the visit when the relapse occurs, and subsequent dosing of tildrakizumab will be given 4 weeks after treatment re-initiation, and every 12 weeks thereafter through Week 64. PARTIAL RESPONDERS: Participants initially randomized to tildrakizumab who achieved a PASI response of ≥50% but \<75% improvement from baseline will be assigned a treatment regimen as described below, with their first dose started at Week 28. * Participants initially randomized to tildrakizumab 200 mg will remain on tildrakizumab 200 mg every 12 weeks. * Participants initially randomized to tildrakizumab 100 mg will be re-randomized to either remain on tildrakizumab 100 mg every 12 weeks or to receive tildrakizumab 200 mg every 12 weeks. * Participants initially randomized to placebo who achieved ≥50% improvement from baseline in PASI will receive tildrakizumab (200 or 100 mg) according to their re-randomized treatment assignment at Week 12 and continue on this treatment every 12 weeks through Week 64. NON-RESPONDERS: Participants who did not achieve at least 50% improvement from baseline PASI at Week 28 will be discontinued from the study. EXTENSION: Participants will receive tildrakizumab 200 mg or 100 mg every 12 weeks through Extension Week 192, depending on the treatment received at the time of completion of the base study.