Has received a live-virus vaccination within 30 days of planned treatment start. Seasonal flu vaccines that do not contain live virus are permitted. Received a live-virus vaccination within 30 days of planned treatment start. Has received a live vaccine within 30 days of the planned start of study therapy. Seasonal flu vaccines that do not contain live virus are permitted. Live-virus vaccination -4 weeks NOTE: Seasonal flu vaccines that do not contain live virus are not an exclusion. Received live-virus vaccination within 30 days (seasonal flu vaccine allowed if non-live virus) Has received a live-virus vaccine within 30 days of planned start of study drug. Seasonal flu vaccines that do not contain live virus are permitted. Has received a live-virus vaccination within 28 days of planned treatment start. Seasonal flu vaccines that do not contain live virus are permitted. Live-virus vaccination within 30 days of the first dose of mRNA-4157 or pembrolizumab. Seasonal flu vaccines that do not contain live virus are permitted Has received a live-virus vaccination within 30 days of planned treatment start. Has received a live-virus vaccination within 30 days of planned treatment start Has received a live-virus vaccination within 30 days of planned treatment start. Seasonal flu vaccines that do not contain live virus are permitted Has received a live-virus vaccination within 30 days of planned start of study therapy. Seasonal flu vaccines that do not contain live virus are permitted. Has received a live-virus vaccination within 30 days of planned treatment start. Note: Seasonal flu vaccines that do not contain live virus are permitted. Has received or will receive a live vaccine within 30 days prior to the first administration of study medication; seasonal flu vaccines that do not contain live virus are permitted Has received a live-virus vaccination within 30 days of planned treatment start. Seasonal flu vaccines that do not contain live virus are permitted. Has received a live-virus vaccination within 30 days of planned treatment start. Previous treatment with live vaccine within 30 days of planned start of study drugs (seasonal flu vaccines that do not contain a live virus are permitted). Has received a live-virus vaccination within 30 days prior to start of study drug treatment. a. Seasonal flu and other inactivated vaccines that do not contain live virus are permitted. Receipt of live vaccination within 28 days before randomization; seasonal flu vaccines that do not contain live virus are permitted Has received a live-virus vaccination within 30 days of planned treatment start. Note: Seasonal flu vaccines that do not contain live virus are permitted. Received a live-virus vaccination within 30 days of planned start of study medication. Has received a live vaccine within 30 days of the planned start of study therapy. Seasonal flu vaccines that do not contain live virus are permitted. Have received a live vaccine within 30 days prior to enrollment. Seasonal flu vaccines that do not contain live virus are permitted. Has received a live-virus vaccination within 30 days of planned treatment start Participant has received a live-virus vaccination within 30 days of planned treatment start. Seasonal flu vaccines that do not contain live virus are permitted. Has received a live-virus vaccination within 30 days of planned treatment start. Seasonal flu vaccines that do not contain live virus are permitted Has received a live-virus vaccination within 30 days of planned start of study therapy. Seasonal flu vaccines that do not contain live virus are permitted. Patients must not have received any live vaccine within 30 days prior to registration; seasonal flu vaccines that do not contain live virus are permitted