key inclusion criteria: parts a and b both: * adults ≥50 years of age on the day of the randomization visit (day 1) who are primarily responsible for self-care and activities of daily living. participants may have one or more chronic medical diagnoses, but should be medically stable as assessed by: absence of changes in medical therapy within 1 month due to treatment failure or toxicity; absence of medical events qualifying as saes within 1 month of the planned vaccination on day 1; and absence of known, current, and life-limiting diagnoses which, in the opinion of the investigator, would make completion of the protocol unlikely. * able to comply with study requirements, including access to transportation for study visits. part b only: * fully vaccinated for covid-19 with an approved primary series according to the locally authorized or approved regimen. if the most recent covid-19 vaccine was part of a primary series, it must be ≥ 150 days before (or less per local guidance) day 1. if the most recent covid-19 vaccine was a booster dose, it must be ≥ 120 days before (or less per local guidance) day 1. part c: * participants at part c study sites who have been enrolled in part b (groups 4 and 5) of this study; have immunogenicity blood sampling at part b baseline and day 29; completed the day 211/end-of-study visits for part b; were included in the per-protocol (pp) set; and received 1 dose of mrna-1345 at least 12 months (but no later than 15 months) prior to the time of enrollment. * able to comply with study requirements, including access to transportation for study visits. key

key inclusion criteria: parts a and b both: * adults ≥50 years of age on the day of the randomization visit (day 1) who are primarily responsible for self-care and activities of daily living. participants may have one or more chronic medical diagnoses, but should be medically stable as assessed by: absence of changes in medical therapy within 1 month due to treatment failure or toxicity; absence of medical events qualifying as saes within 1 month of the planned vaccination on day 1; and absence of known, current, and life-limiting diagnoses which, in the opinion of the investigator, would make completion of the protocol unlikely. * able to comply with study requirements, including access to transportation for study visits. part b only: * fully vaccinated for covid-19 with an approved primary series according to the locally authorized or approved regimen. if the most recent covid-19 vaccine was part of a primary series, it must be ≥ 150 days before (or less per local guidance) day 1. if the most recent covid-19 vaccine was a booster dose, it must be ≥ 120 days before (or less per local guidance) day 1. part c: * participants at part c study sites who have been enrolled in part b (groups 4 and 5) of this study; have immunogenicity blood sampling at part b baseline and day 29; completed the day 211/end-of-study visits for part b; were included in the per-protocol (pp) set; and received 1 dose of mrna-1345 at least 12 months (but no later than 15 months) prior to the time of enrollment. * able to comply with study requirements, including access to transportation for study visits. key

key inclusion criteria: parts a and b both: adults ≥50 years of age on the day of the randomization visit (day 1) who are primarily responsible for self-care and activities of daily living. participants may have one or more chronic medical diagnoses, but should be medically stable as assessed by: absence of changes in medical therapy within 1 month due to treatment failure or toxicity; absence of medical events qualifying as saes within 1 month of the planned vaccination on day 1; and absence of known, current, and life-limiting diagnoses which, in the opinion of the investigator, would make completion of the protocol unlikely. able to comply with study requirements, including access to transportation for study visits. part b only: fully vaccinated for covid-19 with an approved primary series according to the locally authorized or approved regimen. if the most recent covid-19 vaccine was part of a primary series, it must be ≥ 150 days before (or less per local guidance) day 1. if the most recent covid-19 vaccine was a booster dose, it must be ≥ 120 days before (or less per local guidance) day 1. part c: participants at part c study sites who have been enrolled in part b (groups 4 and 5) of this study; have immunogenicity blood sampling at part b baseline and day 29; completed the day 211/end-of-study visits for part b; were included in the per-protocol (pp) set; and received 1 dose of mrna-1345 at least 12 months (but no later than 15 months) prior to the time of enrollment. able to comply with study requirements, including access to transportation for study visits. key

key inclusion criteria: parts a and b both: adults ≥50 years of age on the day of the randomization visit (day 1) who are primarily responsible for self-care and activities of daily living. participants may have one or more chronic medical diagnoses, but should be medically stable as assessed by: absence of changes in medical therapy within 1 month due to treatment failure or toxicity; absence of medical events qualifying as saes within 1 month of the planned vaccination on day 1; and absence of known, current, and life-limiting diagnoses which, in the opinion of the investigator, would make completion of the protocol unlikely. able to comply with study requirements, including access to transportation for study visits. part b only: fully vaccinated for covid-19 with an approved primary series according to the locally authorized or approved regimen. if the most recent covid-19 vaccine was part of a primary series, it must be ≥ 150 days before (or less per local guidance) day 1. if the most recent covid-19 vaccine was a booster dose, it must be ≥ 120 days before (or less per local guidance) day 1. part c: participants at part c study sites who have been enrolled in part b (groups 4 and 5) of this study; have immunogenicity blood sampling at part b baseline and day 29; completed the day 211/end-of-study visits for part b; were included in the per-protocol (pp) set; and received 1 dose of mrna-1345 at least 12 months (but no later than 15 months) prior to the time of enrollment. able to comply with study requirements, including access to transportation for study visits. key

key inclusion criteria: parts a and b both: adults ≥50 years of age on the day of the randomization visit (day 1) who are primarily responsible for self-care and activities of daily living. participants may have one or more chronic medical diagnoses, but should be medically stable as assessed by: absence of changes in medical therapy within 1 month due to treatment failure or toxicity; absence of medical events qualifying as saes within 1 month of the planned vaccination on day 1; and absence of known, current, and life-limiting diagnoses which, in the opinion of the investigator, would make completion of the protocol unlikely. able to comply with study requirements, including access to transportation for study visits. part b only: fully vaccinated for covid-19 with an approved primary series according to the locally authorized or approved regimen. if the most recent covid-19 vaccine was part of a primary series, it must be ≥ 150 days before (or less per local guidance) day 1. if the most recent covid-19 vaccine was a booster dose, it must be ≥ 120 days before (or less per local guidance) day 1. key

key inclusion criteria: parts a and b both: adults ≥50 years of age on the day of the randomization visit (day 1) who are primarily responsible for self-care and activities of daily living. participants may have one or more chronic medical diagnoses, but should be medically stable as assessed by: absence of changes in medical therapy within 1 month due to treatment failure or toxicity; absence of medical events qualifying as saes within 1 month of the planned vaccination on day 1; and absence of known, current, and life-limiting diagnoses which, in the opinion of the investigator, would make completion of the protocol unlikely. able to comply with study requirements, including access to transportation for study visits. part b only: fully vaccinated for covid-19 with an approved primary series according to the locally authorized or approved regimen. if the most recent covid-19 vaccine was part of a primary series, it must be ≥ 150 days before (or less per local guidance) day 1. if the most recent covid-19 vaccine was a booster dose, it must be ≥ 120 days before (or less per local guidance) day 1. key