A Phase 2 Randomized, Active-controlled, Observer-blind Study to Assess the Safety, Reactogenicity, and Immunogenicity of a Booster Dose of Investigational COVID-19 mRNA Vaccines in Healthy Adults Who Previously Received a Complete Primary Vaccination Series With or Without Booster Dose(s)

Part A: Geometric Mean Titer (GMT) of Serum Neutralization Titers Against Pseudovirus Bearing SARS-CoV-2 Strain XX Spike Protein

时间窗: At Day 29

Part A: GMT of Serum Neutralization Titers Against Pseudovirus Bearing SARS-CoV-2 Strain XY Spike Protein

时间窗: At Day 29

Part B: Number of Participants Reporting Any Solicited Administration Site AEs

时间窗: Day 1 to Day 7

Assessed solicited administration site events included injection site redness (erythema), pain, swelling and lymphadenopathy (defined as localized axillary, cervical or supraclavicular swelling or tenderness ipsilateral to the injection arm). Any = occurrence of the event regardless of intensity grade.

Part B: GMT of Serum Neutralization Titers Against Pseudovirus Bearing SARS-CoV-2 Strain XY Spike Protein

时间窗: At Day 29

Part A: Number of Participants Reporting Any Solicited Administration Site Adverse Events (AEs)

时间窗: Day 1 to Day 7

Assessed solicited administration site events included injection site redness (erythema), pain, swelling and lymphadenopathy (defined as localized axillary, cervical or supraclavicular swelling or tenderness ipsilateral to the injection arm). Any = occurrence of the event regardless of intensity grade.

Part A: Number of Participants Reporting Any Solicited Systemic AEs

时间窗: Day 1 to Day 7

Assessed solicited systemic events included fever, chills, headache, myalgia (muscle pain), arthralgia (joint pain), and fatigue (tiredness). Fever is defined as body temperature is higher than or equal to (\>=) 38ºC; preferred location for measuring the temperature is oral. Any = occurrence of the event regardless of intensity grade.

Part A: Number of Participants Reporting Any Unsolicited AEs

时间窗: Day 1 to Day 28

An unsolicited AE is an AE that is either not included in the list of solicited events or can be included in the list of solicited events but with an onset outside the specified period of follow-up for solicited events. Unsolicited AEs include both serious and nonserious AEs. Any = occurrence of the event regardless of intensity grade.

Part A: Number of Participants Reporting Any Medically Attended Adverse Events (MAAEs), Serious Adverse Events (SAEs) and AEs of Special Interest (AESIs)

时间窗: Day 1 to Day 181

An SAE refers to any untoward medical occurrence that results in death, is life-threatening, requires inpatient hospitalization or extends existing hospitalization, causes persistent or significant disability/incapacity, involves a congenital anomaly/birth defect in a participant's offspring, includes an abnormal pregnancy outcome, or occurs in any other situation per the investigator's judgement. An MAAE results in a visit to a medical professional, such as televisits, physician's office visits, urgent care visits, emergency rooms visits, or hospitalizations. AESIs are severe or non-severe predefined AEs of scientific and medical concern specific to the product/program. This study noted the following AESIs: potential immune-mediated disease (pIMDs), lab-confirmed moderate to severe COVID-19, myocarditis and pericarditis, anaphylaxis, or severe hypersensitivity within 24 hours post-intervention. "Any" indicates the occurrence of the event regardless of its intensity grade.

Part B: Number of Participants Reporting Any Solicited Systemic AEs

时间窗: Day 1 to Day 7

Assessed solicited systemic events included fever, chills, headache, myalgia (muscle pain), arthralgia (joint pain), and fatigue (tiredness). Fever is defined as body temperature \>= 38ºC; preferred location for measuring the temperature is oral. Any = occurrence of the event regardless of intensity grade.

Part B: Number of Participants Reporting Any Unsolicited AEs

时间窗: Day 1 to Day 28

An unsolicited AE is an AE that is either not included in the list of solicited events or can be included in the list of solicited events but with an onset outside the specified period of follow-up for solicited events. Unsolicited AEs include both serious and nonserious AEs. Any = occurrence of the event regardless of intensity grade.

Part B: Number of Participants Reporting Any MAAEs, SAEs and AESIs

时间窗: Day 1 to Day 181