mRNA-1010

Solicited ARs were recorded daily using electronic diaries (eDiaries). Local ARs: injection site pain, erythema (redness), swelling/induration (hardness); and axillary (underarm) swelling or tenderness ipsilateral to the side of injection. Systemic ARs: fever, headache, fatigue, myalgia, arthralgia, nausea/vomiting, and chills. Note, not all solicited ARs were considered adverse events (AEs). Investigator reviewed whether the solicited AR was also to be recorded as an AE. A summary of all Serious Adverse Events (SAEs) and Other Adverse Events (nonserious) regardless of causality is located in the 'Reported Adverse Events' Section.

An unsolicited AE was defined as any AE reported by the participant that was not specified as a solicited AR in the protocol. An AE was defined as any untoward medical occurrence associated with the use of a drug in humans, whether or not considered drug related. A summary of all SAEs and Other Adverse Events (nonserious) regardless of causality is located in the 'Reported Adverse Events' Section.

MAAEs were AEs that led to an unscheduled visit to a healthcare provider. SAEs were AEs that resulted in death, were life threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in persistent or significant incapacity or substantial disruption of the ability to conduct normal life functions, a congenital anomaly or birth defect, or was a medically important event. AESIs were AEs (serious or nonserious) of scientific and medical concern specific to the Sponsor's product or program for which ongoing monitoring and immediate notification by the investigator to the Sponsor was required. A summary of all SAEs and Other Adverse Events (nonserious) regardless of causality is located in the 'Reported Adverse Events' Section.

Protocol-defined ILI: At least 1 systemic symptom (temperature \>37.2 degrees celsius \[°C\] \[\>99.0 degrees Fahrenheit {°F}\], chills, feverish, tiredness, headaches, or myalgia) and at least 1 respiratory symptom (sore throat, cough, sputum production, wheezing, or difficulty breathing).

Influenza A strains included H1N1 and H3N2 and influenza B strains included Victoria and Yamagata strains. Antibody values reported as below lower limit of quantification (LLOQ) were replaced by 0.5\*LLOQ. Values greater than the upper limit of quantification (ULOQ) were converted to the ULOQ.

Influenza A strains included H1N1 and H3N2 and influenza B strains included Victoria and Yamagata strains. Seroconversion rate was defined as the percentage of participants with either a Baseline HAI titer \<1:10 and a postbaseline titer ≥1:40 or a Baseline HAI titer ≥1:10 and a minimum 4-fold rise in postbaseline HAI antibody titer. Antibody values reported as below LLOQ were replaced by 0.5\*LLOQ. Values greater than the ULOQ were converted to the ULOQ.

An MAAE was an AE that led to an unscheduled visit to a healthcare practitioner. An AESI was an AE (serious or nonserious) of scientific and medical concern specific to the Sponsor's product or program, for which ongoing monitoring and immediate notification by the Investigator to the Sponsor was required. An SAE was defined as any AE that resulted in death, was life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in disability, was a congenital anomaly/birth defect, or was an important medical event. A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the "Reported Adverse Events" section.

Solicited ARs (local and systemic) were collected in an electronic diary (eDiary). Local ARs included: injection site pain, injection site erythema (redness), injection site swelling/induration (hardness), and axillary (underarm) swelling or tenderness ipsilateral to the side of injection. Systemic ARs included: fever, headache, fatigue, myalgia, arthralgia, nausea/vomiting, and chills. All solicited ARs considered causally related to injection were graded 0-4 (per Toxicity Grading Scale for Healthy Adult and Adolescent Volunteers Enrolled in Preventive Vaccine Clinical Trials); lower score indicates lower severity, and a higher score indicates greater severity. Note, not all solicited ARs were considered adverse events (AEs). The Investigator reviewed whether the solicited AR was also to be recorded as an AE. A Summary of serious AEs (SAEs) and nonserious AEs ("Other"), regardless of causality, is located in the "Reported Adverse Events" section.

An SAE was defined as any AE that resulted in death, was life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in disability/permanent damage, was a congenital anomaly/birth defect, or was an important medical event. AESIs included thrombocytopenia, new onset of or worsening of the protocol specified neurologic diseases, anaphylaxis, and myocarditis/pericarditis. An MAAE is an AE that lead to an unscheduled visit to an healthcare practitioner. This included visits to a study site for unscheduled assessments (for example, abnormal laboratory follow-up, and/or coronavirus disease 2019 \[COVID-19\] and visits to healthcare practitioners external to the study site (for example, urgent care, primary care physician). Number of participants with SAEs, AESIs, MAAEs, and AEs leading to discontinuation up to the end of study (Day 361) are reported in this outcome measure.

Protocol-defined ILI: The presence of body temperature ≥37.5 degrees celsius (°C) (≥99.5 degrees fahrenheit \[°F\]), accompanied by at least one of the respiratory illness symptoms (sore throat, cough, sputum production, wheezing, or difficulty breathing) and a positive Reverse Transcription Polymerase Chain Reaction (RT-PCR) for influenza.

Seasonal influenza A strains included H1N1 and H3N2 and seasonal influenza B strains included Victoria-lineage and Yamagata-lineage.

Seasonal influenza A strains included H1N1 and H3N2 and seasonal influenza B strains included Victoria-lineage and Yamagata-lineage. Seroconversion was defined as either a Baseline HAI titer \<1:10 and a post-Baseline titer ≥1:40 or a Baseline HAI titer ≥1:10 and a minimum 4-fold rise in post-Baseline HAI Ab titer.

An AE was defined as any untoward medical occurrence associated with the use of a drug in humans, whether or not considered drug related. Any abnormal laboratory test result (hematology, clinical chemistry, or prothrombin time \[PT\]/partial thromboplastin time \[PTT\]) or other safety assessment (for example, electrocardiogram, radiological scan, vital sign measurement), including one that worsened from baseline and was considered clinically significant in the medical and scientific judgment of the Investigator was recorded as an AE. Number of participants with unsolicited AEs (SAEs and non-serious AEs) up to 28 days post-vaccination are reported in this outcome measure.

An SAE was defined as any AE that resulted in death, was life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in disability/permanent damage, was a congenital anomaly/birth defect, or was an important medical event. AESIs included thrombocytopenia, new onset of or worsening of the protocol specified neurologic diseases, anaphylaxis, and myocarditis/pericarditis. An MAAE is an AE that lead to an unscheduled visit to an healthcare practitioner. This included visits to a study site for unscheduled assessments (for example, abnormal laboratory follow-up, and/or coronavirus disease 2019 \[COVID-19\] and visits to healthcare practitioners external to the study site (for example, urgent care, primary care physician). Number of participants with SAEs, AESIs, MAAEs, and AEs leading to discontinuation up to the end of study (Day 181) are reported in this outcome measure.

Seroresponse is defined as a Day 29 titer \> 1:40 if baseline is \< 1:10 or a minimum 4-fold rise if baseline is \>1:10 in anti-HA antibodies measured by HAI assay.

Seroresponse is defined as post-baseline titer ≥4-fold if baseline is ≥LLOQ or ≥4×LLOQ if baseline titer is \<LLOQ in neutralizing antibody titers measured by RSV neutralization assay, at Day 29.

Seroconversion is defined as either a pre-vaccination HAI titer \<1:10 and a post-vaccination titer ≥1:40 or a pre-vaccination HAI titer ≥1:10 and a minimum 4-fold rise in post-vaccination HAI antibody titer.

Solicited ARs (local and systemic) were collected in the electronic diary (eDiary). Local ARs included: pain at injection site, erythema (redness) at injection site, swelling/induration (hardness) at injection site, and localized axillary swelling or tenderness ipsilateral to the injection arm. Systemic ARs included: headache, fatigue, myalgia (muscle aches all over the body), arthralgia (aching in several joints), nausea/vomiting, rash, body temperature (potentially fever), and chills. Solicited ARs (local and systemic) considered causally related to injection were graded 0-4; lower score indicates lower severity and a higher score indicates greater severity. Note, not all solicited ARs were considered adverse events (AEs). The Investigator reviewed whether the solicited AR was also to be recorded as an AE. Summary of serious AEs (SAEs) and nonserious AEs ("Other"), regardless of causality, is in Reported "Adverse Events" section and presented by Phase/dose group.

An SAE was defined as any AE that resulted in death, is life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in disability/permanent damage, was a congenital anomaly/birth defect, or was an important medical event. AESIs included thrombocytopenia, new onset of or worsening of the protocol specified neurologic diseases, anaphylaxis, and myocarditis/pericarditis. An MAAE is an AE that leads to an unscheduled visit to an healthcare practitioner. This would include visits to a study site for unscheduled assessments (for example, abnormal laboratory follow-up, and/or COVID-19 and visits to healthcare practitioners external to the study site (for example, urgent care, primary care physician). A summary of SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the Reported "Adverse Events" section and presented by each Phase and dose group separately.

Seasonal influenza A included H1N1 and H3N2 and seasonal influenza B strains included Victoria-lineage and Yamagata-lineage. GMT 95% confidence interval (CI) was calculated based on the t-distribution of the log-transformed values then back transformed to the original scale for presentation.

Humoral immunogenicity relative to that of the active comparator (afluria quadrivalent) was assessed against vaccine-matched Influenza A and B strains at Day 29. Seasonal influenza A included H1N1 and H3N2 and seasonal influenza B strains included Victoria-lineage and Yamagata-lineage. GMT 95% CI was calculated based on the t-distribution of the log-transformed values then back transformed to the original scale for presentation.

Humoral immunogenicity relative to that of the active comparator (afluria quadrivalent) was assessed against vaccine-matched Influenza A and B strains at Day 29. Seasonal influenza A included H1N1 and H3N2 and seasonal influenza B strains included Victoria-lineage and Yamagata-lineage. GMT 95% CI was calculated based on the t-distribution of the log-transformed values then back transformed to the original scale for presentation.

Seasonal influenza A included H1N1 and H3N2 and seasonal influenza B strains included Victoria-lineage and Yamagata-lineage. Seroconversion at a participant level was defined as (a) if lower limit of quantification (LLOQ) was 1:10, a post-baseline titer ≥1:40 if baseline was \<1:10 or a 4-fold or greater rise from baseline if baseline was ≥1:10 in anti-HA antibodies; or (b) if LLOQ was \>1:10, a post-baseline titer ≥4 x LLOQ if baseline was \<LLOQ, or 4-fold or greater increase from baseline if baseline was ≥LLOQ in anti-HA antibodies.

The GMFR measures the changes in immunogenicity titers or levels from Baseline within participants. Seasonal influenza A included H1N1 and H3N2 and seasonal influenza B strains included Victoria-lineage and Yamagata-lineage. 95% CI was calculated based on the difference in the log-transformed values for GMFR, then back transformed to the original scale for presentation.

Humoral immunogenicity relative to that of the active comparator (afluria quadrivalent) was assessed against vaccine-matched Influenza A and B strains at Day 29. Seasonal influenza A included H1N1 and H3N2 and seasonal influenza B strains included Victoria-lineage and Yamagata-lineage. Seroconversion at a participant level was defined as (a) if LLOQ was 1:10, a post-baseline titer ≥1:40 if baseline was \<1:10 or a 4-fold or greater rise from baseline if baseline was ≥1:10 in anti-HA antibodies; or (b) if LLOQ was \>1:10, a post-baseline titer ≥4 x LLOQ if baseline was \<LLOQ, or 4-fold or greater increase from baseline if baseline was ≥LLOQ in anti-HA antibodies. 95% CI was calculated using the Clopper-Pearson method

Humoral immunogenicity relative to that of the active comparator (afluria quadrivalent) was assessed against vaccine-matched Influenza A and B strains at Day 29. Seasonal influenza A included H1N1 and H3N2 and seasonal influenza B strains included Victoria-lineage and Yamagata-lineage. Seroconversion at a participant level was defined as (a) if LLOQ was 1:10, a post-baseline titer ≥1:40 if baseline was \<1:10 or a 4-fold or greater rise from baseline if baseline was ≥1:10 in anti-HA antibodies; or (b) if LLOQ was \>1:10, a post-baseline titer ≥4 x LLOQ if baseline was \<LLOQ, or 4-fold or greater increase from baseline if baseline was ≥LLOQ in anti-HA antibodies. 95% CI was calculated using the Clopper-Pearson method