A Phase 2, Randomized, Observer-Blind, Placebo-Controlled, Dose Confirmation Study to Evaluate the Safety, Tolerability, and Immunogenicity of Zika Vaccine mRNA-1893 in Adults Aged 18 Through 65 Years and Living in Endemic and Non-Endemic Flavivirus Areas

Number of Participants With Solicited Local and Systemic Reactogenicity Adverse Reactions (ARs)

Time Frame: Up to 7 days post-vaccination

Solicited ARs (local and systemic) were collected in the electronic diary. Local ARs included: pain, erythema (redness), swelling/induration (hardness). Systemic ARs included: headache, fatigue, myalgia (muscle aches all over the body), arthralgia (aching in several joints), nausea/vomiting, body temperature (potentially fever), and chills. A summary of all serious adverse events (SAEs) and all nonserious adverse events (AEs) ("Other"), regardless of causality, is in Reported "Adverse Events" section.

Geometric Mean Titer (GMT) of Zika Virus (ZIKV)-Specific Neutralizing Antibodies (nAbs) at Day 57, as Measured by 50% Plaque Reduction Neutralization Test (PRNT50)

Time Frame: Day 57

Antibody values reported as below the lower limit of quantification (LLOQ) were replaced by 0.5 \* LLOQ. Values that were greater than the upper limit of quantification (ULOQ) were converted to the ULOQ. LLOQ=91; ULOQ=24814. 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.

Number of Participants With Unsolicited Adverse Events (AEs)

Time Frame: Up to 28 days post-vaccination

An unsolicited AE was defined as any untoward medical occurrence associated with the use of a drug in humans, whether or not considered drug related. Unsolicited AEs were AEs that were not included in the protocol-defined solicited ARs. A treatment-emergent adverse event (TEAE) was defined as any AE not present before exposure to vaccine or any AE already present that worsened in intensity or frequency after exposure. A summary of all SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the Reported "Adverse Events" section.

Percentage of Participants With Seroconversion at Day 57, as Measured by PRNT50

Time Frame: Day 57

Seroconversion was defined as an increase in ZIKV-specific nAb titer from below the LLOQ to a titer equal to or above LLOQ, or an increase of at least 4-fold in ZIKV-specific nAb titer in participants with pre-existing nAb titers. LLOQ=91; ULOQ=24814.

Percentage of Participants With Seroconversion at Day 57, as Measured by PRNT80

Time Frame: Day 57

Seroconversion was defined as an increase in ZIKV-specific nAb titer from below the LLOQ to a titer equal to or above LLOQ, or an increase of at least 4-fold in ZIKV-specific nAb titer in participants with pre-existing nAb titers. LLOQ=91; ULOQ=24814.

Number of Participants With Serious Adverse Events (SAEs), AEs of Special Interest (AESIs)

Time Frame: Day 1 through Day 196 for Main Study and Day 197 through Day 700 for Extension Period

An SAE was defined as any AE that resulted in death, was life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, persistent or significant incapacity or substantial disruption of the ability to conduct normal life functions, was a congenital anomaly/birth defect, or was an important medical event. 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 were required. A summary of all SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the Reported "Adverse Events" section.

Number of Participants With Medically Attended AEs (MAAEs)

Time Frame: Day 1 through Day 196 for Main Study and Day 197 through Day 700 for Extension Period

An MAAE was an AE that led to an unscheduled visit to a healthcare practitioner. Note that the generation of the tables for the MAAE data for the Main Study occurred after the start of the Extension Period. Therefore, some of the MAAE data for this outcome measure may appear both in the Main Study and the Extension Period. A summary of all SAEs and all nonserious AEs ("Other"), regardless of causality, is located in the Reported "Adverse Events" section.

GMT of ZIKV-specific nAbs at Day 57, as Measured by 80% Plaque Reduction Neutralization Test (PRNT80)

Time Frame: Day 57

Antibody values reported as below the LLOQ were replaced by 0.5 \* LLOQ. Values that were greater than the ULOQ were converted to the ULOQ. LLOQ=91; ULOQ=24814. 95% CI was calculated based on the t-distribution of the log-transformed values, then back transformed to the original scale for presentation.