The QMG is a 13 item ordinal scale which measures ocular, bulbar, extremity fatigue and strength, along with respiratory function. The total QMG score was calculated by adding the score of each of the 13 individual QMG questions. Possible scoring on the QMG range from 0 (normal) to 39 (severe). A lower score indicates a better clinical outcome. The QMG score at Baseline is the average of the screening and Week 0 Baseline scores. Change from Baseline was calculated by subtracting the Baseline value from the post-baseline value. The differences in adjusted least square means were presented (Belimumab 10 mg/kg minus Placebo). A negative treatment difference indicates benefit relative to placebo. The analysis method was Mixed-Model Repeated Measures adjusted for Treatment, Visit, Baseline QMG Score, Treatment by Visit, and Baseline QMG Score by Visit.

Intent-to-Treat (ITT) Population includes participants in the Safety Population who has provided any post treatment efficacy assessment.

The QMG is a 13 item ordinal scale which measures ocular, bulbar, extremity fatigue and strength, along with respiratory function. The total QMG score was calculated by adding the score of each of the 13 individual QMG questions. Possible scoring on the QMG range from 0 (normal) to 39 (severe). A lower score indicates a better clinical outcome. The QMG score at Baseline is the average of the screening and Week 0 Baseline scores. Proportions compared using exact analyses stratified by the observed median Baseline score (less than or equal to [<=] median, greater than [ >] median). Exact odds ratio, double the exact one-sided p-value and exact confidence intervals were presented. Participants with missing data were assumed to have a negative response.

ITT Population.

The QMG is a 13 item ordinal scale which measures ocular, bulbar, extremity fatigue and strength, along with respiratory function. The total QMG score was calculated by adding the score of each of the 13 individual QMG questions. Possible scoring on the QMG range from 0 (normal) to 39 (severe). A lower score indicates a better clinical outcome. The QMG score at Baseline is the average of the screening and Week 0 Baseline scores. Proportions compared using exact analyses stratified by the observed median Baseline score (<=median, > median). Exact odds ratio, double the exact one-sided p-value and exact confidence interval were presented. Participants with missing data were assumed to have a worsening response.

ITT Population.

A sustained response during the treatment phase is when a participant improves by >=3 points from Baseline at Week 12, and the participant maintains at least a 3 point improvement from Baseline through Week 24. The QMG is a 13 item ordinal scale which measures ocular, bulbar, extremity fatigue and strength, along with respiratory function. The total QMG score was calculated by adding the score of each of the 13 individual QMG questions. Possible scoring on the QMG range from 0 (normal) to 39 (severe). A lower score indicates a better clinical outcome. The QMG score at Baseline is the average of the screening and Week 0 Baseline scores. Odds ratios are calculated by Cochran-Mantel-Haenszel method stratified by the observed median baseline score (<= median, > median). Wald confidence intervals and p-values were presented.

ITT Population.

A sustained response during the treatment phase is when a participant improves by >=3 points from Baseline at Week 12, and the participant maintains at least a 3 point improvement from Baseline through Week 24. The QMG is a 13 item ordinal scale which measures ocular, bulbar, extremity fatigue and strength, along with respiratory function. The total QMG score was calculated by adding the score of each of the 13 individual QMG questions. Possible scoring on the QMG range from 0 (normal) to 39 (severe). A lower score indicates a better clinical outcome. The QMG score at Baseline is the average of the screening and Week 0 Baseline scores.

As per the criteria documented in the Reporting and Analysis Plan these analyses were not conducted since <50% of subjects met the criteria (i.e. had the event in question).

The QMG is a 13 item ordinal scale which measures ocular, bulbar, extremity fatigue and strength, along with respiratory function. Total QMG score was calculated by adding the score of each of the 13 individual QMG questions. Possible scoring on the QMG range from 0 (mild) to 39 (severe). A lower score indicates a better clinical outcome. The QMG score at baseline (BL) is the average of the screening and Week 0 BL scores. Change from BL was calculated by subtracting the BL value from the post-BL value. The differences in adjusted least square means are presented (Belimumab 10 mg/kg minus Placebo). A negative trt difference indicates benefit relative to placebo. Analysis method was Mixed-Model Repeated Measures adjusted for Trt, Visit, BL QMG Score, Trt by Visit and BL QMG Score by Visit. Only follow-up visits are presented but the analysis also includes all trt phase visits. Only those par. available at indicated time points (represented by n=X in the category titles) were analyzed.

ITT Population.

The total MGC score was calculated by adding the score of each of the 10 individual MGC questions. Possible total MGC scores range from 0 (normal) to 50 (severe). A lower score indicates a better clinical outcome. Baseline is defined as the participants last available assessment prior to initiation of study IV infusion. Change from Baseline was calculated by subtracting the Baseline value from the post-baseline value. The differences in adjusted least square means are presented (Belimumab 10 mg/kg minus Placebo). A negative treatment difference indicates benefit relative to placebo. The analysis method was Mixed-Model Repeated Measures adjusted for Treatment, Visit, Baseline MGC Score, Treatment by Visit, and Baseline MGC Score by Visit.

ITT Population.

The total MGC score was calculated by adding the score of each of the 10 individual MGC questions. Possible total MGC scores range from 0 (normal) to 50 (severe). A lower score indicates a better clinical outcome. Baseline is defined as the participants last available assessment prior to initiation of study IV infusion. Proportions compared using exact analyses stratified by the observed median baseline score (<= median, > median). Exact odds ratios, double the exact one-sided p-values and exact confidence intervals were presented. Participants with missing data were assumed to have a negative response.

ITT Population.

The total MGC score was calculated by adding the score of each of the 10 individual MGC questions. Possible total MGC scores range from 0 (normal) to 50 (severe). A lower score indicates a better clinical outcome. Baseline is defined as the participants last available assessment prior to initiation of study IV infusion. Proportions compared using exact analyses stratified by the observed median baseline score (<= median, > median). Exact odds ratios, double the exact one-sided p-values and exact confidence intervals were presented. Participants with missing data were assumed to have a worsening response.

ITT Population.

AA sustained response during the treatment phase is when a participant improves by >=3 points from Baseline at Week 12, and the participant maintains at least a 3 point improvement from Baseline through Week 24. The total MGC score was calculated by adding the score of each of the 10 individual MGC questions. Possible total MGC scores range from 0 (normal) to 50 (severe). A lower score indicates a better clinical outcome. Baseline is defined as the participants last available assessment prior to initiation of study IV infusion. Odds ratios are calculated by Cochran-Mantel-Haenszel method without adjusting for any strata. Wald confidence intervals and p-values were presented.

ITT Population.

A sustained response during the treatment phase is when a participant improves by >=3 points from Baseline at Week 12, and the participant maintains at least a 3 point improvement from Baseline through Week 24. The total MGC score was calculated by adding the score of each of the 10 individual MGC questions. Possible total MGC scores range from 0 (normal) to 50 (severe). A lower score indicates a better clinical outcome. Baseline is defined as the participants' last available assessment prior to initiation of study intravenous (IV) infusion.

As per the criteria documented in the Reporting and Analysis Plan these analyses were not conducted since <50% of subjects met the criteria (i.e. had the event in question).

The total MGC score was calculated by adding the score of each of the 10 individual MGC questions. Possible total MGC scores range from 0 (normal) to 50 (severe). A lower score indicates a better clinical outcome. Baseline is defined as the participant's last available assessment prior to initiation of study IV infusion. Change from Baseline was calculated by subtracting the Baseline value from the post-BL value. The differences in adjusted least square means are presented (Belimumab 10 mg/kg minus Placebo). A negative treatment difference indicates benefit relative to placebo. The analysis method was Mixed-Model Repeated Measures adjusted for Treatment, Visit, Baseline MGC Score, Treatment by Visit, and Baseline MGC Score by Visit. Only follow-up visits are presented but the analysis also includes all treatment phase visits. Only those participants available at the indicated time points (represented by n=X in the category titles) were analyzed.

ITT Population.

Myasthenia Foundation of America-post intervention status assesses whether subjects can be categorized as being in a status of Minimal Manifestation (MM), Pharmacologic Remission (PR) or Complete Remission (CR). Only MM and PR were assessed in this study as CR is not achievable based on the definition.

The Reporting and Analysis Plan pre-specified that these analyses would not be conducted since during a review of blinded data it was identified that the MGFA scale had been inconsistently performed across sites and any statistical analyses would not be interpretable.

Myasthenia Foundation of America (MGFA) post intervention status (PIS) assesses whether subjects can be categorized as being in a status of Minimal Manifestation (MM), Pharmacologic Remission (PR) or Complete Remission (CR). Only MM and PR were assessed in this study as CR is not achievable based on the definition.

The Reporting and Analysis Plan pre-specified that these analyses would not be conducted since during a review of blinded data it was identified that the MGFA scale had been inconsistently performed across sites and any statistical analyses would not be interpretable.

Myasthenia Foundation of America (MGFA) post intervention status (PIS) assesses whether subjects can be categorized as being in a status of Minimal Manifestation (MM), Pharmacologic Remission (PR) or Complete Remission (CR). Only MM and PR were assessed in this study as CR is not achievable based on the definition.

The Reporting and Analysis Plan pre-specified that these analyses would not be conducted since during a review of blinded data it was identified that the MGFA scale had been inconsistently performed across sites and any statistical analyses would not be interpretable.

Myasthenia Foundation of America (MGFA) post intervention status (PIS) assesses whether subjects can be categorized as being in a status of Minimal Manifestation (MM), Pharmacologic Remission (PR) or Complete Remission (CR). Only MM and PR were assessed in this study as CR is not achievable based on the definition.

The Reporting and Analysis Plan pre-specified that these analyses would not be conducted since during a review of blinded data it was identified that the MGFA scale had been inconsistently performed across sites and any statistical analyses would not be interpretable.

Myasthenia Foundation of America (MGFA) post intervention status (PIS) assesses whether subjects can be categorized as being unchanged, improved or worsened.

The Reporting and Analysis Plan pre-specified that these analyses would not be conducted since during a review of blinded data it was identified that the MGFA scale had been inconsistently performed across sites and any statistical analyses would not be interpretable.

The total MG-ADL score was calculated by adding the score of each of the 8 individual MG-ADL questions. Possible total MG-ADL scores ranges from 0 (normal) to 24 (severe). A lower score indicates a better clinical outcome. Baseline is defined as the participants last available assessment prior to initiation of study IV infusion. Change from Baseline was calculated by subtracting the Baseline value from the post-baseline value. The differences in adjusted least square means are presented (Belimumab 10 mg/kg minus Placebo). A negative treatment difference indicates benefit relative to placebo. The analysis method was Mixed-Model Repeated Measures adjusted for Treatment, Visit, Baseline MG-ADL Score, Treatment by Visit, and Baseline MG-ADL Score by Visit. Only those participants available at the indicated time points (represented by n=X in the category titles) were analyzed.

ITT Population.

The total MG-ADL score was calculated by adding the score of each of the 8 individual MG-ADL questions. Possible total MG-ADL scores range from 0 (normal) to 24 (severe). A lower score indicates a better clinical outcome. Baseline is defined as the participant's last available assessment prior to initiation of study IV infusion. Change from Baseline was calculated by subtracting the Baseline value from the post-Baseline value. The differences in adjusted least square means are presented (Belimumab 10 mg/kg minus Placebo). A negative treatment difference indicates benefit relative to placebo. The analysis method was Mixed-Model Repeated Measures adjusted for Treatment, Visit, Baseline MG-ADL Score, Treatment by Visit, and Baseline MG-ADL Score by Visit. Only follow-up visits are presented but the analysis also includes all treatment phase visits. Only those participants available at the indicated time points (represented by n=X in the category titles) were analyzed.

ITT Population.