The number of participants who achieve IMACS DOI (International Myositis Assessment and Clinical Studies definition of improvement) without rescue medication at week 24.

The IMACS DOI is: An improvement of >/= 20% from baseline in 3 IMACS core measures, no more than 2 IMACS core measure scores worsen by >/= 25% from baseline, and no more than 2 IMACS core measure scores worsen by >/= 25% from baseline.

IMACS core measures are: Physician Global Assessment of Disease Activity (PGA), Patient (Subject) Global Assessment of Disease Activity (SGA), Manual Muscle Test (MMT-8), Health Assessment Questionnaire-Disability Index (HAQ-DI), Muscle Enzyme levels, Myositis Disease Activity Assessment Tool (MDAAT) Extramuscular Global Activity.

All treated participants excluding participants with relevant protocol deviations

The adjusted mean change from baseline in the Health Assessment Questionnaire-Disability Index (HAQ-DI). HAQ-DI is a patient-reported outcome measuring disability by asking a total of 20 questions in eight categories of function: dressing, arising, eating, walking, hygiene, reach, grip, and activities. If an aid or device is used or if help is required from another individual, then the minimum score for that section is 2. The highest component score in each category determines the score for the category and scores are averaged to give the disability index. The HAQ scale ranges from 0 (no difficulties) to 3 (unable to do).

All treated participants with both baseline and post-baseline measurements

The adjusted mean change from baseline in Myositis FI-2 scores is assessing muscle endurance impairment by testing specific muscle groups. The 3 Score average includes shoulder flexion, hip flexion, and head lift. Each muscle group is scored as the number of correctly performed repetitions with 60 maximal number of repetitions.

The total score is based on hip flexion, shoulder flexion (R/L) and neck divided by 3 (range 0-60 repetitions).

All treated participants with both baseline and post-baseline measurements

The adjusted mean change from baseline in the Myositis Disease Activity Assessment Tool (MDAAT) assessment of extra-muscular uses a 100 mm Visual Analog Scale (VAS) scale. This VAS assesses the overall extra-muscular clinical features based upon: 1) The presence of clinical features or symptoms within the previous 4 weeks that are due to active disease. 2) The judgment that the feature is due to the myositis disease process. 3) The concept that disease activity is defined as a potentially reversible finding. 4) A clinical, functional, and laboratory assessments.

The scoring is performed by the investigator and ranges from 0 (absent extra-muscular disease activity) to 100 (maximum extra-muscular disease activity).

All treated participants with both baseline and post-baseline measurements

The Myositis Response Criteria (MRC) is a continuous total improvement score from baseline (range 0-100) based on the sum of the absolute percent change in the 6 core domains (weighted) used in the IMACS DOI (International Myositis Assessment and Clinical Studies definition of improvement)

IMACS core measures are: Physician Global Assessment of Disease Activity (PGA), Patient (Subject) Global Assessment of Disease Activity (SGA), Manual Muscle Test (MMT-8), Health Assessment Questionnaire-Disability Index (HAQ-DI), Muscle Enzyme levels, Myositis Disease Activity Assessment Tool (MDAAT) Extramuscular Global Activity.

The total improvement score ranges between 0 and 100 percent corresponds to the degree of improvement, with higher scores corresponding to a greater degree of improvement ( >/= 20 represents minimal improvement, a score of >/= 40 represents moderate improvement, and a score of >/= 60 represents major improvement).

All treated participants with both baseline and post-baseline measurements

The number of treated participants experiencing an adverse event. An Adverse Event (AE) is defined as any new untoward medical occurrence or worsening of a preexisting medical condition in participants administered a study drug and that does not necessarily have a causal relationship with the treatment.

All treated participants

The number of treated participants experiencing a Serious Adverse Event (SAE). A Serious Adverse Event (SAE) is any untoward medical occurrence that at any dose: results in death, is life-threatening, requires inpatient hospitalization or causes prolongation of existing hospitalization, results in persistent or significant disability/incapacity, is a congenital anomaly/birth defect, or is an important medical event.

All treated participants

The number of treated participants experiencing adverse events of special interest: infections, malignancies, autoimmune events, local injection site reactions, and systemic injection reactions.

All treated participants

The number of participants experiencing laboratory test abnormalities. Laboratory analysis was performed on the following: hematology, liver and kidney function, electrolytes, other chemistry testing (glucose, protein, cardiac), and urine chemistry. Only tests with participants experiencing abnormalities were reported.

All treated participants with at least one laboratory result for each analyte

The number of treated participants experiencing an adverse event. An Adverse Event (AE) is defined as any new untoward medical occurrence or worsening of a preexisting medical condition in participants administered a study drug and that does not necessarily have a causal relationship with the treatment.

All participants who received at least one dose of abatacept. Participants are summarized under their actual Double-Blind treatment groups, but during the Open-Label and Long-Term Extension Periods all participants received abatacept.

The number of treated participants experiencing a Serious Adverse Event (SAE). A Serious Adverse Event (SAE) is any untoward medical occurrence that at any dose: results in death, is life-threatening, requires inpatient hospitalization or causes prolongation of existing hospitalization, results in persistent or significant disability/incapacity, is a congenital anomaly/birth defect, or is an important medical event.

All participants who received at least one dose of abatacept. Participants are summarized under their actual Double-Blind treatment groups, but during the Open-Label and Long-Term Extension Periods all participants received abatacept.

The number of treated participants experiencing adverse events of special interest: infections, malignancies, autoimmune events, local injection site reactions, and systemic injection reactions.

All participants who received at least one dose of abatacept. Participants are summarized under their actual Double-Blind treatment groups, but during the Open-Label and Long-Term Extension Periods all participants received abatacept.

The number of participants experiencing laboratory test abnormalities. Laboratory analysis was performed on the following: hematology, liver and kidney function, electrolytes, other chemistry testing (glucose, protein, cardiac), and urine chemistry. Only tests with participants experiencing abnormalities were reported. For participants who enter the Japan open-label extension period or long-term extension period, assessments after the first dose in the open-label period and before the first dose date in the subsequent period are included. For participants who prematurely discontinue the open-label period or complete the open-label period but do not enter the Japan open-label extension period or long-term term extension period, assessments after the first dose in the open-label period and up to 56 days post last dose are included.

All participants who received at least one dose of abatacept and with at least one laboratory result for each analyte. Participants are summarized under their actual Double-Blind treatment groups, but during the Open-Label Period all participants received abatacept.

The number of participants experiencing laboratory test abnormalities. Laboratory analysis was performed on the following: hematology, liver and kidney function, electrolytes, other chemistry testing (glucose, protein, cardiac), and urine chemistry. Only tests with participants experiencing abnormalities were reported. For participants who completed/discontinued the Long-Term Extension Period, assessments after the first dose in the Long-Term Extension Period and up to 56 days post last dose in the Long-Term Extension Period are included.

All participants who received at least one dose of abatacept and with at least one laboratory result for each analyte. Participants are summarized under their actual Double-Blind treatment groups, but during the Long-Term Open Label Period all participants received abatacept.