One-year PFS rate is defined as the percentage (%) of participants surviving 1 year after randomization without progression to multiple myeloma or death estimated by the Kaplan-Meier method and based on the International Myeloma Working Group (IMWG) calcium, renal, anemia, and bone lesions (CRAB) criteria. Progressive disease (PD) is defined as presence of an M- component in serum plus clonal plasma cells in the bone marrow plus 1 or more of the following: Calcium elevation (greater than [>] 11.5 milligram per deciliter [mg/dL] [> 2.88 millimoles per liter {mmol/L}]); Renal insufficiency (creatinine > 2 mg/dL [177 micromoles per liter or more]; Anemia (hemoglobin less than [<] 10 gram per deciliter [g/dL] or 2 g/dL lower than lower limit of normal [LLN] [hemoglobin < 6.5 mmol/L or 1.25 mmol/L lower than LLN]); Bone disease (lytic lesions or osteopenia).

Intent-to-treat (ITT) population included participants who were randomly assigned to siltuximab or placebo treatment group based on an integrated voice response system (IVRS).

PDIR is defined as percentage of participants who meet any of following criteria occurring within 6 months of start of treatment. a) CRAB criteria: true progression events, b) Serum M-protein: increase by 25 % compared with baseline at 2 consecutive assessments, c) Magnetic resonance imaging: unequivocal increase in focal bone lesions, d) Immunoparesis: decrease by 25% compared with baseline of 2 other non-affected immunoglobulin (Ig) (IgG, IgM, IgA) at 2 consecutive assessments, e) Hemoglobin: decrease of 1.5 g/dL (with at least 1 read below LLN) at 2 consecutive assessments, with no other identifiable cause. PD is defined as presence of M-component in serum plus clonal plasma cells in bone marrow plus 1 or more of following: Calcium elevation (> 11.5 mg/dL [> 2.88 mmol/L]); Renal insufficiency (creatinine >2 mg/dL [177 micro mol/L or more]); Anemia (hemoglobin <10 or 2 g/dL lower than LLN) [hemoglobin < 6.5 or 1.25 mmol/L lower than LLN]); Bone disease (lytic lesions or osteopenia).

Response evaluable population included participants who had a diagnosis of high-risk smoldering multiple myeloma (SMM) and received at least 1 dose of siltuximab/placebo treatment. In addition, participants were to have at least 1 post-baseline disease assessment.

PFS is defined as the time between randomization and initial documented PD according to the CRAB - International Myeloma Working Group (IMWG) criteria or date of death, whichever occurs first. PD is defined as presence of an M-component in serum plus clonal plasma cells in the bone marrow plus 1 or more of the following: Calcium elevation (> 11.5 mg/dL [> 2.88 mmol/L]); Renal insufficiency (creatinine > 2 mg/dL [177 [micro mol/L or more]); Anemia (<10 g/dL or 2 g/dL) lower than LLN) [hemoglobin < 6.5 mmol/L or 1.25 mmol/L lower than LLN]); Bone disease (lytic lesions or osteopenia).](streamdown:incomplete-link)

ITT population included participants who were randomly assigned to siltuximab or placebo treatment group based on an IVRS.

Serum M-protein response is defined as a decrease of greater than or equal to (>=) 50% in serum M-protein compared with baseline at 2 consecutive assessments.

ITT population included participants who were randomly assigned to siltuximab or placebo treatment group based on an IVRS.

Time to worsening in EORTC-QLQ-C30 (physical function scale) is defined as time between randomization and first documentation of a worsening in EORTC-QLQ-C-30. Worsening in the EORTC-QLQ-C30 is defined as 10 points decrease from baseline. It comprises module with 30 items. Questionnaire includes 9 multi-item scales: 5 functional scales (physical, role, cognitive, emotional, and social), 3 symptom scales (fatigue, pain, nausea/vomiting), a global health and quality of life scale, and a number of single items assessing symptoms (dyspnea, loss of appetite, insomnia, constipation, diarrhoea). Instrument contains 28 items using a Likert scale with 4 response options: "Not at All," "A Little," "Quite a Bit," "Very Much" (scored 1-4). Two additional items use response options (1-7): 1=Very Poor, to 7=Excellent. All scale and item scores were linearly transformed to be in range from 0-100. A higher score represents a higher (better) level of functioning, or a higher (worse) level of symptoms.

ITT population included participants who were randomly assigned to siltuximab or placebo treatment group based on an IVRS who had 10 points decrease from baseline in the physical function scale.

Time to worsening in the BPI worst item is defined as the time between randomization and the first documentation of a worsening in the BPI worst item. It has 2 domains reflecting pain severity and pain interference with domains of functioning and well-being. The selected item refers to the "worst" pain the patient has experienced over the past 24 hours. This item has been found to be most responsive to interference with key domains of functioning and well-being and may be used as a single item. Responses are provided on an 11-point numeric rating scale ranging from 0 "no pain" to 10 "pain as bad as you can imagine". Responses are described as mild (1 to 4), moderate (5 to 6) and severe (7 to 10). Worsening in the BPI worst item is defined as 2 points increase from baseline.

ITT population included participants who were randomly assigned to siltuximab or placebo treatment group based on an IVRS.

Number of participants who progressed to symptomatic multiple myeloma with stage III of International Staging System (ISS) or abnormal cytogenetic findings were assessed. The ISS system consists of stage I: beta2-microglobulin < 3.5 milligram per liter (mg/L) and albumin >= 3.5 gram (g)/100 ml; stage II: neither stage I nor stage III and stage III: beta2-microglobulin >= 5.5 mg/L.

The data was not collected and analyzed for this outcome measure as per the change in planned analysis.

Best response to first subsequent anti-myeloma therapy was assessed by physician report at 6-month intervals and classified as: complete response (CR) (negative immunofixation on the serum and urine, disappearance of any soft tissue plasmacytomas and < 5% plasms cells (PCs) in bone marrow); stringent CR (CR plus a normal FLC ratio, absence of clonal cells in bone marrow); near CR (< 5% PCs in a bone marrow aspirate, no increase in lytic bone lesions); very good partial response (VGPR) (serum and urine component detectable by immunofixation but not on electrophoresis, or >= 90% reduction in serum M-protein plus urine M-protein level <100 mg per 24 hour); partial response (PR): >= 50 reduction of serum M-protein, reduction in 24-hour urinary M-protein by >=90 % or to < 200 mg/24 hours); minimal response (>=25% but <= 49% reduction of serum M-protein and reduction in urine M-protein by 50%-89%); stable disease (not meeting criteria for CR, VGPR, PR, or PD); PD; not evaluable and unknown.

The data was not collected and analyzed for this outcome measure as per the change in planned analysis.

OS is defined as the time between randomization and death due to any cause.

ITT population included participants who were randomly assigned to siltuximab or placebo treatment group based on an IVRS.