This study is a randomized, double-blind, double-dummy, parallel group, multicenter, non-inferiority study. The study will enroll adult and adolescent asthmatic subjects who are currently receiving mid dose inhaled corticosteroids (ICS) plus long-acting beta2-agonist (LABA) (equivalent to fluticasone propionate [FP]/salmeterol 250/50 microgram [mcg]twice daily [BD]), either via a fixed dose combination product or through separate inhalers. The study consists of a LABA washout period of 5 days and a run-in period of 4 weeks, followed by a treatment period of 24 weeks, and a follow up contact period of one week. The total duration of the study is 30 weeks. Approximately 1461 subjects will be randomized to one of the following three treatments (487 per treatment): fluticasone furoate (FF)/vilanterol (VI) 100/25 mcg once daily (OD) in the evening (PM) via ELLIPTA™ inhaler plus placebo BD via ACCUHALER™/DISKUS™; FP/salmeterol 250/50 mcg BD via ACCUHALER/DISKUS inhaler plus placebo OD (PM) via ELLIPTA inhaler; FP 250 mcg BD via ACCUHALER/DISKUS inhaler plus placebo OD (PM) via ELLIPTA inhaler. In addition, all subjects will be supplied with albuterol/salbutamol inhalation aerosol to use as needed to treat acute asthma symptoms. This study will determine if FF/VI 100/25 mcg OD via ELLIPTA inhaler is non-inferior to FP/salmeterol 250/50 mcg BD via ACCUHALER/DISKUS inhaler in adult and adolescent asthmatic subjects already adequately controlled on a twice-daily ICS/LABA.

SERETIDE, ELLIPTA, ACCUHALER, RELVAR, and DISKUS are trademarks of the GlaxoSmithKline Group of Companies.

Inclusion Criteria

• Subjects must give their signed and dated written informed consent to participate prior to commencing any study related activities.
• Subjects must be outpatients >=12 years of age at Visit 1 who have had a diagnosis of asthma, as defined by the National Institutes of Health, for at least 12 weeks prior to Visit 1 (Note: Countries with local restrictions prohibiting enrollment of adolescents will enroll subjects >=18 years of age only).
• Subjects may be male or an eligible female. An eligible female is defined as having non-childbearing potential or having childbearing potential and a negative urine pregnancy test at Screening and agrees to use an acceptable method of birth control consistently and correctly.
• Subjects must have a FEV1 of >=80% of the predicted normal value.
• Subjects are eligible if they have received mid dose ICS plus LABA (equivalent to FP/salmeterol 250/50 twice daily or an equivalent combination via separate inhalers) for at least the 12 weeks immediately preceding Visit 1.
• All subjects must be able to replace their current SABA treatment with albuterol/salbutamol aerosol inhaler at Visit 1 for use, as needed, for the duration of the study. Subjects must be able to withhold albuterol/salbutamol for at least 6 hours prior to study visits.
• If in the opinion of the investigator the subject's asthma is well controlled. Exclusion Criteria
• History of Life-Threatening Asthma, defined for this protocol as an asthma episode that required intubation and/or associated with hypercapnea, respiratory arrest or hypoxic seizures within the last 5 years.
• Culture-documented or suspected bacterial or viral infection of the upper or lower respiratory tract, sinus or middle ear that is not resolved within 4 weeks of Visit 1 and led to a change in asthma management or in the opinion of the Investigator, expected to affect the subject's asthma status or the subject's ability to participate in the study.
• Any asthma exacerbation requiring oral corticosteroids within 12 weeks of Visit 1 or resulting in an overnight hospitalization requiring additional treatment for asthma within 6 months prior to Visit 1.
• A subject must not have current evidence of Atlectasis, Bronchopulmonary dysplasia, Chronic bronchitis, Chronic obstructive pulmonary disease, Pneumonia, Pneumothorax, Interstitial lung disease, or any evidence of concurrent respiratory disease other than asthma
• A subject must not have any clinically significant, uncontrolled condition or disease state that, in the opinion of the investigator, would put the safety of the subject at risk through study participation or would confound the interpretation of the results if the condition/disease exacerbated during the study.
• A subject must not have used any investigational drug within 30 days prior to Visit 1 or within five half-lives (t½) of the prior investigational study, whichever is longer of the two.
• Any adverse reaction including immediate or delayed hypersensitivity to any beta2-agonist, sympathomimetic drug, or any intranasal, inhaled, or systemic corticosteroid therapy. Known or suspected sensitivity to the constituents of RELVAR™ ELLIPTA inhaler, SERETIDE™ ACCUHALER/DISKUS inhaler or FP 250.
• History of severe milk protein allergy.
• Administration of prescription or non-prescription medication that would significantly affect the course of asthma, or interact with study drug.
• A subject must not be using or require the use of immunosuppressive medications during the study.
• A subject will not be eligible if he/she or his/her parent or legal guardian has any infirmity, disability, disease, or geographical location which seems likely (in the opinion of the Investigator) to impair compliance with any aspect of this study protocol, including visit schedule and completion of the daily diaries.
• Current tobacco smoker or has a smoking history of 10 pack-years (20 cigarettes/day for 10 years). A subject may not have used inhaled tobacco products or inhaled marijuana within the past 3 months (e.g., cigarettes, cigars, electronic cigarettes, or pipe tobacco).
• A subject will not be eligible for this study if he/she is an immediate family member of the participating investigator, sub-investigator, study coordinator, or employee of the participating investigator.