AUCtau is defined as concentration of drug over time (the area under the concentration versus time curve over the dosing interval).

The Lead-in Phase Pharmacokinetic (PK) Analysis Set included all PK lead-in phase participants with available data who received at least 1 dose of study drug and whom at least one non-missing PK concentration data value is available from the PK Lead-in Phase intensive sampling.

AUCtau is defined as concentration of drug over time (the area under the concentration versus time curve over the dosing interval).

Participants in the Lead-in Phase PK Analysis Set with available data were analyzed.

AUCtau is defined as concentration of drug over time (the area under the concentration verses time curve over the dosing interval).

Participants in the Lead-in Phase PK Analysis Set with available data were analyzed.

An AE is any untoward medical occurrence in a clinical study participant administered a medicinal product, which does not necessarily have a causal relationship with the treatment. An AE can therefore be any unfavorable and/or unintended sign, symptom, or disease temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product. TEAEs were defined as 1 or both of the following: Any AEs with an onset date on or after the study drug start date and no later than 30 days after permanent discontinuation of study drug and/or Any AEs leading to premature discontinuation of study drug.

The Safety Analysis Set included all participants who were enrolled into the study and received at least 1 dose of study drug (SOF/VEL FDC).

Participants in the Lead-in Phase PK Analysis Set with available data were analyzed.

An AE is any untoward medical occurrence in a clinical study participant administered a medicinal product, which does not necessarily have a causal relationship with the treatment. An AE can therefore be any unfavorable and/or unintended sign, symptom, or disease temporally associated with the use of a medicinal product, whether or not considered related to the medicinal product.

Participants in the Lead-in Phase PK Analysis Set were analyzed.

SVR12 was defined as HCV RNA < the lower limit of quantitation (LLOQ; ie, 15 IU/mL) at 12 weeks after stopping study treatment.

The Full Analysis Set included all participants who were enrolled into the study and received at least 1 dose of study drug (SOF/VEL FDC).

SVR4 was defined as HCV RNA < LLOQ (ie, 15 IU/mL) at 4 weeks after stopping study treatment.

Participants in the Full Analysis Set were analyzed.

SVR 24 was defined as HCV RNA < LLOQ (ie, 15 IU/mL) at 24 weeks after stopping study treatment.

Participants in the Full Analysis Set were analyzed.

Virologic failure was defined as: On-treatment virologic failure - Breakthrough (confirmed HCV RNA ≥ LLOQ after having previously had HCV RNA < LLOQ while on treatment), or Rebound (confirmed > 1 log10 IU/mL increase in HCV RNA from nadir while on treatment), or Non-response (HCV RNA persistently ≥ LLOQ through 8 weeks of treatment); Virologic relapse: Confirmed HCV RNA ≥ LLOQ during the posttreatment period having achieved HCV RNA < LLOQ at last on-treatment visit.

Participants in the Full Analysis Set were analyzed.

Percentage of participants with HCV RNA < LLOQ while on treatment by analysis visit.

Participants in the Full Analysis Set were analyzed.

Drug-resistant substitutions were analyzed as part of the Virology Study. Plasma samples were collected and stored for potential HCV sequencing. Impact on the treatment outcomes of SVR12 and SVR24 were observed during the study. Baseline deep sequencing of the HCV nonstructural protein (NS)5A and NS5B genes was performed for all participants at the first time point after virologic failure if the plasma or serum sample was available. Pretreatment full-length NS5A deep sequencing data were obtained at a 15% assay cutoff for the Resistance Analysis Population which covered all NS5A and NS5B nucleoside inhibitor (NI) resistance-associated variants (RAVs).

The Resistance Analysis Population with all adolescent or pediatric participants in the Safety Analysis Set with a virologic outcome with available data were analyzed.

Participants in the Full Analysis Set with available data were analyzed.

To evaluate the effect of treatment with SOF/VEL on general and disease-specific health-related QoL, the PedsQL™ Pediatric QoL Inventory V4.0 Short Form (SF15) was completed at Day 1, end of treatment, early termination (if applicable), and posttreatment Weeks 12 and 24. The SF15 questionnaire represented 4 domains: physical, emotional, social, and school functioning, with the emotional, social, and school functioning domains representing the psychosocial health summary. Neuropsychiatric assessment was conducted using the PedsQL™ Pediatric QoL Inventory V4.0 SF15 psychosocial domain-related scores. Items were calculated and transformed into an overall score with a range of 0 to 100 points, with more points indicating better QoL.

Participants in the Full Analysis Set with available data were analyzed.

An age- and sex-specific percentile was derived for each weight, height, and body mass index (BMI) measurement according to the statistical analysis system (SAS) program available on the Centers for Disease Control and Prevention (CDC) website using the year 2000 growth charts.

Participants in the Safety Analysis Set with available data were analyzed.

An age- and sex-specific percentile was derived for each weight, height, and BMI measurement according to the SAS program available on the CDC website using the year 2000 growth charts.

Participants in the Safety Analysis Set with available data were analyzed.

Tanner Pubertal Staging was assessed for pubic hair growth and genitalia development (males) and for pubic hair growth and breast development (females) in stages 1 to 5. Tanner stages were used to evaluate the onset and progression of pubertal changes from stage 1 (pre-pubertal) to stage 5 (adult). If a participant had reached Tanner stage 5, no further Tanner pubertal stage assessments were to be completed. Pubic hair growth: Tanner stages (1: No hair, 2: Downy hair, 3: More coarse and curly hair, 4: Adult-like hair quality; 5: Hair extends to the medial surface of the thighs); Breast development: Tanner stages (1: No glandular tissue, 2: Breast bud forms,3: More elevated, outside areola, 4: Increased breast size,5: Final adult-size breasts); Genitalia development: Tanner stages (1: Testes, scrotum, and penis about same size, 2: Enlargement of scrotum, testes and penis, 3: Enlargement of penis, 4: Penis size enlargement, 5: Genitalia adult in size and shape).

Participants in the Safety Analysis Set with available data were analyzed. Overall Number of Participants Analyzed for each arm is the total of male plus female participants.

Mid-parental height was calculated as the average of the biological father's and mother's heights. For boys, the sex-adjusted mid-parental height was calculated by adding 2.5 inches or 6.5 cm to the mean of the parents' heights. For girls, 2.5 inches or 6.5 cm was subtracted from the mean of the parents' heights.

Participants in the Safety Analysis Set with available data were analyzed.

Bone age was determined based on x-ray of the left wrist, hand, and fingers. Baseline value is the last available value on or prior to first dose date of study drug.

Participants in the Full Analysis Set with available data were analyzed.

A SOF/VEL FDC swallowability assessment was performed using placebo tablets at baseline.

Participants in the Full Analysis Set with available data were analyzed. Swallowability assessment was performed in 12 to <18 years old and 6 to < 12 Years old only.

Acceptability was assessed by numeric response marked on line between numbers 0 - 100. Higher scores indicate better acceptability and/or palatability.

Participants in the Full Analysis Set with available data were analyzed.

Acceptability was assessed by numeric response marked on line between numbers 0 - 100. Higher scores indicate better acceptability and/or palatability.

Participants in the Full Analysis Set with available data were analyzed.