A double-blind, randomized, multicenter, controlled study to evaluate the immunogenicity, safety, and tolerability of varicella vaccine (VARIVAX™) passage extension 34 (PE34) process administered concomitantly with measles, mumps, and rubella vaccine (M-M-R™ II)

ABSTRACT

The varicella vaccine passage extension (VAR-PE) process was undertaken to extend the availability of varicella zoster virus (VZV)-containing vaccines. This study (V210-A03; NCT03239873) assessed the immunogenicity, safety, and tolerability of VAR-PE process in comparison with varicella vaccine commercial product 2016 (VAR) randomized 1:1 in 600 healthy children 12 to 23 months of age administered concomitantly with measles-mumps-rubella (MMR) vaccine. The VZV seroconversion rate at 6 weeks Postdose 1 in the PP population was 100% for both groups. VZV antibody response rates and GMTs of VZV antibodies to VAR-PE induced and were non-inferior to those induced by VAR 6 weeks Postdose 1. From Day 1 through Day 42, adverse events (AEs) were reported by 81.3% of participants Postdose 1 and 67.9% Postdose 2. From Day 1 through Day 42 Postdose 1, injection-site AEs related to varicella vaccine were reported by 31.1% and 29.7% of participants in VAR-PE and VAR, respectively, and Postdose 2, by 25.7% and 25.5% of participants in the VAR-PE and VAR groups, respectively. Systemic AEs were generally comparable for the 2 vaccination groups, with the exception of pyrexia and otitis media higher in VAR-PE, and diarrhea and teething higher in VAR. The incidence of systemic AEs was generally lower Postdose 2 compared with Postdose 1.

KEYWORDS:

• Varicella
• safety
• immunogenicity
• measles
• mumps
• rubella

Introduction

Varicella (chickenpox) is a common and highly contagious childhood infectious disease caused by primary infection with varicella zoster virus (VZV), with seasonal occurrence and peak incidence in temperate climates during late winter and early spring.¹ In the prevaccination era, 90% of the cases occurred in children 1 to 14 years of age, with 60% of these cases among children 5 to 9 years, and during this time it was estimated that varicella resulted in ~10,000 hospitalizations and 100 deaths per year, primarily in immunocompetent children and adults.² Typically, varicella is a self-limiting illness characterized by fever, malaise, and generalized papulovesicular rash, with subsequent crusting and resolution over 5 to 6 days.^1,2 During primary infection, VZV establishes latent infection in the dorsal root ganglia; reactivation may result in the development of herpes zoster.^1,2 The most common complication is bacterial superinfection of skin lesions; less common though more severe complications include viral or bacterial pneumonia, septic shock, secondary bacterial arthritis and fasciitis, thrombocytopenia, nephritis, uveitis, orchitis, purpura fulminans, and Reye Syndrome.^3–6

Varicella vaccine (VARIVAX™: Varicella Virus Vaccine Live [Oka/Merck], Merck & Co., Inc., Kenilworth, NJ, USA) was first licensed in the US in 1995. Following licensure, vaccine coverage has increased to an estimated 90% of the pediatric population in the US, thereby reducing varicella incidence by up to 91.6% and varicella-related hospitalizations by 75%–88%.^7–10 During the first 12 years after licensure of varicella vaccine in the US, varicella-related mortality decreased by 88%, including a 97% reduction in individuals younger than 20 years of age, and a 96% reduction in individuals younger than 50 years of age.¹¹ Additionally, epidemiologic data have demonstrated that varicella vaccine not only affords long-term protection over 15 years against varicella, but also imparts community protection to unvaccinated individuals in settings where vaccine adoption is widespread.^12,13

Following the recommendation of a 2-dose varicella vaccination schedule by the Advisory Committee on Immunization Practices, and the approval of other VZV-containing products (ZOSTAVAX™ and ProQuad™, Merck & Co., Inc., Kenilworth, NJ, USA), the demand for VZV-containing vaccines has increased. The varicella vaccine passage extension (VAR-PE) process will extend the availability of VZV (Oka/Merck)-containing vaccines and is not expected to change the safety profile or effectiveness of the vaccine. The purpose of this Phase 3, randomized, double-blind, multicenter, controlled study (V210-A03; NCT03239873) was to assess the immunogenicity, safety, and tolerability of VAR-PE process in comparison with varicella vaccine commercial product 2016 (VAR) in healthy children who are between 12 to 23 months of age administered concomitantly with measles-mumps-rubella (MMR) vaccine (M-M-R™ II; Merck & Co., Inc., Kenilworth, NJ, USA).

Results

Participants

Overall, 599/600, (99.8%) of randomized participants received Dose 1 of study vaccine, 93.2% (558/599) received both Dose 1 and Dose 2 (Figure 1). Approximately 90.2% of participants completed the study (received both doses, had all blood samples collected, and completed the 42-day safety data after each vaccination). Overall, 9.8% of participants discontinued the study with the most common reasons for discontinuation being lost to follow-up (n = 31; 5.2%) and withdrawal by parent/guardian (n = 25; 4.2%).

Figure 1. Participant disposition

*Some participants may have discontinued from study vaccine but remained in the trial and completed all study visits.

Participants across both groups were similar with respect to gender, age, and race (Table 1). The majority of vaccinated participants in each vaccination group (VAR-PE: 88.0%; VAR: 86.0%) were seronegative at baseline with an initial VZV antibody titer <1.25 glycoprotein enzyme-linked immunosorbent assay (gpELISA) units/mL. Overall, 37.5% and 42.8% of participants received any medical therapy within 14 days prior to Doses 1 and 2, respectively; the most common medication classes being analgesics (12.0% before Dose 1; 13.8% before Dose 2), antibacterials (10.8% before Dose 1; 8.4% before Dose 2), and anti-inflammatory/antirheumatic products (7.5% before Dose 1; 12.0% before Dose 2). Comparable proportion of participants across both vaccination groups received any medical therapy during the study.

Table 1. Demographics

	VAR-PE (N = 300)		VAR (N = 300)
	n	(%)	n	(%)
Gender
Male	147	(49.0)	173	(57.7)
Female	153	(51.0)	127	(42.3)
Age (months)
Mean (SD)	13.0 (1.4)		13.2 (1.7)
Median	12.0		12.0
Range	12 to 19		12 to 23
Race
White	237	(79.0)	239	(79.7)
Black	35	(11.7)	23	(7.7)
Other^†	28	(9.3)	38	(12.7)

VAR-PE = Varicella vaccine PE + measles-mumps-rubella vaccine

VAR = Varicella vaccine CP2016 + measles-mumps-rubella vaccine

† = Asian, multi-racial, Native American, or Pacific Islander

N = Number of participants randomized in the vaccination group

n = Number of participants in each category

SD = Standard deviation

Immunogenicity

The results demonstrate non-inferiority of VAR-PE compared to VAR based on the VZV antibody response rate at 6 weeks Postdose 1 in the Per-Protocol (PP) population, as the lower bound of the 2-sided 95% CI for the treatment difference in response rate (VAR-PE – VAR) excluded a decrease of 10 percentage points or more (p < .001) (Table 2). Acceptability of the immune response was demonstrated based on the VZV antibody response at 6 weeks Postdose 1 in the PP population, as the lower bound of the one-sided 95% CIs of the antibody response rate was greater than 76% (p < .001) (Table 2). In the PP population, antibody geometric mean titers (GMTs) for VZV at 6 weeks Postdose 1 in the PP population were comparable (Table 2). The VZV seroconversion rate (defined as the proportion of participants with baseline VZV antibody titer <1.25 gpELISA units/mL and with postvaccination VZV antibody titer ≥1.25 gpELISA units/mL) at 6 weeks Postdose 1 in the PP population was 100% for both groups.

Table 2. Summary of VZV response rates and geometric mean titers (GMTs) at 6 weeks postdose 1

	VAR-PE (N = 299)		VAR (N = 300)
Parameter	n	Observed Response (95% CI)	n	Observed Response (95% CI)	Risk Difference^†/GMT Ratio^‡ (95% CI)	Conclusion
% ≥5 gpELISA units/mL	245	98.4% (95.9, 99.6)	239	98.3% (95.8, 99.5)	0.0 (−2.7, 2.8)	Non-inferior P < .001
GMT		18.5 (17.1, 20.1)		19.0 (17.6, 20.5)	1.0 (0.9, 1.1)	Non-inferior P < .001

VAR-PE = Varicella vaccine PE + measles-mumps-rubella vaccine

VAR = Varicella vaccine CP2016 + measles-mumps-rubella vaccine

† Risk difference = VAR-PE – VAR. The 2-sided 95% CI is based on the natural log-transformed titers and the t-distribution. The conclusion of non-inferiority (similarity) is based on the lower bound of the 2-sided 95% CI for the GMT ratio being > 0.67, which corresponds to a one-sided p-value < 0.025.

‡ GMT ratio = VAR-PE/VAR. The 2-sided 95% CI is based on the natural log-transformed titers and the t-distribution. The conclusion of non-inferiority (similarity) is based on the lower bound of the 2-sided 95% CI for the GMT ratio being > 0.67, which corresponds to a one-sided p-value < 0.025.

N = Number of participants vaccinated in the vaccination group at Dose 1

n = Number of participants with seronegative antibody titer (<1.25gpELISA units/mL) at baseline and postvaccination serology contributing to the per-protocol analysis

gpELISA = Glycoprotein enzyme-linked immunosorbent assay

VZV = Varicella-zoster virus

CI = Confidence interval

GMT = Geometric mean titer

VZV = varicella-zoster virus

A total of 71 participants (VAR-PE: 31; VAR:40) were identified as seropositive at baseline (VZV antibody ≥1.25 gpELISA units/mL) and excluded from the PP analysis. The range of antibody titers for these participants at baseline was 1.252 to 26.43 gpELISA units/mL. Postvaccination, results were comparable between the 2 vaccination groups in the antibody response rates (proportion of participants ≥5 gpELISA units/mL), GMTs, geometric mean fold rise (GMFR), and proportion of participants with ≥4-fold rise at 6 weeks Postdose 1 compared to baseline (Supplemental Table 1).

Immunogenicity was also assessed in a supportive analysis which included all participants with valid serology results regardless of protocol deviations, and results were comparable to the PP population with respect to antibody response rate and GMTs.

Where postvaccination immunogenicity was not able to be assessed due to discontinuation or due to a missing serology result, the baseline antibody titer range was (<0.625, 6.095).

Safety

Adverse events (AEs) were reported by 81.3% of participants Postdose 1 and 67.9% Postdose 2 (Table 3). The incidence of participants with injection-site or vaccine-related systemic AEs was comparable for the vaccination groups. A total of 12 participants, 6 in each vaccination group, reported a total of 15 serious AEs (SAEs) from Day 1 through the end of the study (Day 180 Postdose 2) (Supplemental Table 2). No SAEs were considered by the investigator to be vaccine-related and no participants discontinued study vaccine or died during the primary safety follow-up period (Day 1 to Day 42 Postdose 1 or 2).

Table 3. Adverse Experience (AE) Summary (Days 1 to 42 Postdose 1 and 2)

	VAR-PE (N = 300)		VAR (N = 300)
Parameter	n	(%)	n	(%)
Postdose 1
Participants with follow-up	299		300
With one or more AE	247	(82.6)	240	(80.0)
With vaccine-related* AEs	138	(46.2)	135	(45.0)
Injection-site AEs^†	101	(33.8)	97	(32.3)
Systemic AEs^†	69	(23.1)	66	(22.0)
With serious AEs	3	(1.0)	0	(0.0)
Serious vaccine-related AEs	0	(0.0)	0	(0.0)
Who died^‡	0	(0.0)	0	(0.0)
Discontinued due to an AE	0	(0.0)	0	(0.0)
Postdose 2
Participants with follow-up	276		282
With one or more AE	191	(69.2)	188	(66.7)
With vaccine-related* AEs	95	(34.4)	94	(33.3)
Injection-site AEs^†	78	(28.3)	78	(27.7)
Systemic AEs^†	28	(10.1)	33	(11.7)
With serious AEs	1	(0.4)	1	(0.4)
Serious vaccine-related AEs	0	(0.0)	0	(0.0)
Who died^‡	0	(0.0)	0	(0.0)
Discontinued due to an AE	0	(0.0)	0	(0.0)

VAR-PE = Varicella vaccine PE + measles-mumps-rubella vaccine

VAR = Varicella vaccine CP2016 + measles-mumps-rubella vaccine

*Determined by the investigator to be possibly, probably, or definitely related to the vaccine

†Includes those adverse experiences occurring with an overall incidence of 5% or more in the study population

‡No Subject died throughout the course of this study

N = Number of participants randomized and vaccinated in the vaccination group

n = Number of participants in each category

The same subject may appear in different categories, but counted only once in each category

All injection-site AEs were considered related to the vaccination. From Day 1 through Day 42 Postdose 1, injection-site AEs related to varicella vaccine were reported by 33.8% and 32.3% of participants in VAR-PE and VAR, respectively, and Postdose 2, by 28.3% and 27.7% of participants in the VAR-PE and VAR groups, respectively (Table 3). The incidences of solicited injection-site AEs from Day 1 to Day 5 Postdose 1 and Postdose 2 were comparable (Table 4).

Table 4. Summary of participants with VRC-solicited injection site varicella vaccine related adverse events

	VAR-PE		VAR
	n	(%)	n	(%)
Days 1 to 5 Postdose 1
Participants with follow-up	299		300
with ≥1 VRC-solicited injection-site AE	66	(22.1)	63	(21.0)
Erythema	29	(9.7)	32	(10.7)
Pain	40	(13.4)	38	(12.7)
Swelling	9	(3.0)	17	(5.7)
Days 1 to 5 Postdose 2
Participants with follow-up	276		282
with ≥1 VRC-solicited injection-site AE	70	(25.4)	71	(25.2)
Erythema	54	(19.6)	56	(19.9)
Pain	24	(8.7)	29	(10.3)
Swelling	28	(10.1)	23	(8.2)

VAR-PE = Varicella vaccine PE + measles-mumps-rubella vaccine

VAR = Varicella vaccine CP2016 + measles-mumps-rubella vaccine

VRC = vaccine report card

From Day 1 through 42 Postdose 1, systemic AEs were reported for the majority of participants and the incidences of systemic AEs were generally comparable for the 2 vaccination groups, with the exception of pyrexia and otitis media which were higher in VAR-PE, and diarrhea and teething which were higher in VAR (Table 5). Overall, 22.5% of participants experienced vaccine-related systemic AEs from Day 1 to Day 42 Postdose 1. The proportions of participants with vaccine-related systemic AEs were comparable for the vaccination groups (VAR-PE: 23.1%; VAR: 22.0%). The most commonly reported vaccine-related systemic AEs for each vaccination group were pyrexia (7.5%) and irritability (7.2%).

Table 5. Summary of Participants with Systemic Adverse Events (incidence ≥10% in either group)

	VAR-PE		VAR
	n	%	n	%
Days 1 to 42 Postdose 1
Participants with follow-up	299		300
with ≥1 systemic AE	229	76.6	223	74.3
Pyrexia	70	23.4	51	17.0
Irritability	38	12.7	40	13.3
Cough	36	12.0	43	14.3
Rhinorrhea	33	11.0	32	10.7
Otitis media	30	10.0	13	4.3
Upper respiratory tract infection	24	8.0	30	10.0
Diarrhea	22	7.4	44	14.7
Teething	21	7.0	36	12.0
Nasopharyngitis	21	7.0	31	10.3
Days 1 to 42 Postdose 2
Participants with follow-up	276		282
with ≥1 systemic AE	168	60.9	168	59.6
Pyrexia	44	15.9	34	12.1
Rhinorrhea	28	10.1	16	5.7

VAR-PE = Varicella vaccine PE + measles-mumps-rubella vaccine

VAR = Varicella vaccine CP2016 + measles-mumps-rubella vaccine

The incidence of systemic AEs was generally lower Postdose 2 compared with Postdose 1 (Table 5). From Day 1 through Day 42 Postdose 2, the most frequently reported systemic AEs were pyrexia (VAR-PE: 15.9%; VAR: 12.1%) and rhinorrhea (VAR-PE: 10.1%; VAR: 5.7%). Overall, 10.9% of participants experienced vaccine-related systemic AEs from Day 1 to Day 42 Postdose 2. The proportions of participants with vaccine-related systemic AEs were comparable for the vaccination groups (VAR-PE: 10.1%; VAR: 11.7%). The most commonly reported vaccine-related systemic AEs for each vaccination group were pyrexia (3.8%) and irritability (3.2%).

Across all day ranges, the incidence of fever, defined as temperature ≥102.2°F (≥39.0°C) oral equivalent, was comparable for the 2 vaccination groups and no significant differences were observed (Table 6). The incidence of fever was lower Postdose 2 compared with Postdose 1. The highest rate of fever was observed between Day 6 to Day 14 Postdose 1 and between Days 29 to 42 Postdose 2.

Table 6. Analysis of rates of fever by Day Range between vaccination groups (all participants as treated population)

Day Range		VAR-PE	VAR	Risk Difference(95% CI) ^†
Days 1 to 42 Postdose 1		N = 299	N = 300
Days 1 to 5	Participants with Temperature Follow-Up	297	296
	Max Temp (oral equivalent): ≥102.2 °F [≥39.0°C]	6 (2.0%)	3 (1.0%)	1.0 (−1.2, 3.4)
Days 6 to 14	Participants with Temperature Follow-Up	291	295
	Max Temp (oral equivalent): ≥102.2 °F [≥39.0°C]	13 (4.5%)	15 (5.1%)	−0.6 (−4.3, 3.0)
Days 15 to 28	Participants with Temperature Follow-Up	290	291
	Max Temp (oral equivalent): ≥102.2 °F [≥39.0°C]	11 (3.8%)	10 (3.4%)	0.4 (−2.9, 3.6)
Days 29 to 42	Participants with Temperature Follow-Up	285	286
	Max Temp (oral equivalent): ≥102.2 °F [≥39.0°C]	10 (3.5%)	6 (2.1%)	1.4 (−1.4, 4.5)
Days 1 to 42 Postdose 2		N = 276	N = 282
Days 1 to 5	Participants with Temperature Follow-Up	265	282
	Max Temp (oral equivalent): ≥102.2 °F [≥39.0°C]	1 (0.4%)	3 (1.1%)	−0.7 (−2.9, 1.1)
Days 6 to 14	Participants with Temperature Follow-Up	269	273
	Max Temp (oral equivalent): ≥102.2 °F [≥39.0°C]	6 (2.2%)	3 (1.1%)	1.1(−1.2, 3.8)
Days 15 to 28	Participants with Temperature Follow-Up	265	275
	Max Temp (oral equivalent): ≥102.2 °F [≥39.0°C]	8 (3.0%)	6 (2.2%)	0.8 (−2.1, 3.9)
Days 29 to 42	Participants with Temperature Follow-Up	259	274
	Max Temp (oral equivalent): ≥102.2 °F [≥39.0°C]	10 (3.9%)	6 (2.2%)	1.7 (−1.3, 5.0)

VAR-PE = Varicella vaccine PE + measles-mumps-rubella vaccine.

VAR = Varicella vaccine CP2016 + measles-mumps-rubella vaccine

^†The 2-sided 95% CI is calculated using the Miettinen and Nurminen unconditional asymptotic method.

The incidence of vaccine-specific rashes was low and comparable for the vaccination groups from Day 1 to Day 42 Postdose 1 (VAR-PE: 3.3%; VAR: 3.3%) and Postdose 2 (VAR-PE: 2.2%; VAR: 0.7%) with no significant differences observed for the 2 vaccination groups. The incidence of vaccine-specific rashes was generally higher Postdose 1 compared with Postdose 2 (Supplemental Table 3). None of these rashes were considered a SAE by the study investigator. No participants in either vaccination group experienced mumps-like symptoms from Day 1 to Day 42 Postdose 1 or Postdose 2.

Discussion

Varicella vaccine is indicated for active immunization for the prevention of varicella in individuals 12 months of age and older. The Varicella Passage Extension process is part of a multi-stage process designed to ensure ongoing supply of VZV (Oka/Merck)-containing vaccines.¹⁴ This process change was not expected to impact the safety profile or effectiveness of the vaccine. Strengths of the trial include the randomized, double-blind, comparator-controlled design in order to assess both immunogenicity, safety, and tolerability. Another strength was that this trial was performed in healthy children who were between 12 to 23 months of age, which was the recommended age group for the first dose of VZV-containing vaccine. Immmunogenicity was assessed twice during the study, prior to enrollment and Postdose 1, but not Postdose 2. This may be viewed as a limitation; however, this schema was chosen as not to be overly prohibitive for the participants and their parents/guardians, and importantly, PD1 immunogenicity data supports broad use globally. In a previous study comparing one versus two injections of VAR in healthy children 1–12 years of age, the seroconversion rate was 98.2% after one injection and 99.9% after two injections. Additionally, >99% of these children who received either one or two doses of VAR maintained antibody to varicella at 1 year postvaccination with geometric mean titers of 19.5 and 31.2, respectively.¹⁵

The results of this study demonstrate non-inferiority of immune responses induced by vaccinations with VAR-PE compared to VAR, including both antibodies assessed by gpELISA, as well as GMTs, 6 weeks postvaccination. The immune responses met the statistical criteria for acceptability. Vaccination with VAR-PE process product is well tolerated and has a safety profile comparable to that of VAR.

The results from this study demonstrate that VAR-PE is non-inferior with respect to immunogenicity and has a comparable safety and tolerability profile compared to VAR. The results support the passage extension manufacturing change in order to ensure the availability of VZV (Oka/Merck)-containing vaccines.

Methods

Design

This was a randomized, double-blind clinical trial conducted in 35 sites within the US from October 2017 to April 2019. The protocol was conducted in accordance with principles of Good Clinical Practice (GCP), including obtaining written informed consent from each participant’s parent(s) or legal guardian(s) prior to study entry, and was approved by the Institutional Review Board applicable to each study site. Randomization occurred centrally using an automated web/phone system. Subjects were assigned randomized treatment in a 1:1 ratio to VAR-PE or VAR, without stratification.

Participants

Healthy children 12 to 23 months of age with a negative history for varicella, herpes zoster, measles, mumps, and rubella, and without prior immunization against these diseases were eligible for the study. Exclusion criteria included receipt of any inactivated vaccine within 14 days, or any live vaccine within 30 days, prior to study entry; history of seizure disorder; febrile illness within 72 hours prior to study entry; or any congenital or acquired immune deficiency, neoplastic disease, or immunosuppression.

Participants were allocated to a vaccination group using a randomized schedule generated by the study statistician. Participants were randomized to receive either two 0.5 ml subcutaneous doses of VAR-PE vaccine 3 months apart or two 0.5 ml subcutaneous doses of VAR vaccine 3 months apart, concomitantly with MMR vaccine. The study was designed to have approximately 600 participants randomized in a 1:1 ratio to either one of the two groups. Based on approximately 300 participants per group, and with an expected evaluability rate of 80%, the study provided 94.1% power for demonstrating noninferiority of the response rate, 95.8% power for demonstrating noninferiority of the GMT, and >99.9% power for demonstrating acceptability of the response rate at an overall 2-sided 5% α-level.

Vaccines

Varicella vaccine is manufactured using MRC-5 cells inoculated with the Oka/Merck strain of the VZV. The virus is harvested into a stabilizer solution followed by sonication and clarification by filtration. The dispensed drug substance is diluted with stabilizer solution, filled into vials and lyophilized. The lyophilized vaccine pellet is reconstituted with sterile diluent prior to administration. VAR-PE is manufactured using an additional passage extension which establishes an additional stock seed, which is then the process input to manufacture the varicella drug substance at increased passage level. The end result is an increase of the seed supply to be used to manufacture VZV (Oka/Merck)-containing vaccines. Both VAR-PE and VAR have the same dose, as VAR-PE is formulated to contain the same potency as VAR at the time of release. Additionally, this manufacturing change is not expected to change the immunogenicity or safety profile of the vaccine.

Blinded, single-dose vials of VAR-PE and VAR were supplied to the clinical sites, were indistinguishable in appearance, and stored at 2º to 8ºC. Open-label, single-dose vials of MMR-II vaccine and sterile diluent for reconstitution were also supplied to the clinical sites and handled according to the package inserts.

Immunogenicity

The primary immunogenicity analyses were based on the Per-Protocol (PP) population. The PP population was defined as participants who received 1 dose of vaccine according to their vaccination group assignment, adhered to study instructions, and provided serum samples within the appropriate day ranges. The PP population excluded participants who had important protocol deviations with the potential to impact the immunogenicity analyses (not compliant with immunogenicity specimen collection, n = 8; receipt of systemic steroids 7 days prior to vaccination or during the 42-day safety follow-up period, n = 6; receipt of improperly stored study treatment, n = 2; improper reconstitution of vaccine, n = 1), as well as participants who did not receive any study vaccine, n = 1, did not have postvaccination immunogenicity assessments, n = 45, or had seropositive status at baseline, n = 77.

Serum samples collected before and 6 weeks Postdose 1 were tested for VZV antibody titers for varicella evaluated by gpELISA method.^16–18 Immunogenicity was evaluated by response rates at each time point and GMTs of antibodies to each virus were evaluated 6 weeks Postdose 1. Serum samples were not collected 6 weeks Postdose 2 for antibody measurement.

The primary immunogenicity objectives were: (1) to demonstrate that a single dose of VAR-PE process induces VZV antibody responses 6 weeks Postdose 1 that are noninferior to those induced by VAR; and (2) to demonstrate that a single dose of VAR-PE process induces an acceptable VZV antibody response 6 weeks Postdose 1. Response rate was defined as the proportion of participants with VZV antibody titer ≥5 gpELISA units/mL 6 weeks Postdose 1, among participants who were seronegative to VZV (antibody titer <1.25 gpELISA units/mL) at baseline.

The secondary immunogenicity objective was to summarize the VZV antibody responses after a single dose of VAR-PE and after a single dose of VAR. The endpoint for participants who were seronegative to VZV (antibody titer <1.25 gpELISA units/mL) at baseline, the seroconversion rate was defined as the proportion of participants with VZV antibody titer ≥1.25 gpELISA units/mL 6 weeks Postdose 1. The endpoint for participants who were seropositive to VZV (antibody titer ≥1.25 gpELISA units/mL) at baseline was defined as the GMFR and the proportion of participants achieving ≥4-fold rise in antibody titer from baseline to Postdose 1.

Safety

All participants who received a vaccine dose were included in the safety analysis. The primary safety objective was to assess the safety and tolerability of the first and second doses of VAR-PE compared to VAR. Using an electronic Vaccination Report Card (eVRC), parents and/or guardians recorded solicited injection-site reactions (redness, swelling, and pain/tenderness) of any intensity or size were recorded from Day 1 through Day 5 postvaccination. Additionally, unsolicited injection-site AEs, systemic AEs, elevated temperatures (≥102.2°F [≥39.0°C] oral equivalent), and rashes (varicella-like, zoster-like, measles-like, or mumps-like) were recorded on the eVRC through 42 days following each dose. All participants were followed for serious AEs (SAEs) throughout the trial duration and medically-attended AEs from Day 133 through Day 271 (Day 180 Postdose 2).

Statistical methods

For the primary immunogenicity hypothesis based on antibody response rate, VAR-PE was considered non-inferior to VAR if the lower bound of the 2-sided 95% CI (Miettinen and Nurminen unconditional asymptotic method) for the difference in rates (VAR-PE minus VAR) excluded a decrease of 10 percentage points or more. For the primary immunogenicity hypothesis based on GMT ratios, VAR-PE was considered non-inferior to VAR if the lower bound of the 2-sided 95% CI of the GMT ratio (VAR-PE/VAR) was >0.67. For the primary safety hypothesis, VAR-PE was considered acceptable if the lower bound of the 2-sided 95% CI for the response rate was above 76.0%.

Abbreviations

AE	=	Adverse experience
CI	=	Confidence interval
VAR	=	Varicella vaccine commercial product 2016
GCP	=	Good Clinical Practices
GMFR	=	Geometric mean fold rise
GMT	=	Geometric mean titer
gpELISA	=	Glycoprotein enzyme-linked immunosorbant assay
HZ	=	Herpes zoster
MMR	=	Measles, mumps, and rubella vaccine
VAR-PE	=	Varicella vaccine passage extension 34 product
PP	=	Per-Protocol
SAE	=	Serious adverse experience
US	=	United States
eVRC	=	electronic Vaccination Report Card
VZV	=	Varicella-zoster virus

Author contributions

Peter Silas and Edward Zissman: enrollment of participants and/or data collection, analysis and interpretation of data, and preparation of manuscript. Julie Gardner, Shanjun Helian, Andrew Lee, and Heather Platt: study concept and design, analysis and interpretation of data, and preparation of manuscript.

Disclosure of potential conflicts of interest

Peter Silas and Edward Zissman were investigators for the sponsor supported by research grants.

Julie Gardner, Shanjun Helian, Andrew Lee, and Heather Platt are employees of the sponsors and may hold stock and/or stock options from the sponsors.

Acknowledgments

The authors would like to thank: All the participants who participated in this study and their parents or legal guardian; the Protocol A03 Study group: G Adams, DM Brandon, SG Christensen, L Chu, RA Clifford, MG Cruz, O deValle, CA Duffy, BK Eberhard, JE Ervin, DJ Finn, M Harper, RD Hartvickson, AY Ituriaga, WH Johnston, KA Julien, JL Kirstein, ML Leonardi, J Ley, K Palapurwala, JT Peterson, AG Pruitt, MA Rausch, B Rizzardi, KG Rouse, RE Rupp, R Schumacher, SD Senders, JS Shepard, PE Silas, L Soylu, V Statler, WP Stewart, MA Turner, LB Weiner, JL Williams, PP Wisman, and EN Zissman; and Jon E. Stek and Karyn Davis of Merck & Co., Inc., Kenilworth, NJ, USA for their editorial assistance.

Supplementary Material

Supplemental data for this article can be accessed online at http://doi.org/10.1080/21645515.2020.1743122.

Additional information

Funding

This study was funded by Merck & Co., Inc., USA (sponsor). Although the sponsor formally reviewed a penultimate draft, the opinions and conclusion expressed are those of the only and do not reflect those of the sponsor. All co-authors approved the final version of the manuscript.