14.1 Efficacy Against V. cholerae Challenge
Study 2 was a randomized, double-blind, saline placebo-controlled V. cholerae challenge study conducted in the US. Subjects 18 through 45 years of age (N=197) with no prior history of cholera infection or travel to a cholera-endemic area in the previous 5 years were randomized according to a 1:1 ratio to receive one dose of VAXCHORA or placebo. In order to identify the subset of subjects to be challenged, an unblinded statistician prepared four randomly ordered lists of subjects per site, one list each for vaccine recipients with blood type O, vaccine recipients with non-O blood types, placebo recipients with blood type O, and placebo recipients with non-O blood types. This was done to maintain a minimum of 60% blood group O subjects in each treatment group. Individuals with type O blood are less likely to be infected with V. cholerae, but are at risk for developing severe cholera if infected. Each site was provided with a blinded version of the four lists specific to its site and advised on the number of subjects from each list to challenge. In the event that a subject was determined to be ineligible for challenge, the site was instructed to select the next subject from the same list as the ineligible subject
The challenges were split into 2 cohorts for 10 day and 3 month challenges. Subjects were admitted to an inpatient unit. Subjects had nothing by mouth from midnight before ingestion of the challenge strain, except for water, and had nothing by mouth for 90 minutes after ingestion of the challenge strain. Approximately 1 minute prior to challenge, subjects ingested 120 mL sodium bicarbonate (NaHCO3) buffer. The oral challenge consisted of 1 x 105 CFU live wild type V. cholerae El Tor Inaba N16961 in 30 mL NaHCO3 buffer at 10 days or 3 months post-vaccination. The co-primary objectives were to demonstrate the efficacy of a single dose of VAXCHORA in the prevention of moderate to severe diarrhea following challenge at 10 days and 3 months post-vaccination. Moderate to severe diarrhea was defined as cumulative diarrheal purge ≥ 3 liters (L) within 10 days after challenge. Diarrheal stool was defined as ≥ 2 unformed stools (takes shape of container) collected during a 48 hour period ≥ 200 grams (g) or a single unformed stool ≥ 300 g. Subjects were instructed to collect every stool from the time of challenge until discharge from the inpatient unit. Nursing staff or study personnel inspected all stool, graded the consistency of the stool and calculated the total weight of diarrheal stool per day. Weight of stool was converted to volume using the formula 1 g=1 mL. VAXCHORA recipients challenged at 10 days post-vaccination and VAXCHORA recipients challenged at 3 months post-vaccination were compared with a pooled group of placebo (saline) recipients challenged at 10 days or 3 months post-vaccination.
Of the 95 VAXCHORA recipients, 68 were challenged; 35 were challenged at 10 days post- vaccination and 33 were challenged at 3 months post-vaccination. Of the 102 placebo recipients, 66 were challenged; 33 were challenged at 10 days post-vaccination and 33 at 3 months post- vaccination. Among all randomized subjects, the mean age was 31.0 years. Overall, the mean age of the challenge population was 31.4 years. More males were in the vaccine group (71.6%) compared to the placebo group (54.9%). The majority of randomized subjects were Black (67.5%), 29.4% were White, 0.5% were American Indian/Alaskan Native, 0.5% were Asian, and 2.0% were other. There were 4.6% Hispanic or Latino participants. Overall 50.3% had blood type O. Among subjects selected for either challenge cohort, more males were challenged in the vaccine group (76.5%) compared to the placebo group (57.6%). The majority (70.9%) of the challenge population were Black, 25.4% were White, 0.7% were American Indian/Alaskan Native, 0.7% were Asian, and 2.2% were other. There were 3.7% Hispanic or Latino participants. Overall, 56.0% of challenged subjects had blood type O.
Vaccine efficacy against the occurrence of moderate to severe diarrhea at 10 days post-vaccination was 90.3% [95% CI 62.7%, 100.0%] and at 3 months post-vaccination was 79.5% [95% CI 49.9%, 100.0%] (Table 3).
Table 3: Vaccine Efficacy in the Prevention of Moderate to Severe Diarrhea Following Challenge with V. cholerae O1 El Tor Inaba at 10 Days and 3 Months Post-Vaccination (Intent-to-Treat Population)
14.2 Immunogenicity
Vibriocidal Antibody Against the Vaccine Strain (classical Inaba)
A vibriocidal antibody assay was used to measure serum levels of neutralizing antibodies against the vaccine strain.
Study 2 was a randomized, double-blind, saline placebo-controlled V. cholerae challenge study conducted in adults 18 through 45 years of age. In the subset of subjects challenged in Study 2, 91% [95% CI 82%, 97%] of vaccinees seroconverted prior to challenge and 9% developed moderate to severe cholera following challenge, while 2% of placebo recipients seroconverted prior to challenge and 59% developed moderate to severe cholera following challenge. (Seroconversion was defined as a ≥ 4-fold rise in serum vibriocidal antibody from baseline to 10 days post-vaccination.) Based on the observed association between seroconversion and protection from V. cholerae disease, seroconversion rate at 10 days post-vaccination was used to evaluate response to vaccination in other age groups.
Study 1 was a randomized, double-blind, saline placebo-controlled safety and immunogenicity study conducted in the US and Australia. A total of 3146 subjects 18 through 45 years of age not previously exposed to cholera were randomized 8:1 to receive one dose of VAXCHORA or placebo. The mean age was 29.9 years; 45.2% were male; 68.3% were White, 25.6% were Black, 2.0% were Asian, 1.9% were multiracial, 1.4% were other, 0.4% were American Indian/Alaskan Native, and 0.3% were Native Hawaiian/Pacific Islander. There were 10.0% Hispanic or Latino participants.
In this study, the rates of seroconversion were 93.5% [95% CI 92.5%, 94.4%] in vaccine recipients and 4% [95% CI 2%, 7%] in placebo recipients at 10 days post-vaccination.
Study 4 was a randomized, double-blind, placebo-controlled safety and immunogenicity study conducted in the US. A total of 398 subjects 46 through 64 years of age with no prior history of cholera infection or travel to a cholera-endemic area in the previous 5 years were randomized 3:1 to receive one dose of VAXCHORA or placebo. Overall, the mean age of the randomized population was 53.8 years; 45.7% were male; 74.9% were White, 21.9% were Black, 1.8% were American Indian/Alaskan Native, 0.5% were Asian, 0.5% were multiracial, 0.3% were Native Hawaiian/Pacific Islander, and 0.3% were other. There were 7.5% Hispanic or Latino participants.
Seroconversion rates at 10 days post-vaccination by classical Inaba vibriocidal antibody among 46 through 64 year old subjects in Study 4 were compared to those in 18 through 45 year old subjects in Study 1. VAXCHORA recipients from Study 1 were in the same age group as those in Study 2, the V. cholerae challenge study.
Adults 46 through 64 years were shown to have a non-inferior rate of seroconversion by classical Inaba vibriocidal antibody at 10 days post-vaccination compared to adults 18 through 45 years of age (Table 4).
Table 4: Vibriocidal Antibody Seroconversion Against Classical Inaba V. cholerae Vaccine Strain at 10 Days Post-Vaccination in Adults 46 through 64 Years of Age (Study 4) Compared to Adults 18 through 45 Years of Age (Study 1) [Bridging Analysis Population]
Vibriocidal Antibody Against Classical Ogawa, El Tor Inaba and El Tor Ogawa
V. cholerae serogroup O1 consists of four major subtypes: classical Inaba, classical Ogawa, El Tor Inaba and El Tor Ogawa. Serum vibriocidal antibody against the three types of V. cholerae not contained in the vaccine, namely classical Ogawa, El Tor Inaba and El Tor Ogawa, was also measured in Study 2 and Study 4. The percentages of vaccine recipients who seroconverted against each of the 4 major biotype/serotypes of V. cholerae serogroup O1 at 10 days post- vaccination (71.4% to 91.4%) are shown in Table 5.
Table 5: Seroconversion Rates 10 Days Post-Vaccination for the Four Major V. cholerae O1 Serogroup Biotypes and Serotypes in Studies 2 and 4 [Immunogenicity Evaluable Population]
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