Serostatus cutoff levels and fold increase to define seroresponse to recombinant vesicular stomatitis virus - Zaire Ebola virus envelope glycoprotein vaccine: An evidence-based analysis

rVSVΔG-ZEBOV-GP Vaccine Is Highly Immunogenic and Efficacious Across a Wide Dose Range in a Nonhuman Primate EBOV Challenge Model

The recombinant vesicular stomatitis virus-Zaire Ebolavirus envelope glycoprotein vaccine (rVSVΔG-ZEBOV-GP) was highly effective against Ebola virus disease in a ring vaccination trial conducted during the 2014-2016 outbreak in Guinea and is licensed by regulatory agencies including US FDA, EMA, and prequalified by WHO. Vaccination studies in a nonhuman primate (NHP) model guided initial dose selection for clinical trial evaluation. We summarize two dose-ranging studies with the clinical-grade rVSVΔG-ZEBOV-GP vaccine candidate to assess the impact of dose level on immune responses and efficacy in an NHP Ebola virus (EBOV) challenge model. Forty-six cynomolgus macaques were vaccinated with a wide range of rVSVΔG-ZEBOV-GP doses and challenged 42 days later intramuscularly with 1000 pfu EBOV. Vaccination with rVSVΔG-ZEBOV-GP induced relatively high levels of EBOV-specific IgG and neutralizing antibodies, measured using the same validated assays as used in rVSVΔG-ZEBOV-GP clinical trials. Similar responses were observed across dose groups from 1 × 108 to 1 × 102 pfu. A single vaccination conferred 98% protection from lethal intramuscular EBOV challenge across all dose groups. These results demonstrate that robust antibody titers are induced in NHPs across a wide range of rVSVΔG-ZEBOV-GP vaccine doses, correlating with high levels of protection against death from EBOV challenge.

Durability of Immunogenicity and Protection of rVSV∆G-ZEBOV-GP Vaccine in a Nonhuman Primate EBOV Challenge Model

The rVSVΔG-ZEBOV-GP vaccine demonstrated efficacy in preventing Ebola virus (EBOV) disease in a ring vaccination clinical trial conducted during the 2014-2016 West Africa outbreak and is licensed by regulatory agencies, including the US FDA and the EMA. Here, we present two studies that evaluated the durability of immunogenicity and protection from an EBOV challenge up to ~12 months following vaccination with rVSVΔG-ZEBOV-GP in nonhuman primates (NHPs). Cynomolgus macaques were vaccinated with either one or two doses of rVSVΔG-ZEBOV-GP or a saline control and were challenged intramuscularly with EBOV at a target dose of 1000 pfu at ~4 months (Study 1) or ~8 or ~12 months (Study 2) after the last vaccination. All vaccinated animals developed robust ZEBOV-GP-specific IgG and neutralizing antibody titers, which were sustained until the last time point tested prior to the challenge. The majority of animals (88-93%) challenged with EBOV at ~4 or ~8 months post-vaccination survived, whereas the survival rate was lower (53%) in animals challenged ~12 months post-vaccination. These results demonstrate that both one-dose and two-dose regimens of the rVSVΔG-ZEBOV-GP vaccine induced durable ZEBOV-GP-specific antibody titers in NHPs and provided high levels of protection against a lethal EBOV challenge up to ~8 months post-vaccination. In this stringent challenge model, decreased protection was observed at ~12 months post-vaccination despite sustained antibody levels.

Humoral Immunogenicity of mRNA-1345 RSV Vaccine in Older Adults

BACKGROUND

The mRNA-1345 vaccine demonstrated efficacy against respiratory syncytial virus (RSV) disease with acceptable safety in adults aged ≥60 years in the ConquerRSV trial. Here, humoral immunogenicity results from the trial are presented.

METHODS

This phase 2/3 trial randomly assigned adults (≥60 years) to mRNA-1345 50-µg encoding prefusion F (preF) glycoprotein (n = 17 793) vaccine or placebo (n = 17 748). RSV-A and RSV-B neutralizing antibody (nAb) and preF binding antibody (bAb) levels at baseline and day 29 postvaccination were assessed in a per-protocol immunogenicity subset (PPIS; mRNA-1345, n = 1515; placebo, n = 333).

RESULTS

Day 29 nAb geometric mean titers (GMTs) increased 8.4-fold against RSV-A and 5.1-fold against RSV-B from baseline. Seroresponses (4-fold rise from baseline) in the mRNA-1345 groups were 74.2% and 56.5% for RSV-A and RSV-B, respectively. Baseline GMTs were lower among participants who met the seroresponse criteria than those who did not. mRNA-1345 induced preF bAbs at day 29, with a pattern similar to nAbs. Day 29 antibody responses across demographic and risk subgroups were generally consistent with the overall PPIS.

CONCLUSIONS

mRNA-1345 enhanced RSV-A and RSV-B nAbs and preF bAbs in adults (≥60 years) across various subgroups, including those at risk for severe disease, consistent with its demonstrated efficacy in the prevention of RSV disease.

CLINICAL TRIALS REGISTRATION

NCT05127434.

SEROLOGICAL RESPONSE TO CANINE DISTEMPER VACCINATION IN WILD CAUGHT RACCOONS (PROCYON LOTOR)

Canine distemper virus (CDV) is a well-known RNA virus that affects domestic dogs and all families of wild terrestrial carnivores. Spillover infections from wildlife to domestic animals are mitigated by preventive vaccination, but there is limited information on the off-label use of veterinary vaccines for wildlife like raccoons (Procyon lotor). Twenty wild-caught raccoons were inoculated with a commercial recombinant DNA canarypox-vectored CDV vaccine, applying a regimen of two serial doses by SC route with an interval of 25-28 days between doses. The CDV serum virus neutralizing antibody (VNA) baseline titers and the postvaccination titers were measured at fixed time points. Forty percent (8/20) of the wild-caught raccoons had CDV VNA titers of 1:8 or greater upon intake, and all but a single individual were juvenile animals. Approximately one month following the first vaccine dose, 8% (1/12) of raccoons seronegative at baseline had serum CDV VNA titers of 1:24 or greater. Approximately one month following the booster vaccine dose, 67% (8/12) of raccoons seronegative at baseline had serum CDV VNA titers of 1:24 or greater. Among raccoons with CDV VNA titers greater than or equal to 1:8 at baseline, 13% (1/8) demonstrated a fourfold or greater rise in titer one month after the first vaccine dose, whereas 38% (3/8) reached the same threshold one month after the booster dose. The presence of naturally acquired CDV VNA in juvenile raccoons at the time of vaccination may have interfered with the humoral VNA response. A regimen of at least two serially administered SC vaccine doses may be immunogenic for raccoons, but further investigation of alternative routes, regimens, and CDV vaccine products is also warranted for this species.

Immunogenicity and Vaccine Shedding After 1 or 2 Doses of rVSVΔG-ZEBOV-GP Ebola Vaccine (ERVEBO®): Results From a Phase 2, Randomized, Placebo-controlled Trial in Children and Adults

BACKGROUND

The rVSVΔG-ZEBOV-GP vaccine (ERVEBO®) is a single-dose, live-attenuated, recombinant vesicular stomatitis virus vaccine indicated for the prevention of Ebola virus disease (EVD) caused by Zaire ebolavirus in individuals 12 months of age and older.

METHODS

The Partnership for Research on Ebola VACcination (PREVAC) is a multicenter, phase 2, randomized, double-blind, placebo-controlled trial of 3 vaccine strategies in healthy children (ages 1-17) and adults, with projected 5 years of follow-up (NCT02876328). Using validated assays (GP-ELISA and PRNT), we measured antibody responses after 1-dose rVSVΔG-ZEBOV-GP, 2-dose rVSVΔG-ZEBOV-GP (given on Day 0 and Day 56), or placebo. Furthermore, we quantified vaccine virus shedding in a subset of children's saliva using RT-PCR.

RESULTS

In total, 819 children and 783 adults were randomized to receive rVSVΔG-ZEBOV-GP (1 or 2 doses) or placebo. A single dose of rVSVΔG-ZEBOV-GP increased antibody responses by Day 28 that were sustained through Month 12. A second dose of rVSVΔG-ZEBOV-GP given on Day 56 transiently boosted antibody concentrations. In vaccinated children, GP-ELISA titers were superior to placebo and non-inferior to vaccinated adults. Vaccine virus shedding was observed in 31.7% of children, peaking by Day 7, with no shedding observed after Day 28 post-dose 1 or any time post-dose 2.

CONCLUSIONS

A single dose of rVSVΔG-ZEBOV-GP induced robust antibody responses in children that was non-inferior to the responses induced in vaccinated adults. Vaccine virus shedding in children was time-limited and only observed after the first dose. Overall, these data support the use of rVSVΔG-ZEBOV-GP for the prevention of EVD in at-risk children. Clinical Trials Registration. The study is registered at ClinicalTrials.gov (NCT02876328), the Pan African Clinical Trials Registry (PACTR201712002760250), and the European Clinical Trials Register (EudraCT number: 2017-001798-18).

Cellular and humoral immunity to Ebola Zaire glycoprotein and viral vector proteins following immunization with recombinant vesicular stomatitis virus-based Ebola vaccine (rVSVΔG-ZEBOV-GP)

While effective at preventing Zaire ebolavirus (ZEBOV) disease, cellular immunity to ZEBOV and vector-directed immunity elicited by the recombinant vesicular stomatitis virus expressing ZEBOV glycoprotein (rVSVΔG-ZEBOV-GP) vaccine remain poorly understood. Sera and peripheral blood mononuclear cells were collected from 32 participants enrolled in a prospective multicenter study [ClinicalTrials.gov NCT02788227] before vaccination and up to six months post-vaccination. IgM and IgG antibodies, IgG-producing memory B cells (MBCs), and T cell reactivity to ZEBOV glycoprotein (ZEBOV-GP), vesicular stomatitis virus-Indiana strain (VSV-I) matrix (M) protein, and VSV-I nucleoprotein (NP) were measured using ELISA, ELISpot, and flow cytometry, respectively. 11/32 (34.4%) participants previously received a different investigational ZEBOV vaccine prior to enrollment and 21/32 (65.6%) participants were ZEBOV vaccine naïve. Both ZEBOV vaccine naïve and experienced participants had increased ZEBOV-GP IgG optical densities (ODs) post-rVSVΔG-ZEBOV-GP vaccination while only ZEBOV vaccine naïve participants had increased ZEBOV-GP IgM ODs. Transient IgM and IgG antibody responses to VSV-I M protein and NP were observed in a minority of participants. All participants had detectable ZEBOV-GP specific IgG-producing MBCs by 6 months post-vaccination while no changes were observed in the median IgG-producing MBCs to VSV-I proteins. T cell responses to ZEBOV-GP differed between ZEBOV vaccine experienced and ZEBOV vaccine naïve participants. T cell responses to both VSV-I M protein and VSV-I NP were observed, but were of a low magnitude. The rVSVΔG-ZEBOV-GP vaccine elicits robust humoral and memory B cell responses to ZEBOV glycoprotein in both ZEBOV vaccine naïve and experienced individuals and can generate vector-directed T cell immunity. Further research is needed to understand the significance of pre-existing vector and target antigen immunity on responses to booster doses of rVSVΔG-ZEBOV-GP and other rVSV-vectored vaccines.

Immunogenicity of rVSVΔG-ZEBOV-GP Ebola vaccine (ERVEBO®) in African clinical trial participants by age, sex, and baseline GP-ELISA titer: A post hoc analysis of three Phase 2/3 trials

BACKGROUND

ERVEBO®, a live recombinant vesicular stomatitis virus (VSV) vaccine containing the Zaire ebolavirus glycoprotein (GP) in place of the VSV GP (rVSVΔG-ZEBOV-GP), was advanced through clinical development by Merck & Co., Inc., Rahway, NJ, USA in collaboration with multiple partners to prevent Ebola virus disease (EVD) and has been approved for human use in several countries.

METHODS

We evaluated data from three Phase 2/3 clinical trials conducted in Liberia (PREVAIL), Guinea (FLW), and Sierra Leone (STRIVE) during the 2013-2016 West African EVD outbreak to assess immune responses using validated assays. We performed a post hoc analysis of the association of vaccine response with sex, age (18-50 yrs & >50 yrs), and baseline (BL) GP-enzyme-linked immunosorbent assay (ELISA) titer (<200 & ≥200 EU/mL), including individual study (PREVAIL, FLW, or STRIVE) data and pooled data from all 3 studies. The endpoints were total IgG antibody response (EU/mL) measured by the GP-ELISA and neutralizing antibody response measured by the plaque reduction neutralization test (PRNT) to rVSVΔG-ZEBOV-GP at Days 28, 180, and 365 postvaccination.

RESULTS

In the overall pooled population, in all subgroups, and in each trial independently, GP-ELISA and PRNT geometric mean titers increased from BL, generally peaking at Day 28 and persisting through Day 365. Immune responses were greater in women and participants with BL GP-ELISA ≥ 200 EU/mL, but did not differ across age groups.

CONCLUSION

These data demonstrate that rVSVΔG-ZEBOV-GP elicits a robust and durable immune response through 12 months postvaccination in participants regardless of age, sex, or BL GP-ELISA titer. The higher immune responses observed in women and participants with pre-existing immunity are consistent with those described previously and for other vaccines. Trials were registered as follows: PREVAIL: ClinicalTrials.gov NCT02344407; FLW: Pan African Clinical Trials Registry PACTR201503001057193; STRIVE: ClinicalTrials.gov NCT02378753. Protocols V920-009, 011, and 018.

Lessons Learned from the Development and Roll-Out of the rVSVΔG-ZEBOV-GP Zaire ebolavirus Vaccine to Inform Marburg Virus and Sudan ebolavirus Vaccines

This review describes key aspects of the development of the rVSVΔG-ZEBOV-GP Ebola vaccine and key activities which are continuing to further expand our knowledge of the product. Extensive partnerships and innovative approaches were used to address the various challenges encountered during this process. The rVSVΔG-ZEBOV-GP Ebola vaccine was initially approved by the European Medicines Agency and prequalified by the World Health Organization in November 2019. It was approved by the United States Food and Drug Administration in December 2019 and approved in five African countries within 90 days of prequalification. The development resulted in the first stockpile of a registered Ebola vaccine that is available to support outbreak response. This also provides insights into how the example of rVSVΔG-ZEBOV-GP can inform the development of vaccines for Sudan ebolavirus, Marburg virus, and other emerging epidemic diseases in terms of the types of approaches and data needed to support product registration, availability, and the use of a filovirus vaccine.

Adenovirus-based vaccines-a platform for pandemic preparedness against emerging viral pathogens

Zoonotic viruses continually pose a pandemic threat. Infection of humans with viruses for which we typically have little or no prior immunity can result in epidemics with high morbidity and mortality. These epidemics can have public health and economic impact and can exacerbate civil unrest or political instability. Changes in human behavior in the past few decades-increased global travel, farming intensification, the exotic animal trade, and the impact of global warming on animal migratory patterns, habitats, and ecosystems-contribute to the increased frequency of cross-species transmission events. Investing in the pre-clinical advancement of vaccine candidates against diverse emerging viral threats is crucial for pandemic preparedness. Replication-defective adenoviral (Ad) vectors have demonstrated their utility as an outbreak-responsive vaccine platform during the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic. Ad vectors are easy to engineer; are amenable to rapid, inexpensive manufacturing; are relatively safe and immunogenic in humans; and, importantly, do not require specialized cold-chain storage, making them an ideal platform for equitable global distribution or stockpiling. In this review, we discuss the progress in applying Ad-based vaccines against emerging viruses and summarize their global safety profile, as reflected by their widespread geographic use during the SARS-CoV-2 pandemic.

Baseline Asymptomatic Malaria Infection and Immunogenicity of Recombinant Vesicular Stomatitis Virus-Zaire Ebola Virus Envelope Glycoprotein

BACKGROUND

The effect of malaria infection on the immunogenicity of the recombinant vesicular stomatitis virus-Zaire Ebola virus envelope glycoprotein (GP) vaccine (rVSVΔG-ZEBOV-GP) (ERVEBO) is unknown.

METHODS

The Sierra Leone Trial to Introduce a Vaccine Against Ebola (STRIVE) vaccinated 7998 asymptomatic adults with rVSVΔG-ZEBOV-GP during the 2014-2016 Ebola epidemic. In STRIVE's immunogenicity substudy, participants provided blood samples at baseline and at 1, 6, and 9-12 months. Anti-GP binding and neutralizing antibodies were measured using validated assays. Baseline samples were tested for malaria parasites by polymerase chain reaction.

RESULTS

Overall, 506 participants enrolled in the immunogenicity substudy and had ≥1 postvaccination antibody titer. Of 499 participants with a result, baseline malaria parasitemia was detected in 73 (14.6%). All GP enzyme-linked immunosorbent assay (ELISA) and plaque reduction neutralization test (PRNT) geometric mean titers (GMTs) at 1, 6, and 9-12 months were above baseline, and 94.1% of participants showed seroresponse by GP-ELISA (≥2-fold rise and ≥200 ELISA units/mL), while 81.5% showed seroresponse by PRNT (≥4-fold rise) at ≥1 postvaccination assessment. In participants with baseline malaria parasitemia, the PRNT seroresponse proportion was lower, while PRNT GMTs and GP-ELISA seroresponse and GMTs showed a trend toward lower responses at 6 and 9-12 months.

CONCLUSION

Asymptomatic adults with or without malaria parasitemia had robust immune responses to rVSVΔG-ZEBOV-GP, persisting for 9-12 months. Responses in those with malaria parasitemia were somewhat lower.

Development of Pandemic Vaccines: ERVEBO Case Study

Preventative vaccines are considered one of the most cost-effective and efficient means to contain outbreaks and prevent pandemics. However, the requirements to gain licensure and manufacture a vaccine for human use are complex, costly, and time-consuming. The 2013-2016 Ebola virus disease (EVD) outbreak was the largest EVD outbreak to date and the third Public Health Emergency of International Concern in history, so to prevent a pandemic, numerous partners from the public and private sectors combined efforts and resources to develop an investigational Zaire ebolavirus (EBOV) vaccine candidate (rVSVΔG-ZEBOV-GP) as quickly as possible. The rVSVΔG-ZEBOV-GP vaccine was approved as ERVEBOTM by the European Medicines Authority (EMA) and the United States Food and Drug Administration (FDA) in December 2019 after five years of development. This review describes the development program of this EBOV vaccine, summarizes what is known about safety, immunogenicity, and efficacy, describes ongoing work in the program, and highlights learnings applicable to the development of pandemic vaccines.

A mixture model to assess the the immunogenicity of an oral rotavirus vaccine among healthy infants in Niger

Analysis of immunogenicity data is a critical component of vaccine development, providing a biological basis to support any observed protection from vaccination. Conventional methods for analyzing immunogenicity data use either post-vaccination titer or change in titer, often defined as a binary variable using a threshold. These methods are simple to implement but can be limited especially in populations experiencing natural exposure to the pathogen. A mixture model can overcome the limitations of the conventional approaches by jointly modeling the probability of an immune response and the level of the immune marker among those who respond. We apply a mixture model to analyze the immunogenicity of an oral, pentavalent rotavirus vaccine in a cohort of children enrolled into a placebo-controlled vaccine efficacy trial in Niger. Among children with undetectable immunoglobulin A (IgA) at baseline, vaccinated children had 5.2-fold (95% credible interval (CrI) 3.7, 8.3) higher odds of having an IgA response than placebo children, but the mean log IgA among vaccinated responders was 0.9-log lower (95% CrI 0.6, 1.3) than among placebo responders. This result implies that the IgA response generated by vaccination is weaker than that generated by natural infection. Multivariate logistic regression of seroconversion defined by ≥ 3-fold rise in IgA similarly found increased seroconversion among vaccinated children, but could not demonstrate lower IgA among those who seroresponded. In addition, we found that the vaccine was less immunogenic among children with detectable IgA pre-vaccination, and that pre-vaccination infant serum IgG and mother's breast milk IgA modified the vaccine immunogenicity. Increased maternal antibodies were associated with weaker IgA response in placebo and vaccinated children, with the association being stronger among vaccinated children. The mixture model is a powerful and flexible method for analyzing immunogenicity data and identifying modifiers of vaccine response and independent predictors of immune response.