Immunological Correlate of Protection by PHV02 Live, Attenuated Recombinant VSV Vaccine Against Nipah Virus Disease Using an Improved Neutralizing Antibody Method, with Considerations for Future Research

Abstract Establishing the immune correlate of protection for a vaccine could be the basis for regulatory approval. The immune correlate of PHV02, a clinical-stage recombinant vesicular stomatitis (rVSV) vaccine against Nipah virus (NiV) disease, was investigated in the African green monkey (AGM) model. Although a mechanistic immune correlate is not required, the focus was on neutralizing antibody, which is the mediator of protection against future viral infection. Two experimental approaches were used to determine the correlation between neutralizing antibody levels, survival, and viral and clinical outcomes after 100% lethal intranasal/intratracheal (IN/IT) challenge with NiV (Bangladesh): 1) reduction in vaccine dose given 28 days before challenge; and 2) challenge at early times after vaccination. In a previous publication of the results of these studies, a neutralization test employing wild type NiV (wt NT) under high (BSL-4) biocontainment was employed. Since this is not sustainable for immunological testing in multiple clinical trials, we developed and qualified a pseudovirus microneutralization test (VNA) using a non-replicating rVSV expressing NiV F and G envelope proteins and luciferase that can be performed at BSL-2. Sera from the two studies in the AGM model were re-tested using the VNA method. There was a high correlation between the VNA and wt NT. We confirmed that reduction in PHV02 vaccine dose to very low levels led to primary vaccine failure rather than a sub-protective level of antibody, a feature common to other replication-competent live vaccines. All AGMs survived when vaccinated with PHV02 at a dose (2 x 10 7 pfu) employed in clinical trials 21, 14 or 7 days before challenge. Neutralizing antibodies by VNA appeared by Day 13–14 after vaccination and increased by Day 20–21, reaching high titers (GMT > 1000). AGMs vaccinated 7 days before challenge had low or undetectable neutralizing antibody titers pre-challenge but also survived; these animals were on the cusp of an immune response when subjected to NiV challenge. AGMs vaccinated on Day − 21 were protected against illness whereas 4 (67%) of 6 survivor AGMs vaccinated on Day − 14 before challenge and 5 (83%) of 6 survivor AGMs vaccinated on Day − 7 before challenge had signs of illness from which they recovered. Thus, the frequency and severity of sub-lethal clinical illness decreased as time between vaccination and challenge was increased. Since high titers of VNA antibody were already present by Day 13/14, other factors such as affinity maturation of neutralizing antibody likely played a role in preventing clinical signs. A simple logistic regression model of the combined studies was used in which the sole explanatory variable was pre-challenge neutralizing antibody titers. For a pre-challenge VNA titer of 1:12 [the lower level of quantitation and representing a two-fold rise (seroresponse) from the imputed baseline of LLOQ/2], the predicted survival probability is 75–100% depending on whether random noise (± 1.0 log2) was added to the pre-challenge VNA titers. It was concluded that PHV02 vaccine elicited rapid onset of protection and that any detectable level of neutralizing antibody was a functional immune correlate of survival. A more conservative VNA titer of 1:24, representing a fourfold rise from baseline is suggested as a conservative immune correlate of protection by PHV02, thus avoiding influence of variability of the assay. By 14–21 days after vaccination VNA GMTs are many fold higher than the 1:24 threshold for protection. Various suggestions for future research aimed at confirming the PHV02 immune correlate are discussed.