Surveillance of rVSV-ZEBOV Vaccine-induced Immunity Against Ebola Virus in Previously Vaccinated Health Care Workers < EBOSURV >

Despite the availability of new effective therapeutics, Ebola Virus Disease (EVD) remains a highly lethal disease, with significant collateral impact on the society. In addition, during the last decades, EVD outbreaks appear to become more and more frequent. The disease, therefore, continues to pose a significant public health concern to many areas in West and Central Africa, most notably in the Democratic Republic of the Congo (DRC), where the majority of outbreaks occur.

Fortunately, safe and effective vaccines are now available and these have become a crucial part of the current outbreak response. Most widely used in the DRC is the rVSV-ZEBOV vaccine (licensed as ErveboTM), which was recently approved for human use by both the US Food and Drug Administration (FDA) and the European Medicines Agency (EMA).

Many questions regarding EVD vaccines in general and rVSV-ZEBOV in particular remain unanswered. Currently, there is no data beyond the 2-year time point for antibody titers and more in depth investigations on IgG subclass restrictions or cellular immunity are lacking. However, previous data from non-human primate (NHP) models collectively indicate that Ebola virus (EBOV)-specific CD4+ T cells play a critical role in generating the protective antibody responses and could therefore play a vital part upon re-challenge or prove a vital predictor of poor immunity generation, durability and vaccine failure. These findings argue for an integrated and comprehensive assessment of both durable circulating nAb as well as memory CD4 T cell responses and B cell compartments to fully assess the quality of the long-lived recall responses after a single dose rVSV-ZEBOV vaccination. In addition, there are no post marketing studies that have assessed long-lived immunity outside of a clinical trial environment in routine vaccination campaigns.

Recent events have showed that acquiring such data is of the utmost importance. During the 10th EVD outbreak in DRC in North-Kivu, a person, previously vaccinated with rVSV-ZEBOV, still developed the disease and even relapsed several months later (9). These unresolved questions on the persistence of anti-EBOV immune responses are therefore highly relevant.

Due to concerns on the durability of the immune response after vaccination with rVSV-ZEBOV, a booster dose might be necessary to adequately protect front line health care workers. In this context, the INRB Goma performed a small pilot survey among their personnel, during which they found low binding antibody titers in their previously vaccinated staff (unpublished results). As in October 2021 a new EVD outbreak in North-Kivu was declared, the INRB Goma, in consultation with WHO, decided to, therefore, revaccinate some of their staff that are part of the outbreak response. This would be the first time that the rVSV-ZEBOV vaccine is administered as a booster dose, and documenting the immune response following boostering will be crucial.