Protective efficacy of Newcastle disease virus expressing soluble trimeric hemagglutinin against highly pathogenic H5N1 influenza in chickens and mice

Negative-Strand RNA Virus-Vectored Vaccines

Vectored RNA vaccines offer a variety of possibilities to engineer targeted vaccines. They are cost-effective and safe, but replication competent, activating the humoral as well as the cellular immune system.This chapter focuses on RNA vaccines derived from negative-strand RNA viruses from the order Mononegavirales with special attention to Newcastle disease virus-based vaccines and their generation. It shall provide an overview on the advantages and disadvantages of certain vector platforms as well as their scopes of application, including an additional section on experimental COVID-19 vaccines.

Live Recombinant NDV-Vectored H5 Vaccine Protects Chickens and Domestic Ducks From Lethal Infection of the Highly Pathogenic H5N6 Avian Influenza Virus

The H5 subtype highly pathogenic avian influenza virus (HPAIV) has been introduced to South Korea every 2 or 3 years via wild migratory waterfowls, causing devastating damages to the poultry industry. Although most damages and economic losses by HPAIV are focused on chicken layers, domestic ducks are known to play a major role in the farm-to-farm transmission. However, most HPAIV vaccine studies on poultry have been performed with oil-emulsion inactivated vaccines. In this study, we developed a live recombinant Newcastle disease virus (NDV)-vectored vaccine against H5 HPAIV (rK148/ES2-HA) using a previously established NDV vaccine strain (K148/08) isolated from a wild mallard duck. The efficacy of the vaccine when administered via the oculonasal route or as a spray was evaluated against lethal H5 HPAIV infection in domestic ducks and chickens. Oculonasal inoculation of the rK148/ES2-HA in chickens and ducks elicited antibody titers against HPAIV as early as 1 or 2 week after the single dose of vaccination, whereas spray vaccination in ducks elicited antibodies against HPAIV after the booster vaccination. The chickens and ducks vaccinated with rK148/ES2-HA showed high survival rates and low viral shedding after H5N6 HPAIV challenge. Collectively, vaccination with rK148/ES2-HA prevented lethal infection and decreased viral shedding in both chickens and ducks. The vaccine developed in this study could be useful in suppressing the viral shedding in H5 HPAIV outbreaks, with the ease of vaccine application and fast onset of immunity.

A Yellow Fever 17D Virus Replicon-Based Vaccine Platform for Emerging Coronaviruses

The tremendous global impact of the current SARS-CoV-2 pandemic, as well as other current and recent outbreaks of (re)emerging viruses, emphasize the need for fast-track development of effective vaccines. Yellow fever virus 17D (YF17D) is a live-attenuated virus vaccine with an impressive efficacy record in humans, and therefore, it is a very attractive platform for the development of novel chimeric vaccines against various pathogens. In the present study, we generated a YF17D-based replicon vaccine platform by replacing the prM and E surface proteins of YF17D with antigenic subdomains from the spike (S) proteins of three different betacoronaviruses: MERS-CoV, SARS-CoV and MHV. The prM and E proteins were provided in trans for the packaging of these RNA replicons into single-round infectious particles capable of expressing coronavirus antigens in infected cells. YF17D replicon particles expressing the S1 regions of the MERS-CoV and SARS-CoV spike proteins were immunogenic in mice and elicited (neutralizing) antibody responses against both the YF17D vector and the coronavirus inserts. Thus, YF17D replicon-based vaccines, and their potential DNA- or mRNA-based derivatives, may constitute a promising and particularly safe vaccine platform for current and future emerging coronaviruses.

Generation and Evaluation of Recombinant Thermostable Newcastle Disease Virus Expressing the HA of H9N2 Avian Influenza Virus

H9N2 avian influenza virus (AIV) has become endemic in many countries, causing great economic losses when co-infected with other pathogens. So far, several live vaccines based on Newcastle disease virus (NDV) vectors expressing influenza hemagglutinin (HA) have been developed. However, the thermostable recombinant NDV is rarely reported. In this study, using a thermostable NDV rAHR09 strain as the vector, three recombinant NDVs expressing native HA, chimeric HA ectodomain with transmembrane domain/C-terminal cytoplasmic tail domain from fusion protein of NDV, and HA ectodomain were generated, designated rAHR09-HA, rAHR09-HAF, and rAHR09-HAE. The MDT value of three recombinant NDVs was above 120 h, their ICPI value was about 0.03, and the recombinant NDVs were still infectious when treated for 100 min under 56 °C, which demonstrated that the recombinant NDVs kept the lentogenic and thermostable nature of rAHR09. The immunization data showed that rAHR09-HA and rAHR09-HAF induced a higher HI antibody titer against H9N2 AIV and NDV. After being challenged with H9N2 AIV, the rAHR09-HA and rAHR09-HAF could significantly reduce the virus shedding in cloacal and tracheal swab samples. Our results suggest that rAHR09-HA and rAHR09-HAF might be vaccine candidates against H9N2 AIV.

Coexpression of soluble and membrane-bound avian influenza virus H5 by recombinant Newcastle disease virus leads to an increase in antigen levels

Newcastle disease virus (NDV) vectors expressing avian influenza virus (AIV) haemagglutinin (HA) of subtype H5 simultaneously protect chickens from Newcastle disease (ND) as well as avian influenza (AI). The expressed, membrane-bound surface protein HA is incorporated into virions while soluble HA has been described as a potent antigen. We tested whether co-expression of both HA variants from the same NDV vector increased the overall level of HA, which could be important for optimal immunogenicity. Recombinant NDVsolH5_H5 co-expressed membrane-bound H5 of highly pathogenic (HP) AIV H5N1, detectable in infected cells, and soluble H5, which was secreted into the supernatant. This virus was compared to recombinant NDV that express either membrane-bound (rNDVH5) or soluble H5 (rNDVsolH5). Replication in embryonated chicken eggs (ECEs) and in cell culture, as well as pathogenicity in ECEs, was not influenced by the second heterologous transcriptional unit. However, the co-expression of soluble H5 with membrane-bound H5 increased total protein level about 5.25-fold as detected by MS quantification. Hence, this virus is very interesting as a vaccine virus in chickens against HPAIV infections in situations in which previous H5-expressing NDVs have reached their limit, such as in the face of pre-existing AIV maternal immunity.

A recombinant avian paramyxovirus serotype 3 expressing the hemagglutinin protein protects chickens against H5N1 highly pathogenic avian influenza virus challenge

Highly pathogenic avian influenza (HPAI) is a devastating disease of poultry and a serious threat to public health. Vaccination with inactivated virus vaccines has been applied for several years as one of the major policies to control highly pathogenic avian influenza virus (HPAIV) infections in chickens. Viral-vectored HA protein vaccines are a desirable alternative for inactivated vaccines. However, each viral vector possesses its own advantages and disadvantages for the development of a HA-based vaccine against HPAIV. Recombinant Newcastle disease virus (rNDV) strain LaSota expressing HA protein vaccine has shown promising results against HPAIV; however, its replication is restricted only to the respiratory tract. Therefore, we thought to evaluate avian paramyxovirus serotype 3 (APMV-3) strain Netherlands as a safe vaccine vector against HPAIV, which has high efficiency replication in a greater range of host organs. In this study, we generated rAPMV-3 expressing the HA protein of H5N1 HPAIV using reverse genetics and evaluated the induction of neutralizing antibodies and protection by rAPMV3 and rNDV expressing the HA protein against HPAIV challenge in chickens. Our results showed that immunization of chickens with rAPMV-3 or rNDV expressing HA protein provided complete protection against HPAIV challenge. However, immunization of chickens with rAPMV-3 expressing HA protein induced higher level of neutralizing antibodies compared to that of rNDV expressing HA protein. These results suggest that a rAPMV-3 expressing HA protein might be a better vaccine for mass-vaccination of commercial chickens in field conditions.

Molecular and biological characterization of the immunological potency of Newcastle disease virus oil emulsion-inactivated vaccines prepared from field isolate obtained from vaccinated chickens outbreak

This study was conducted to characterize the immunological parameters of chickens vaccinated with two formulated inactivated vaccines, water in oil (WO) and water in oil in water (WOW), prepared from velogenic Newcastle disease virus (vNDV) genotype VIIj isolated from outbreak among vaccinated chickens. Six groups (G1-G6) of commercial broiler chickens were established (n = 20). The G1-G3 were received homologous (WO and WOW) and heterologous (LaSota) inactivated vaccines, respectively. The G4 was vaccinated with live heterologous (LaSota) vaccine, while G5 and G6 were kept as control positive and control negative non-vaccinated groups. The antibody titers were measured against vNDV and LaSota antigens using hemagglutination inhibition (HI) test, the cytokine gene expressions of IFNγ, IL1β, IL4, IL6, IL8, and IL18 were quantified using real-time RT-PCR, and the virus shedding was titrated on chicken embryo fibroblast cells after challenging by vNDV. The classical clinical signs and 100% mortality were observed only in G5 after vNDV challenging. The highest HI titers were detected in G1, G2, and G3 using NDV/168 antigen with no significant differences among them. These groups showed higher HI titer than G4 (2-4log2). Cytokine gene expression of IFNγ, IL1, IL6, IL8, and IL18 were significantly downregulated in vaccinated chickens with upregulation of IL4 than non-vaccinated challenge group. Viral shedding titers were significantly (0.0001, p ≤ 0.001) reduced in all samples form vaccinated chickens. In conclusion, the prepared vaccines produced highly efficient immunological responses and could be used for controlling the NDV infection.

Innovation in Newcastle Disease Virus Vectored Avian Influenza Vaccines

Highly pathogenic avian influenza (HPAI) and Newcastle disease are economically important avian diseases worldwide. Effective vaccination is critical to control these diseases in poultry. Live attenuated Newcastle disease virus (NDV) vectored vaccines have been developed for bivalent vaccination against HPAI viruses and NDV. These vaccines have been generated by inserting the hemagglutinin (HA) gene of avian influenza virus into NDV genomes. In laboratory settings, several experimental NDV-vectored vaccines have protected specific pathogen-free chickens from mortality, clinical signs, and virus shedding against H5 and H7 HPAI viruses and NDV challenges. NDV-vectored H5 vaccines have been licensed for poultry vaccination in China and Mexico. Recently, an antigenically chimeric NDV vector has been generated to overcome pre-existing immunity to NDV in poultry and to provide early protection of poultry in the field. Prime immunization of one-day-old poults with a chimeric NDV vector followed by boosting with a conventional NDV vector has shown to protect broiler chickens against H5 HPAI viruses and a highly virulent NDV. This novel vaccination approach can provide efficient control of HPAI viruses in the field and facilitate poultry vaccination.

A novel recombinant attenuated Newcastle disease virus expressing H9 subtype hemagglutinin protected chickens from challenge by genotype VII virulent Newcastle disease virus and H9N2 avian influenza virus

Newcastle disease virus (NDV) and H9 subtype avian influenza virus (AIV) are two avian pathogens across the globe. Inasmuch as most poultry flocks worldwide are vaccinated with a live low-virulence or attenuated NDV vaccine, we embarked on the development of vaccine prototypes that would have dual specificities and would allow a single immunization against both avian influenza (AI) and Newcastle disease (ND). Therefore, in the present work, a cloned full-length copy of the genome of the lentogenic NDV strain rmNA-1 was selected as a backbone vector to construct three chimeric NDVs that expressed (i) the ORF encoding the HA, (ii) the ectodomain of HA fused with the transmembrane domain and cytoplasmic tail regions derived from the NDV F protein and (iii) the ectodomain of HA fused with a short GS linker and the GCN4 sequences, and designated as rmNA-H9, rmNA-H9F, and rmNA-H9 (ECTO), respectively. rmNA-H9, rmNA-H9F, and rmNA-H9 (ECTO) stably expressed the modified HA gene for 10 egg passages and the three recombinants were found innocuous to chickens. The insertion of the chimeric HA-F, rather than HA-ECTO or ORF of HA, resulted in a recombinant virus with enhanced incorporation of the HA protein into the viral surface. A single immunization of SPF chickens with the three recombinants induced NDV- and AIV H9-specific antibodies, and protected chickens against a challenge with a lethal dose of velogenic NDV or AIV H9N2. Remarkably, non-shedding of influenza virus and higher levels of H9 subtype HI titers were observed 7 days post challenge (dpc) in rmNA-H9F vaccinated chickens, than other recombinants. Furthermore, a prime-boost vaccination of chickens with rmNA-H9F induced higher levels of NDV- and H9- HI and secretory IgA, as well as reduced viral shedding and virus-induced gross lesions, compared with the commercial vaccine. Therefore, the recombinant rmNA-H9F is a promising bivalent vaccine candidate against NDV and H9 subtype AIV in chickens.

Efficacy of recombinant measles virus expressing highly pathogenic avian influenza virus (HPAIV) antigen against HPAIV infection in monkeys

Highly pathogenic avian influenza virus (HPAIV) is a serious threat not only to domestic fowls but also to humans. Vaccines inducing long-lasting immunity against HPAIV are required. In the present study, we generated recombinant measles virus (MV) expressing the hemagglutinin protein of HPAIV without the multibasic site necessary for its pathogenicity in chickens using the backbone of an MV vaccine strain (rMV-Ed-H5HA) or a wild-type MV-derived mutant (rMV-HL-Vko-H5HA). We examined protective efficacy of the candidate vaccines in the monkey infection model by the challenge with a HPAIV (H5N1). Cynomolgus monkeys inoculated with the candidate vaccines produced both anti-H5 HA and anti-MV antibodies. They recovered earlier from influenza symptoms than unvaccinated monkeys after the challenge with the HPAIV strain. Chest radiography and histopathological analyses confirmed less severe pneumonia in the vaccinated monkeys. Vaccination tended to suppress viral shedding and reduced the interleukin-6 levels in the lungs. Furthermore, the vaccination with rMV-Ed-H5HA of monkeys with pre-existing anti-MV immunity induced the production of anti-H5 HA antibodies. These results suggest that both candidate vaccines effectively reduce disease severity in naïve hosts, and that rMV-Ed-H5HA is a particularly good candidate vaccine against HPAIV infection.

Viral vector-based influenza vaccines

Antigenic drift of seasonal influenza viruses and the occasional introduction of influenza viruses of novel subtypes into the human population complicate the timely production of effective vaccines that antigenically match the virus strains that cause epidemic or pandemic outbreaks. The development of game-changing vaccines that induce broadly protective immunity against a wide variety of influenza viruses is an unmet need, in which recombinant viral vectors may provide. Use of viral vectors allows the delivery of any influenza virus antigen, or derivative thereof, to the immune system, resulting in the optimal induction of virus-specific B- and T-cell responses against this antigen of choice. This systematic review discusses results obtained with vectored influenza virus vaccines and advantages and disadvantages of the currently available viral vectors.

Comprehensive Analysis and Characterization of Linear Antigenic Domains on HN Protein from Genotype VII Newcastle Disease Virus Using Yeast Surface Display System

Circulation of genotype VII Newcastle disease virus (NDV) has posed a great threat for the poultry industry worldwide. Antibodies against Hemagglutinin-neuraminidase (HN), a membrane protein of NDV with critical roles in NDV infection, have been reported to provide chickens protection from NDV infection. In this study, we comprehensively analyzed the in vivo antibody responses against the linear antigenic domains of the HN protein from genotype VII NDV using a yeast surface display system. The results revealed four distinct regions of HN, P1 (1-52aa), P2 (53-192aa), P3 (193-302aa) and P4 (303-571aa), respectively, according to their antigenic potency. Analysis by FACS and ELISA assay indicated P2 to be the dominant linear antigenic domain, with the immunogenic potency to protect the majority of chickens from NDV challenge. In contrast, the P1, P3 and P4 domains showed weak antigenicity in vivo and could not protect chickens from NDV challenge. These results provide important insight into the characteristic of humoral immune responses elicited by HN of NDV in vivo.

Vaccines against influenza A viruses in poultry and swine: Status and future developments

Influenza A viruses are important pathogens with a very broad host spectrum including domestic poultry and swine. For preventing clinical disease and controlling the spread, vaccination is one of the most efficient tools. Classical influenza vaccines for domestic poultry and swine are conventional inactivated preparations. However, a very broad range of novel vaccine types ranging from (i) nucleic acid-based vaccines, (ii) replicon particles, (iii) subunits and virus-like particles, (iv) vectored vaccines, or (v) live-attenuated vaccines has been described, and some of them are now also used in the field. The different novel approaches for vaccines against avian and swine influenza virus infections are reviewed, and additional features like universal vaccines, novel application approaches and the "differentiating infected from vaccinated animals" (DIVA)-strategy are summarized.

Vaccine self-assembling immune matrix is a new delivery platform that enhances immune responses to recombinant HBsAg in mice

Vaccination remains the most effective public health tool to prevent infectious diseases. Many vaccines are marginally effective and need enhancement for immunocompromised, elderly, and very young populations. To enhance immunogenicity, we exploited the biphasic property of the (RADA)4 synthetic oligopeptide to create VacSIM (vaccine self-assembling immune matrix), a new delivery method. VacSIM solution can easily be mixed with antigens, organisms, and adjuvants for injection. Postinjection, the peptides self-assemble into hydrated nanofiber gel matrices, forming a depot with antigens and adjuvants in the aqueous phase. We believe the depot provides slow release of immunogens, leading to increased activation of antigen-presenting cells that then drive enhanced immunogenicity. Using recombinant hepatitis B virus surface antigen (rHBsAg) as a model immunogen, we compared VacSIM delivery to delivery in alum or complete Freund's adjuvant (CFA). Delivery of the rHBsAg antigen to mice via VacSIM without adjuvant elicited higher specific IgG responses than when rHBsAg was delivered in alum or CFA. Evaluating IgG subtypes showed a mixed Th1/Th2 type response following immunization with VacSIM, which was driven further toward Th1 with addition of CpG as the adjuvant. Increased specific IgG endpoint titers were observed in both C57BL/6 and BALB/c mice, representative of Th1 and Th2 environments, respectively. Restimulation of splenocytes suggests that VacSIM does not cause an immediate proinflammatory response in the host. Overall, these results suggest that VacSIM, as a new delivery method, has the potential to enhance immunogenicity and efficacy of numerous vaccines.

Protective efficacy of a single dose of baculovirus hemagglutinin-based vaccine in chickens and ducks against homologous and heterologous H5N1 virus infections

Outbreaks of the highly pathogenic H5N1 virus in poultry and humans are ongoing. Vaccination is an efficient method for prevention of H5N1 infection. Using chickens and ducks, we assessed the efficacy of a vaccine comprising H5N1 hemagglutinin (HA) protein produced in a baculovirus expression system. The immunized chickens and ducks were protected against lethal infection by H5N1 in an antigen dose-dependent manner. Complete protection against homologous challenge and partial protection against heterologous challenge were achieved in chickens immunized with 5 μg HA protein and in ducks immunized with 10 μg HA protein. The IgG antibody subtype was mainly detected in the sera and tissues, including the lungs. The neuraminidase (NA) inhibition assay was negative in immunized chickens and ducks. Our results indicated that the expressed HA protein by baculovirus was immunogenic to both chickens and ducks, and the immunized chickens and ducks were protected from the lethal infections of highly pathogenic H5N1 influenza virus, though ducks required more HA protein than chickens to be protected. Also, baculovirus HA-vaccinated poultry can be differentiated from infected poultry by NA inhibition assay.

Virus-vectored influenza virus vaccines

Despite the availability of an inactivated vaccine that has been licensed for >50 years, the influenza virus continues to cause morbidity and mortality worldwide. Constant evolution of circulating influenza virus strains and the emergence of new strains diminishes the effectiveness of annual vaccines that rely on a match with circulating influenza strains. Thus, there is a continued need for new, efficacious vaccines conferring cross-clade protection to avoid the need for biannual reformulation of seasonal influenza vaccines. Recombinant virus-vectored vaccines are an appealing alternative to classical inactivated vaccines because virus vectors enable native expression of influenza antigens, even from virulent influenza viruses, while expressed in the context of the vector that can improve immunogenicity. In addition, a vectored vaccine often enables delivery of the vaccine to sites of inductive immunity such as the respiratory tract enabling protection from influenza virus infection. Moreover, the ability to readily manipulate virus vectors to produce novel influenza vaccines may provide the quickest path toward a universal vaccine protecting against all influenza viruses. This review will discuss experimental virus-vectored vaccines for use in humans, comparing them to licensed vaccines and the hurdles faced for licensure of these next-generation influenza virus vaccines.

Evaluation of nonspreading Rift Valley fever virus as a vaccine vector using influenza virus hemagglutinin as a model antigen

Virus replicon particles are capable of infection, genome replication and gene expression, but are unable to produce progeny virions, rendering their use inherently safe. By virtue of this unique combination of features, replicon particles hold great promise for vaccine applications. We previously developed replicon particles of Rift Valley fever virus (RVFV) and demonstrated their high efficacy as a RVFV vaccine in the natural target species. We have now investigated the feasibility of using this nonspreading RVFV (NSR) as a vaccine vector using influenza virus hemagglutinin as a model antigen. NSR particles were designed to express either the full-length hemagglutinin of influenza A virus H1N1 (NSR-HA) or the respective soluble ectodomain (NSR-sHA). The efficacies of the two NSR vector vaccines, applied via either the intramuscular or the intranasal route, were evaluated. A single vaccination with NSR-HA protected all mice from a lethal challenge dose, while vaccination with NSR-sHA was not protective. Interestingly, whereas intramuscular vaccination elicited superior systemic immune responses, intranasal vaccination provided optimal clinical protection.

Prevention and Control of Influenza Viruses

The 2003–2004 outbreaks of highly pathogenic avian influenza (HPAI) have proven to be disastrous to the regional poultry industry in Asia, and have raised serious worldwide public health apprehension regarding the steps that should be taken to urgently control HPAI. Control measures must be taken based on the principles of biosecurity and disease management and at the same time making public aware of the precautionary measures at the verge of outbreak. Creation of protection and surveillance zones, various vaccination strategies viz. routine, preventive, emergency, mass and targeted vaccination programmes using live, inactivated and recombinant vaccines are the common strategies adopted in different parts of the globe. The new generation vaccines include recombinant vaccines and recombinant fusion vaccine. The pro-poor disease control programmes, giving compensation and subsidies to the farmers along with effective and efficient Veterinary Services forms integral part of control of HPAI. Following biosecurity principles and vaccination forms integral part of control programme against swine and equine influenza as well. Use of neuraminidase (NA) inhibitors (Zanamivir and Oseltamivir) for the treatment of human influenza has been widely accepted worldwide. The threat of increasing resistance of the flu viruses to these antivirals has evoked interest in the development of novel antiviral drugs for influenza virus such as inhibitors of cellular factors and host signalling cascades, cellular miRNAs, siRNA and innate immune peptides (defensins and cathelicidins). Commercial licensed inactivated vaccines for humans against influenza A and B viruses are available consisting of three influenza viruses: influenza type A subtype H3N2, influenza type A subtype H1N1 (seasonal) virus strain and influenza type B virus strain. As per WHO, use of tetravaccine consisting of antigens of influenza virus serotypes H3N2, H1N1, B and H5 is the most promising method to control influenza pandemic. All healthy children in many countries are required to be vaccinated between 6 and 59 months of age. The seasonal vaccines currently used in humans induce strain-specific humoral immunity as the antibodies. Universal influenza virus vaccines containing the relatively conserved ectodomain of M2 (M2e), M1, HA fusion peptide and stalk domains, NA, NP alone or in combination have been developed which have been shown to induce cross-protection. The T cell-based vaccines are another recent experimental approach that has been shown to elicit broad-spectrum heterosubtypic immunity in the host. As far as HPAI is concerned, various pandemic preparedness strategies have been documented.

Chimeric newcastle disease virus protects chickens against avian influenza in the presence of maternally derived NDV immunity

Newcastle disease virus (NDV), an avian paramyxovirus type 1, is a promising vector for expression of heterologous proteins from a variety of unrelated viruses including highly pathogenic avian influenza virus (HPAIV). However, pre-existing NDV antibodies may impair vector virus replication, resulting in an inefficient immune response against the foreign antigen. A chimeric NDV-based vector with functional surface glycoproteins unrelated to NDV could overcome this problem. Therefore, an NDV vector was constructed which carries the fusion (F) and hemagglutinin-neuraminidase (HN) proteins of avian paramyxovirus type 8 (APMV-8) instead of the corresponding NDV proteins in an NDV backbone derived from the lentogenic NDV Clone 30 and a gene expressing HPAIV H5 inserted between the F and HN genes. After successful virus rescue by reverse genetics, the resulting chNDVFHN PMV8H5 was characterized in vitro and in vivo. Expression and virion incorporation of the heterologous proteins was verified by Western blot and electron microscopy. Replication of the newly generated recombinant virus was comparable to parental NDV in embryonated chicken eggs. Immunization with chNDVFHN PMV8H5 stimulated full protection against lethal HPAIV infection in chickens without as well as with maternally derived NDV antibodies. Thus, tailored NDV vector vaccines can be provided for use in the presence or absence of routine NDV vaccination.