Protection of White Leghorn chickens by U.S. emergency H5 vaccination against clade 2.3.4.4 H5N2 high pathogenicity avian influenza virus

Efficacy of commercial recombinant HVT vaccines against a North American clade 2.3.4.4b H5N1 highly pathogenic avian influenza virus in chickens

The outbreak of clade 2.3.4.4b H5 highly pathogenic avian influenza (HPAI) in North America that started in 2021 has increased interest in applying vaccination as a strategy to help control and prevent the disease in poultry. Two commercially available vaccines based on the recombinant herpes virus of turkeys (rHVT) vector were tested against a recent North American clade 2.3.4.4b H5 HPAI virus isolate: A/turkey/Indiana/22-003707-003/2022 H5N1 in specific pathogen free white leghorn (WL) chickens and commercial broiler chickens. One rHVT-H5 vaccine encodes a hemagglutinin (HA) gene designed by the computationally optimized broadly reactive antigen method (COBRA-HVT vaccine). The other encodes an HA gene of a clade 2.2 virus (2.2-HVT vaccine). There was 100% survival of both chicken types COBRA-HVT vaccinated groups and in the 2.2-HVT vaccinated groups there was 94.8% and 90% survival of the WL and broilers respectively. Compared to the 2.2-HVT vaccinated groups, WL in the COBRA-HVT vaccinated group shed significantly lower mean viral titers by the cloacal route and broilers shed significantly lower titers by the oropharyngeal route than broilers. Virus titers detected in oral and cloacal swabs were otherwise similar among both vaccine groups and chicken types. To assess antibody-based tests to identify birds that have been infected after vaccination (DIVA-VI), sera collected after the challenge were tested with enzyme-linked lectin assay-neuraminidase inhibition (ELLA-NI) for N1 neuraminidase antibody detection and by commercial ELISA for detection of antibodies to the NP protein. As early as 7 days post challenge (DPC) 100% of the chickens were positive by ELLA-NI. ELISA was less sensitive with a maximum of 75% positive at 10DPC in broilers vaccinated with 2.2-HVT. Both vaccines provided protection from challenge to both types of chickens and ELLA-NI was sensitive at identifying antibodies to the challenge virus therefore should be evaluated further for DIVA-VI.

Efficacy of recombinant H5 vaccines delivered in ovo or day of age in commercial broilers against the 2015 U.S. H5N2 clade 2.3.4.4c highly pathogenic avian Influenza virus

BACKGROUND

Avian influenza is a highly contagious, agriculturally relevant disease that can severely affect the poultry industry and food supply. Eurasian-origin H5Nx highly pathogenic avian influenza viruses (HPAIV) (clade 2.3.4.4) have been circulating globally in wild birds with spill over into commercial poultry operations. The negative impact to commercial poultry renewed interest in the development of vaccines against these viruses to control outbreaks in the U.S.

METHODS

The efficacy of three recombinant H5 vaccines delivered in ovo or day of age were evaluated in commercial broilers challenged with the 2015 U.S. H5N2 clade 2.3.4.4c HPAIV. The recombinant vaccines included an alphavirus RNA particle vaccine (RP-H5), an inactivated reverse genetics-derived (RG-H5) and recombinant HVT vaccine (rHVT-AI) expressing H5 hemagglutinin (HA) genes. In the first experiment, in ovo vaccination with RP-H5 or rHVT-AI was tested against HPAI challenge at 3 or 6 weeks of age. In a second experiment, broilers were vaccinated at 1 day of age with a dose of either 107 or 108 RP-H5, or RG-H5 (512 HA units (HAU) per dose).

RESULTS

In experiment one, the RP-H5 provided no protection following in ovo application, and shedding titers were similar to sham vaccinated birds. However, when the RP-H5 was delivered in ovo with a boost at 3 weeks, 95% protection was demonstrated at 6 weeks of age. The rHVT-AI vaccine demonstrated 95 and 100% protection at 3 and 6 weeks of age, respectively, of challenged broilers with reduced virus shedding compared to sham vaccinated birds. Finally, when the RP-H5 and rHVT vaccines were co-administered at one day of age, 95% protection was demonstrated with challenge at either 3 or 6 weeks age. In the second experiment, the highest protection (92%) was observed in the 108 RP-H5 vaccinated group. Significant reductions (p < 0.05) in virus shedding were observed in groups of vaccinated birds that were protected from challenge. The RG-H5 provided 62% protection from challenge. In all groups of surviving birds, antibody titers increased following challenge.

CONCLUSIONS

Overall, these results demonstrated several strategies that could be considered to protected broiler chickens during a H5 HPAI challenge.

Efficacy of inactivated and RNA particle vaccines against a North American Clade 2.3.4.4b H5 highly pathogenic avian influenza virus in chickens

Highly pathogenic avian influenza virus (HPAIV) has caused widespread outbreaks in poultry in the Americas. Because of the duration and extent of these outbreaks, vaccine use may be an additional tool to limit virus spread. Three vaccines were evaluated for efficacy in chickens against a current North American clade 2.3.4.4b H5 HPAIV isolate, A/turkey/Indiana/3703-003/2022 H5N1. The vaccines included: 1) a commercial inactivated reverse genetics (rg) generated H5N1 product with a clade 2.3.4.4c H5 hemagglutinin (HA) (rgH5N1); 2) a commercial alphavirus RNA particle (RP) vaccine with the TK/IN/22 HA; and 3) an in-house inactivated rg produced vaccine with the TK/IN/22 HA and a North American lineage N9 neuraminidase (NA) (SEP-22-N9). Both inactivated vaccines were produced with HA genes that were modified to be low pathogenic and with the remaining genes from the PR8 influenza strain. All vaccines provided 100% protection against mortality and morbidity and all vaccines reduced virus shed by the oropharyngeal and cloacal routes significantly compared to sham vaccinates. However, differences were observed among the vaccines in quantities of virus shed at two- and four-days post challenge (DPC). To determine if infected birds could be identified after vaccination to aid surveillance programs, serum was collected from the RP and SEP-22-N9 vaccine groups at 7, 10, and 14 DPC to detect antibody to the NA and nucleoprotein (NP) of the challenge virus by enzyme linked lectin assay (ELLA) and ELISA. As early as 7DPC ELLA detected antibody in sera from 100% of the chickens in the RP vaccinated group and 70% of the chickens in the SEP-22-N9 vaccinated group. Antibody to the NP was detected by commercial ELISA in more than 50% of the birds in the RP vaccinated group at each time point.

HVT-vectored H7 vaccine protects chickens from lethal infection with the highly pathogenic H7N9 Avian influenza virus

Since mid-2016, the highly pathogenic H7N9 subtype avian influenza virus (AIV) has threatened both public health and the poultry industry. Although a vaccination strategy has been deemed imperative to manage the virus, the most commonly used inactivated vaccines today are susceptible to interference from maternal antibodies and associated with an over-reliance on humoral immunity. In response, we developed a recombination vaccine with the herpesvirus of turkeys (HVT) as the vector to squeeze HPAI H7N9 and assessed its protective efficiency in immunized chickens. By inserting an enhanced green fluorescent protein (EGFP) expression cassette (i.e., MCMV+EGFP+SV40 polyA) into the HVT065 and HVT066 positions, we obtained the recombinant HVT expressing EGFP (i.e., rHVT-EGFP). Electroporation and EGFP tags improved the efficiency of transfection compared with transfection using expression plasmids without any fluorescent labeling and traditional liposomes. Using limiting dilution analysis and ultrasonic cell disruption techniques, we screened and purified a cell-bound herpes virus based on rHVT-EGFP and consequently constructed a recombinant HVT expressing the hemagglutinin (HA) of H7N9 (i.e., rHVT-H7HA), which was able to proliferate similarly to the parental strain, stably pass for at least 15 generations in vitro, and replicate stably in multiple organs in vivo. After chickens were immunized with rHVT-H7HA, the average antibody titers reached up to 3 log2 at 35 d post-vaccination and remained stable. Those results suggest that rHVT-H7HA can protect chickens against H7N9 with a dose-independent immune protection rate of 90% and significantly reduce the lung virus titer 4 d post-challenge.

Poultry Vaccine Technology Platforms

The different technology platforms used to make poultry vaccines are reviewed. Vaccines based on classical technologies are either live attenuated or inactivated vaccines. Genetic engineering is applied to design by deletion, mutation, insertion, or chimerization, genetically modified target microorganisms that are used either as live or inactivated vaccines. Other vaccine platforms are based on one or a few genes of the target pathogen agent coding for proteins that can induce a protective immune response ("protective genes"). These genes can be expressed in vitro to produce subunit vaccines. Alternatively, vectors carrying these genes in their genome or nucleic acid-based vaccines will induce protection by in vivo expression of these genes in the vaccinated host. Properties of these different types of vaccines, including advantages and limitations, are reviewed, focusing mainly on vaccines targeting viral diseases and on technologies that succeeded in market authorization.

Prediction of highly pathogenic avian influenza vaccine efficacy in chickens by comparison of in vitro and in vivo data: A meta-analysis and systematic review

Vaccines for avian influenza (AI) can protect poultry against disease, mortality, and virus transmission. Numerous factors, including: vaccine platform, immunogenicity, and relatedness to the field strain, are known to be important to achieving optimal AI vaccine efficacy. To better understand how these factors contribute to vaccine protection, a systematic meta-analysis was conducted to evaluate efficacy data for vaccines in chickens challenged with highly pathogenic (HP) AI. Data from a total of 120 individual trials from 25 publications were selected and evaluated. Two vaccine criteria were evaluated for their effects on two metrics of protection. The vaccine criteria were: 1) the relatedness of the vaccine antigen and challenge strain in the hemagglutinin 1 domain (HA1) protein sequence; 2) vaccine-induced antibody titers to the challenge virus (VIAC). The metrics of protection were: A) survival of vaccinated chickens vs unvaccinated controls; and B) reduction in oral virus-shedding by vaccinated vs unvaccinated controls 2-4 days post challenge. Three vaccine platforms were evaluated: oil-adjuvanted inactivated whole AI virus, recombinant herpes virus of turkeys (rHVT) vectored, and a non-replicating alpha-virus vectored RNA particle (RP) vaccine. Higher VIAC correlated with greater reduction of virus-shed and vaccine efficacy by all vaccine platforms. Both higher HA1 relatedness and higher VIAC using challenge virus as antigen correlated with better survival by inactivated vaccines and rHVT-vectored vaccines. However, rHVT-vectored and RP based vaccines were more tolerant of variation in the HA1; the relatedness of the HA1 of the vaccine and challenge virus did not significantly correlate with survival with rHVT-vectored vaccines. Protection was achieved with the lowest aa similarity for which there was data, 90-93 % for rHVT vaccines and 88 % for the RP vaccine.

Protective efficacy of a bivalent H5 influenza vaccine candidate against both clades 2.3.2.1 and 2.3.4.4 high pathogenic avian influenza viruses in SPF chickens

Worldwide, high pathogenic avian influenza viruses belonging to clades 2.3.4.4 and 2.3.2.1 have been circulating in both poultry and wild birds. Since 2018, Korea has built a national antigen bank to ensure preparedness in an emergency. In this study, we developed a bivalent vaccine candidate containing antigens derived from two reassortant KA435/2.3.2.1d and H35/2.3.4.4b strains for Korean national antigen bank. We evaluated its immunogenicity and protective efficacy in specific pathogen free chickens. The two vaccine strains, rgKA435-H9N2 PB2/2.3.2.1d and rgH35/2.3.4.4b, both of which were generated successfully by reverse genetics, were highly immunogenic (titres of haemagglutination inhibition: 8.3 and 8.4 log₂, respectively) and showed good protective efficacy (100 and 147 50% protective dose, respectively) against lethal challenge with wild-type virus when delivered as a 1:1 mixture. Notably, the vaccine provided complete protection against viral shedding at a full dose (512 HAU) and a 1/10 dose (51.2 HAU), with no clinical signs, after challenge with H35/2.3.4.4b. The bivalent vaccine developed in this study may reduce the cost of vaccine production and could be used as a H5 subtype avian influenza vaccine candidate against two clades simultaneously.

Live recombinant Newcastle disease virus vectored vaccine expressing the haemagglutinin of H9N2 avian influenza virus suppresses viral replication in chickens

In 2020, the Y280-lineage H9N2 low-pathogenic avian influenza virus (LPAIV) was introduced into South Korea for the first time. Current vaccines are focused on the control of Y439-like viruses; however, there are continuous reports of decrease in egg production and secondary infections caused by Y280-lineage H9N2 LPAI infection in chickens. Therefore, there is an urgent need to develop effective novel vaccines against Y280-lineage H9N2 LPAI. Most commercialized avian influenza vaccines are oil-adjuvanted inactivated vaccines, which are labour-intensive to administer and require higher dosage. In this study, rK148/Y280-HA, a novel recombinant Newcastle disease virus (NDV) vectored vaccine against Y280-lineage H9N2 LPAI, was developed and evaluated using two mass-applicable administration methods, spray vaccination and drinking water vaccination. Regardless of low serum antibody haemagglutination inhibition titres against NDV and Y280-lineage H9N2 LPAI after applying the rK148/Y280-HA vaccine, vaccination with either administration method protected chickens against virulent NDV and Y280-lineage H9N2 LPAIV after the challenge. Taken together, these results indicate that the rK148/Y280 vaccine can be administered using facile mass-application methods to provide protection against the Y280-lineage LPAI.RESEARCH HIGHLIGHTS NDV vectored vaccine harbouring Y280-lineage H9N2 HA protein was successfully generated.NDV vectored vaccine provides protection against NDV.NDV vectored vaccine with H9N2 HA protects against homologous H9N2 LPAIV.

Reverse genetics based H5N2 vaccine provides clinical protection against H5N1, H5N8 and H9N2 avian influenza infection in chickens

The current study aimed to develop broadly protective vaccines for avian influenza. In an earlier study, HA stalk (universal flu vaccine) was found to be broadly protective against different subtypes of influenza virus in mice. Hence, we were interested to know its breadth of protective efficacy either alone or combined with inactivated rgH5N2 (clade 2.3.2.1a) vaccine against challenge viruses of homologous H5N1, heterologous H5N8 (clade 2.3.4.4) and heterosubtypic H9N2 virus in specific pathogen-free chickens. The rgH5N2 vaccine alone or in combination with HA stalk elicited sufficient pre-challenge immunity in the form of haemagglutination inhibiting (HI) antibodies and neutralizing antibodies (MNT) against H5N1, H5N8, and H9N2 in chickens. The rgH5N2 vaccine alone or in combination with HA stalk also attenuated the shedding of H5N1, H5N8 and H9N2 in chickens and protected against the lethal challenge of H5N1 or H5N8. In contrast, all HA stalk immunised chickens died upon H5N1 or H5N8 challenge and H9N2 challenged chickens survived. Our study suggests that the rgH5N2 vaccine can provide clinical protection against H5N1, H5N8 and can attenuate the viral shedding of H9N2 in chickens.

Review of Poultry Recombinant Vector Vaccines

The control of poultry diseases has relied heavily on the use of many live and inactivated vaccines. However, over the last 30 yr, recombinant DNA technology has been used to generate many novel poultry vaccines. Fowlpox virus and turkey herpesvirus are the two main vectors currently used to construct recombinant vaccines for poultry. With the use of these two vectors, more than 15 recombinant viral vector vaccines against Newcastle disease, infectious laryngotracheitis, infectious bursal disease, avian influenza, and Mycoplasma gallisepticum have been developed and are commercially available. This review focuses on current knowledge about the safety and efficacy of recombinant viral vectored vaccines and the mechanisms by which they facilitate the control of multiple diseases. Additionally, the development of new recombinant vaccines with novel vectors will be briefly discussed.

Examination of the protective efficacy of two avian influenza H5 vaccines against clade 2.3.4.4b H5N8 highly pathogenic avian influenza virus in commercial broilers

The highly pathogenic avian influenza (HPAI) H5N8 virus of clade 2.3.4.4 was detected in 2017 in Egypt, which is one of the few countries using vaccination as a control strategy in poultry farms. This study was conducted to evaluate the efficacy of the commercial recombinant turkey herpes virus-H5 (rHVT-H5) vaccine (clade 2.2), alone or in combination with commercial inactivated reverse genetically engineered H5N1 vaccine (rgH5N1) (clade 2.2), in preventing the genetically distinct HPAI H5N8 virus of clade 2.3.4.4b in commercial broiler chickens. Four experimental groups of chickens were used as follows: G1, non-vaccinated and non-challenged; G2, non-vaccinated and challenged; G3, vaccinated with rHVT-H5; and G4, prime-boost vaccinated with rHVT-H5/rgH5N1. For challenge with the Egyptian HPAI H5N8 (2.3.4.4b) virus, the groups were divided into two subgroups (A and B); chickens in subgroups A were challenged at the age of 28 days, whereas those in subgroups B were challenged at the age of 35 days. Results showed that a protective efficacy (survival rate) of 40%-50% was obtained in the vaccinated subgroups A. By delaying challenge for 1 week (subgroups B), a single rHVT-H5 vaccination provided 80% protection, whereas prime-boost vaccination induced full protection and reduced viral shedding very efficiently (1/10 birds and only detected on the 3rd day post challenge) against HPAI H5N8 virus (2.3.4.4b). Moreover, body weight loss improved from 31.39% and 43.65% in G3A and G4A, respectively, to 16.34% and 7.7% in G3B and G4B, respectively. The HI titers obtained in G3A and G4A on the challenge day (28^th d) using H5N8 antigen were 3 and 3.75 log₂ (p > 0.05), respectively, whereas those in G3B and G4B on the challenge day (35^th d) were 6.25 and 6 log₂ (p > 0.05), respectively, which increased post-challenge in all vaccinated subgroups. Therefore, the dual use of vectored rHVT-H5 and inactivated rgH5N1 vaccines in the vaccination schedule in poultry farms is the most efficient tool for preventing the disease (mortality and viral shedding) caused by the genetically distinct virus (clade 2.3.3.4b HPAI H5N8) in combination with strict biosecurity and sanitary measures.

Single dose of multi-clade virus-like particle vaccine protects chickens against clade 2.3.2.1 and clade 2.3.4.4 highly pathogenic avian influenza viruses

Virus-like particles (VLPs) are recognized as an alternative vaccine platform that provide effective protection against various highly pathogenic avian influenza viruses (HPAIVs). Here, we developed multi-clade VLPs expressing two HAs (a chimera of clade 2.3.2.1c and clade 2.3.4.4c HA) within a single vector. We then compared its protective efficacy with that of a monovalent VLP and evaluated its potency against each homologous strain. Chickens vaccinated with the multi-clade VLP shed less virus and were better protected against challenge than birds receiving monovalent vaccines. Single vaccination with a multi-clade VLP resulted in 100% survival, with no clinical symptoms and high levels of pre-challenge protective immunity (7.6–8.5 log₂). Moreover, the multi-clade VLP showed high productivity (128–256 HAU) both in the laboratory and on a large scale, making it cheaper than whole inactivated vaccines produced in eggs. However, the PD₅₀ (protective dose 50%) of the multi-clade VLP against clades 2.3.2.1c and 2.3.4.4c was < 50 PD₅₀ (28 and 42 PD₅₀, respectively), and effective antibody response was maintained for 2–3 months. This multi-clade VLP protects against both clades of HPAI viruses and can be produced in high amounts at low cost. Thus, the vaccine has potential as a pandemic preparedness vaccine.

Development of an antigen-ELISA and a colloidal gold-based immunochromatographic strip based on monoclonal antibodies for detection of avian influenza A(H5) viruses

Avian influenza A(H5) viruses (avian IAVs) pose a major threat to the economy and public health. We developed an antigen-ELISA (ag-ELISA) and a colloidal gold-based immunochromatographic strip for the rapid detection of avian A(H5) viruses. Both detection methods displayed no cross-reactivity with other viruses (e.g., other avian IAVs, infectious bursal disease virus, Newcastle disease virus, infectious bronchitis virus, avian paramyxovirus). The ag-ELISA was sensitive down to 0.5 hemagglutinin (HA) units/100 µL of avian A(H5) viruses and 7.5 ng/mL of purified H5 HA proteins. The immunochromatographic strip was sensitive down to 1 HA unit/100 µL of avian A(H5) viruses. Both detection methods exhibited good reproducibility with CVs < 10%. For 200 random poultry samples, the sensitivity and specificity of the ag-ELISA were 92.6% and 98.8%, respectively, and for test strips were 88.9% and 98.3%, respectively. Both detection methods displayed high specificity, sensitivity, and stability, making them suitable for rapid detection and field investigation of avian A(H5) viruses.

Efficacy of two vaccines against recent emergent antigenic variants of clade 2.3.2.1a highly pathogenic avian influenza viruses in Bangladesh

H5N1 highly pathogenic avian influenza viruses (HPAIVs) have caused outbreaks in poultry in Bangladesh since 2007. While clade 2.2.2 and 2.3.4.2 HPAIVs have not been detected since 2012, clade 2.3.2.1a viruses have caused continuous outbreaks since 2012 despite the use of vaccines. In this study, we evaluated the efficacy of two H5 vaccines licensed in Bangladesh, RE-6 inactivated vaccine, and a recombinant herpesvirus of turkeys vaccine with an H5 insert (rHVT-H5), for protection against recent field viruses in chickens. We selected three viruses for efficacy tests (A/chicken/Bangladesh/NRL-AI-3237/2017, A/crow/Bangladesh/NRL-AI-8471/2017 and A/chicken/Bangladesh/NRL-AI-8323/2017) from 36 H5 viruses isolated from Bangladesh between 2016 and 2018 by comparing the amino acid sequences at five antigenic sites (A-E) and analyzing hemagglutination inhibition (HI) titers with reference antisera. The RE-6 and rHVT-H5 vaccines both conferred 80-100% clinical protection (i.e. reduced morbidity and mortality) against the three challenge viruses with no significant differences in protection. In addition, both vaccines significantly decreased viral shedding from infected chickens as compared to challenge control chickens. Based on these metrics, the current licensed H5 vaccines protected chickens against the recent field viruses. However, the A/crow/Bangladesh/NRL-AI-8471/2017 virus exhibited antigenic divergence including: several unique amino acid changes in antigenic epitope sites A and B and was a serological outlier in cross HI tests as visualized on the antigenic map. The continuing emergence of such antigenic variants which could alter the dominant antigenicity of field viruses should be continuously monitored and vaccines should be updated if field efficacy declines.

Efficacy of recombinant Marek's disease virus vectored vaccines with computationally optimized broadly reactive antigen (COBRA) hemagglutinin insert against genetically diverse H5 high pathogenicity avian influenza viruses

The genetic and antigenic drift associated with the high pathogenicity avian influenza (HPAI) viruses of Goose/Guangdong (Gs/GD) lineage and the emergence of vaccine-resistant field viruses underscores the need for a broadly protective H5 influenza A vaccine. Here, we tested experimental vector herpesvirus of turkey (vHVT)-H5 vaccines containing either wild-type clade 2.3.4.4A-derived H5 inserts or computationally optimized broadly reactive antigen (COBRA) inserts with challenge by homologous and genetically divergent H5 HPAI Gs/GD lineage viruses in chickens. Direct assessment of protection was confirmed for all the tested constructs, which provided clinical protection against the homologous and heterologous H5 HPAI Gs/GD challenge viruses and significantly decreased oropharyngeal shedding titers compared to the sham vaccine. The cross reactivity was assessed by hemagglutinin inhibition (HI) and focus reduction assay against a panel of phylogenetically and antigenically diverse H5 strains. The COBRA-derived H5 inserts elicited antibody responses against antigenically diverse strains, while the wild-type-derived H5 vaccines elicited protection mostly against close antigenically related clades 2.3.4.4A and 2.3.4.4D viruses. In conclusion, the HVT vector, a widely used replicating vaccine platform in poultry, with H5 insert provides clinical protection and significant reduction of viral shedding against homologous and heterologous challenge. In addition, the COBRA-derived inserts have the potential to be used against antigenically distinct co-circulating viruses and future drift variants.

Cross-Protection by Inactivated H5 Prepandemic Vaccine Seed Strains against Diverse Goose/Guangdong Lineage H5N1 Highly Pathogenic Avian Influenza Viruses

The highly pathogenic avian influenza virus (HPAIV) H5N1 A/goose/Guangdong/1996 lineage (Gs/GD) is endemic in poultry across several countries in the world and has caused sporadic lethal infections in humans. Vaccines are important in HPAIV control both for poultry and in prepandemic preparedness for humans. This study assessed inactivated prepandemic vaccine strains in a One Health framework across human and agricultural and wildlife animal health, focusing on the genetic and antigenic diversity of field H5N1 Gs/GD viruses from the agricultural sector and assessing cross-protection in a chicken challenge model. Nearly half (47.92%) of the 48 combinations of vaccine and challenge viruses examined had bird protection of 80% or above. Most vaccinated groups had prolonged mean death times (MDT), and the virus-shedding titers were significantly lower than those of the sham-vaccinated group (P ≤ 0.05). The antibody titers in the prechallenge sera were not predictive of protection. Although vaccinated birds had higher titers of hemagglutination-inhibiting (HI) antibodies against the homologous vaccine antigen, most of them also had lower or no antibody titer against the challenge antigen. The comparison of all parameters and homologous or closely related vaccine and challenge viruses gave the best prediction of protection. Through additional analysis, we identified a pattern of epitope substitutions in the hemagglutinin (HA) of each challenge virus that impacted protection, regardless of the vaccine used. These changes were situated in the antigenic sites and/or reported epitopes associated with virus escape from antibody neutralization. As a result, this study highlights virus diversity, immune response complexity, and the importance of strain selection for vaccine development to control H5N1 HPAIV in the agricultural sector and for human prepandemic preparedness. We suggest that the engineering of specific antigenic sites can improve the immunogenicity of H5 vaccines.IMPORTANCE The sustained circulation of highly pathogenic avian influenza virus (HPAIV) H5N1 A/goose/Guangdong/1996 (Gs/GD) lineage in the agricultural sector and some wild birds has led to the evolution and selection of distinct viral lineages involved in escape from vaccine protection. Our results using inactivated vaccine candidates from the human pandemic preparedness program in a chicken challenge model identified critical antigenic conformational epitopes on H5 hemagglutinin (HA) from different clades that were associated with antibody recognition and escape. Even though other investigators have reported epitope mapping in the H5 HA, much of this information pertains to epitopes reactive to mouse antibodies. Our findings validate changes in antigenic epitopes of HA associated with virus escape from antibody neutralization in chickens, which has direct relevance to field protection and virus evolution. Therefore, knowledge of these immunodominant regions is essential to proactively develop diagnostic tests, improve surveillance platforms to monitor AIV outbreaks, and design more efficient and broad-spectrum agricultural and human prepandemic vaccines.

Identification of Efficacious Vaccines Against Contemporary North American H7 Avian Influenza Viruses

Five vaccines, including four inactivated, whole-virus water-in-oil adjuvanted vaccines and a commercial nonreplicating alphavirus-vectored RNA particle (RP) vaccine were evaluated in chickens for their ability to provide protection against challenge with a recent H7 highly pathogenic avian influenza virus (AIV) from the United States (A/turkey/IN/1403-1/2016 H7N8). One of the inactivated vaccines and the RP vaccine were prepared with A/turkey/IN/16-01571-6/2016 H7N8 low pathogenic AIV (LPAIV; TK/IN/16), which is identical to the challenge virus, except for the proteolytic cleavage site of the hemagglutinin protein. The remaining three inactivated vaccines were prepared with other North American H7 LPAIVs. The hemagglutination inhibition assay was used to evaluate the antigenic relationships among the vaccines and selected recent H7 AIV isolates. All five vaccines provided protection against mortality. The inactivated vaccines reduced virus shedding significantly at 2 and 4 days post challenge compared with sham-vaccinated chickens. In contrast, the RP vaccine did not significantly reduce virus shedding. The inactivated vaccine prepared with TK/IN/16 elicited the highest antibody responses, which suggests it is a strong candidate for use as an antigen for North American H7 AIVs. Antigenic distance calculations showed that the four inactivated vaccine strains and other recent North American H7 isolates are antigenically similar, which suggests that the vaccines evaluated here would be similar enough to provide protection to other North American H7 AIVs. If future H7 outbreaks in poultry warrant vaccination, the field strain can be rapidly evaluated with these antigens and, if adequately related, one of these characterized strains may be used.

Laboratory Methods for Assessing and Licensing Influenza Vaccines for Poultry

Avian influenza (AI) vaccines for poultry are based on hemagglutinin (HA) proteins, and protection is specific to the subtype. An estimated 313 billion doses have been used between 2002 and 2018 for high pathogenicity AI control. No universal vaccines are currently available. The majority of AI vaccines are inactivated whole influenza viruses that are grown in embryonating chicken eggs, emulsified in oil adjuvant systems, and injected subcutaneously or intramuscularly. Live virus-vectored vaccines such as recombinant viruses of fowl pox, Newcastle disease, and herpesvirus of turkeys containing inserts of AI virus HA genes have been used on a more limited basis. Also, vaccines have been licensed or registered based on baculovirus and defective replicating alphavirus (RNA particles) expressing HA protein or DNA vaccine with HA gene insert. In studies to evaluate vaccine efficacy and potency, the protocol design and its implementation should address the biosafety level needed for the work, provide information required for approval by Institutional Biosafety and Animal Care Committees, contain information on seed strain selection, provide needed information on animal subjects and their relevant parameters, and address the selection and use of challenge viruses. Various metrics have been used to directly measure vaccine-induced protection, including prevention of death, clinical signs, and lesions; prevention of decreases in egg production and alterations in egg quality; quantification of the reduction in virus replication and shedding from the respiratory tract and gastrointestinal tracts; and prevention of contact transmission in in vivo poultry experiments. In addition, indirect measures of vaccine potency and protection have been developed and validated against the direct measures and include serological assays in vaccinated poultry and the assessment of the content of HA antigen in the vaccine. These indirect assessments of protection are useful in determining if vaccine batches have a consistent ability to protect. For adequate potency, vaccines should contain 50 mean protective doses of antigen per dose, which corresponds to 0.3-7.8 μg of HA protein in inactivated vaccines, depending on immunogenicity and antigenic relatedness of individual seed strains.

Protective efficacy of vaccines of the Korea national antigen bank against the homologous H5Nx clade 2.3.2.1 and clade 2.3.4.4 highly pathogenic avian influenza viruses

The occurrence of severe outbreaks of highly pathogenic avian influenza in Korea led to establishment of a national antigen bank for emergency preparedness. Here, we developed five vaccines for this bank (clade 2.3.2.1C, clade 2.3.4.4A, B, C, and D) by reverse genetics, inactivated them with formalin, and evaluated the protective efficacy and potency of serial dilutions against lethal homologous challenge in specific-pathogen-free chickens. After vaccination with one dose, each vaccine resulted in 100% survival, with no clinical symptoms, or lack of detectable virus shedding, and high levels of pre-challenge protective immunity (8.4-10.2 log₂). After vaccination with one-tenth of the full dose, protection was similar to that with the full dose. After vaccination with one-hundredth of the initial dose, survival was 20-80%, and all vaccines showed virus shedding. Four vaccines (excluding clade 2.3.2.1C) had satisfactory potency. In antibody-persistence tests, all vaccines maintained long-lasting protective immunity. Our results suggest that inactivated reverse-genetics vaccines genetically matched to outbreak viruses provide adequate protection after a single vaccination.

Newcastle disease virus vectors expressing consensus sequence of the H7 HA protein protect broiler chickens and turkeys against highly pathogenic H7N8 virus

Continuous outbreaks of highly pathogenic avian influenza (HPAI) viruses in commercial poultry have caused devastating losses to domestic poultry with a raising public health concern. The outbreaks of HPAI viruses have increased worldwide, including the North America. Therefore, vaccination has been considered as an alternative strategy for an efficient control of HPAI viruses. In this study, we aimed to generate Newcastle disease virus (NDV) vectored H7 serotype-specific vaccines by expressing the consensus sequence of the HA protein. Conventional NDV strain LaSota vector and a chimeric NDV vector containing the avian paramyxovirus type-2 F and HN protein were able to express the consensus sequence of HA protein. The protective efficacy of vaccines was evaluated in broiler chickens and in turkeys. One-day-old poults were prime immunized with the chimeric vector expressing the HA protein followed by boost immunization with LaSota vector expressing the HA protein or co-expressing the HA and NA proteins. Our vaccine candidates provided complete protection of broiler chickens from mortality and shedding of H7N8 HPAI challenge virus. Turkeys were better protected by boosting with the LaSota vector co-expressing the HA and NA proteins than the LaSota vector expressing only the HA protein. Our study demonstrated a potential use of heterologous prime and boost vaccination strategy to protect poultry against H7 HPAI viruses.

Protection afforded by avian influenza vaccination programmes consisting of a novel RNA particle and an inactivated avian influenza vaccine against a highly pathogenic avian influenza virus challenge in layer chickens up to 18 weeks post-vaccination

The efficacies of an oil adjuvanted-inactivated reverse genetics-derived H5 avian influenza virus (AIV) vaccine and an alphavirus replicon RNA particle (RP) AIV vaccine were evaluated in commercial Leghorn chickens. Challenge utilized A/turkey/MN/12582/2015, an isolate representing the U.S. H5N2 Clade 2.3.4.4 responsible for the 2015 highly pathogenic avian influenza (HPAI) epornitic in commercial poultry the United States. As part of a long-term, 36-week study, chickens were challenged at seven weeks of age after receiving a single vaccination, at 18 weeks of age following a vaccine prime-single boost, and at 36 weeks of age after a prime- double-boost. All vaccine programmes reduced virus oropharyngeal and cloacal shedding and mortality compared to the non-vaccinated control birds; however, chickens receiving at least one administration of the RP vaccine generally had diminished viral shedding especially from the cloacal swabbings. A detectable serum antibody response and protection were observed through 18 weeks post-vaccination. Our data suggest that, in conjunction with a comprehensive eradication, enhanced biosecurity and controlled marketing plan, vaccination programmes of commercial layer chickens with novel RP vaccines may represent an important tool for preventing HPAI-related mortalities and decreasing viral load during a catastrophic influenza outbreak. RESEARCH HIGHLIGHTS Immunization of poultry following a vaccination schedule consisting of inactivated and RNA particle vaccines offered significant protection against lethal disease following HPAIV challenge. Virus shedding was significantly (P < 0.05) reduced in chickens vaccinated with either inactivated and/or recombinant vaccines. Serum antibody titres were not a reliable indicator of protection. An inactivated vaccine containing 384 HAU of the homologous antigen was unable to induce complete protection.

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.

Pathogenicity and Transmissibility of North American H7 Low Pathogenic Avian Influenza Viruses in Chickens and Turkeys

Low pathogenic avian influenza (LPAI) viruses can silently circulate in poultry and wild aquatic birds and potentially mutate into highly pathogenic avian influenza (HPAI) viruses. In the U.S., recent emergence and spread of H7N8 and H7N9 HPAI viruses not only caused devastating losses to domestic poultry but also underscored the capability of LPAI viruses to mutate into HPAI viruses. Therefore, in this study, we evaluated pathogenicity and transmissibility of H7N8 and H7N9 LPAI viruses (the progenitors of HPAI viruses) in chickens and turkeys. We also included H7N2 isolated from an outbreak of LPAI in commercial chickens. H7 viruses replicated more efficiently in the respiratory tract than in the gastrointestinal tract, suggesting that their replication is restricted to the upper respiratory tract. Specifically, H7N2 replicated most efficiently in two-week-old chickens and turkeys. In contrast, H7N8 replicated least efficiently in those birds. Further, replication of H7N2 and H7N9 was restricted in the upper respiratory tract of four-week-old specific-pathogen-free (SPF) and broiler chickens. Despite their restricted replication, the two viruses efficiently transmitted from infected to naïve birds by direct contact, leading to seroconversion of contacted chickens. Our findings suggest the importance of continuous monitoring and surveillance of LPAI viruses in the fields.

Alphavirus-vectored hemagglutinin subunit vaccine provides partial protection against heterologous challenge in pigs

Influenza A virus in swine (IAV-S) is an important pathogen in pigs in the United States, in addition to posing a potential risk to humans through zoonotic events. Intervention strategies continue to be explored to better control virus circulation. Improved surveillance efforts has led to significantly increased sequence data available on circulating strains, vastly improving our understanding of the genetic and antigenic diversity of IAV-S. IAV-S in North America is characterized by repeated spillover events of human viruses into pigs followed by genetic and antigenic diversification. An important gap that needs to be addressed is our understanding of the role that various vaccine platforms have on efficacy against antigenically heterologous challenge. Currently licensed vaccines often update their components to adapt to a dynamic antigenic landscape and newly developed technologies continue to be approved. Hence, it remains critical to test the performance of vaccines against challenge with antigenically distinct viruses. We tested the level of protection conferred by an alphavirus-vectored hemagglutinin (HA) subunit vaccine, delivered as a monovalent or bivalent formulation, against challenge with IAV-S. Monovalent alphavirus-vectored HA vaccines provided efficient protection against challenge with viruses with matched and mismatched HA, although in one mismatched HA challenge group there was a trend for reduced protection. A bivalent vaccine, in which two HA's were simultaneously delivered, was effective in producing antibody response against both antigens and provided protection against challenge. The alphavirus platform is a promising new technology available to swine producers to help reduce the burden of disease caused by IAV-S.

Inactivated H5 Antigens of H5N8 Protect Chickens from Lethal Infections by the Highly Pathogenic H5N8 and H5N6 Avian Influenza Viruses

Introduction

Highly pathogenic Asian H5-subtype avian influenza viruses have been found in poultry and wild birds worldwide since they were first detected in southern China in 1996. Extensive control efforts have not eradicated them. Vaccination prevents such viruses infecting poultry and reduces the number lost to compulsory slaughter. The study showed the efficacy of inactivated H5 vaccine from the H5N8 virus against highly pathogenic H5N8 and H5N6 avian influenza viruses in chickens.

Material and Methods

Reverse genetics constructed an H5 vaccine virus using the HA gene of the 2014 H5N8 avian influenza virus and the rest of the genes from A/PR/8/34 (H1N1). The vaccine viruses were grown in fertilised eggs, partially purified through a sucrose gradient, and inactivated with formalin. Chickens were immunised i.m. with 1 μg of oil-adjuvanted inactivated H5 antigens.

Results

Single dose H5 vaccine recipients were completely protected from lethal infections by homologous H5N8 avian influenza virus and shed no virus from the respiratory or intestinal tracts but were not protected from lethal infections by heterologous H5N6. When chickens were immunised with two doses and challenged with homologous H5N8 or heterologous H5N6, all survived and shed no virus.

Conclusion

Our results indicate that two-dose immunisations of chickens with H5 antigens with oil adjuvant are needed to provide broad protection against different highly pathogenic H5 avian influenza viruses.

Efficacy of a Recombinant Turkey Herpesvirus AI (H5) Vaccine in Preventing Transmission of Heterologous Highly Pathogenic H5N8 Clade 2.3.4.4b Challenge Virus in Commercial Broilers and Layer Pullets

Outbreaks caused by the highly pathogenic avian influenza virus (HPAIV) H5N8 subtype clade 2.3.4.4 were first reported in 2014 in South Korea then spread very rapidly in Asia, to Europe, and for the first time, to North America. Efficacy of a recombinant HVT-AI (H5) vaccine (rHVT-H5) to provide clinical protection as well as to significantly reduce the shedding of an H5N8 challenge virus has already been demonstrated in SPF chickens. The aim of our studies was to test the efficacy of the same rHVT-H5 vaccine in controlling the transmission of a recent Hungarian HPAIV H5N8 challenge virus in commercial chickens. Broilers and layers were vaccinated at day old according to the manufacturer's recommendation and then challenged with a 2017 Hungarian HPAIV H5N8 (2.3.4.4b) isolate at 5 or 7 weeks of age, respectively. Evaluation of clinical protection, reduction of challenge virus shedding, and transmission to vaccinated contact birds was done on the basis of clinical signs/mortality, detection, and quantitation of challenge virus in oronasal and cloacal swabs (regularly between 1 and 14 days postchallenge). Measurement of seroconversion to AIV nucleoprotein was used as an indicator of infection and replication of challenge virus. Our results demonstrated that rHVT-H5 vaccination could prevent the development of clinical disease and suppress shedding very efficiently, resulting in the lack of challenge virus transmission to vaccinated contact chickens, regardless the type of birds. Single immunization with the tested rHVT-H5 vaccine proved to be effective to stop HPAIV H5N8 (2.3.4.4b) transmission within vaccinated poultry population under experimental conditions.

Co-expression of the Hemagglutinin and Neuraminidase by Heterologous Newcastle Disease Virus Vectors Protected Chickens against H5 Clade 2.3.4.4 HPAI Viruses

Avian influenza remains an important zoonotic disease with a significant global impact. The spread of H5 highly pathogenic avian influenza (HPAI) viruses (clade 2.3.4.4) by migratory birds has caused outbreaks in wide geographic regions (Asia, Europe, and North America) with great economic losses during 2014-2015. Efficient vaccines and vaccination approaches are needed to enhance protective immunity against HPAI viruses. Although several vaccination strategies have been developed, none has been satisfactory. Our strategy has been to use avirulent vaccine strain of Newcastle disease virus (NDV) as a vaccine vector for HPAI viruses. For poultry vaccination, we previously generated a new platform of chimeric NDV vector to overcome preexisting maternal antibodies to NDV in poultry. In this study, we have generated vaccine candidates targeting H5 clade 2.3.4.4 HPAI viruses by using our chimeric NDV and conventional NDV strain LaSota vectors for a heterologous prime-boost immunization approach. Co-expression of the HA and NA proteins by our vaccine vectors induced enhanced HPAI virus specific immune responses in specific-pathogen free and broiler chickens prior to challenge. Further, these vaccine candidates efficiently protected broiler chickens from mortality, clinical signs, and shedding of homologous and heterologous H5 HPAI viruses and highly virulent NDV, thus providing a dual vaccination approach in the field.

The Pandemic Threat of Emerging H5 and H7 Avian Influenza Viruses

The 1918 H1N1 Spanish Influenza pandemic was the most severe pandemic in modern history. Unlike more recent pandemics, most of the 1918 H1N1 virus' genome was derived directly from an avian influenza virus. Recent avian-origin H5 A/goose/Guangdong/1/1996 (GsGd) and Asian H7N9 viruses have caused several hundred human infections with high mortality rates. While these viruses have not spread beyond infected individuals, if they evolve the ability to transmit efficiently from person-to-person, specifically via the airborne route, they will initiate a pandemic. Therefore, this review examines H5 GsGd and Asian H7N9 viruses that have caused recent zoonotic infections with a focus on viral properties that support airborne transmission. Several GsGd H5 and Asian H7N9 viruses display molecular changes that potentiate transmission and/or exhibit ability for limited transmission between ferrets. However, the hemagglutinin of these viruses is unstable; this likely represents the most significant obstacle to the emergence of a virus capable of efficient airborne transmission. Given the global disease burden of an influenza pandemic, continued surveillance and pandemic preparedness efforts against H5 GsGd and Asian lineage H7N9 viruses are warranted.

A two dose immunization with an inactivated reassortant H5N2 virus protects chickens against lethal challenge with homologous 2.3.2.1 clade and heterologous 2.2 clade highly pathogenic avian influenza H5N1 viruses

The present study was aimed at generating a reassortant vaccine candidate virus with clade 2.3.2.1 Hemagglutinin (HA) and its evaluation in a challenge study for protection against homologous (2.3.2.1 clade) and heterologous (2.2 clade) highly pathogenic avian influenza (HPAI) H5N1 viruses. Plasmid-based reverse genetics technique was used to rescue a 5 + 3 reassortant H5N2 strain containing the modified HA of H5N1 (clade 2.3.2.1), the Neuraminidase (NA) of H9N2, the Matrix (M) of H5N1 and the internal genes of A/WSN/33 H1N1. In addition, another 6 + 2 reassortant virus containing modified HA from H5N1 (clade 2.3.2.1), the NA from H9N2 and the internal genes of A/WSN/33 H1N1 was also rescued. The 5 + 3 reassortant H5N2 virus could grow to a higher titer in both MDCK cells and chicken eggs compared to the 6 + 2 reassortant H5N2 virus. The vaccine containing the inactivated 5 + 3 reassortant H5N2 virus was used in a two-dose immunization regime which protected specific pathogen free (SPF) chickens against two repeated challenges with homologous 2.3.2.1 clade and heterologous 2.2 clade HPAI H5N1 viruses. The 5 + 3 reassortant H5N2 virus based on clade 2.3.2.1 generated in this study can be effective in protecting chickens in the case of an outbreak caused by antigenically different clade 2.2 HPAI H5N1 viruses and opens the way to explore its applicability as potential vaccine candidate especially in the Asian countries reporting these clades frequently. The study also indicates that sequential immunization can broaden protection level against antigenically diverse strains of H5N1 viruses.