Multiple gene segments control the temperature sensitivity and attenuation phenotypes of ca B/Ann Arbor/1/66

Multiple Vaccines and Strategies for Pandemic Preparedness of Avian Influenza Virus

Avian influenza viruses (AIV) are a continuous cause of concern due to their pandemic potential and devasting effects on poultry, birds, and human health. The low pathogenic avian influenza virus has the potential to evolve into a highly pathogenic avian influenza virus, resulting in its rapid spread and significant outbreaks in poultry. Over the years, a wide array of traditional and novel strategies has been implemented to prevent the transmission of AIV in poultry. Mass vaccination is still an economical and effective approach to establish immune protection against clinical virus infection. At present, some AIV vaccines have been licensed for large-scale production and use in the poultry industry; however, other new types of AIV vaccines are currently under research and development. In this review, we assess the recent progress surrounding the various types of AIV vaccines, which are based on the classical and next-generation platforms. Additionally, the delivery systems for nucleic acid vaccines are discussed, since these vaccines have attracted significant attention following their significant role in the fight against COVID-19. We also provide a general introduction to the dendritic targeting strategy, which can be used to enhance the immune efficiency of AIV vaccines. This review may be beneficial for the avian influenza research community, providing ideas for the design and development of new AIV vaccines.

Strain development of A/H1N1pdm09 candidate vaccine viruses for the 2021–22 northern hemisphere influenza season

The vaccine effectiveness (VE) of the A/H1N1pdm09 component of the 2017–18 quadrivalent live attenuated influenza vaccine (QLAIV) was improved by performing rational haemagglutinin (HA) mutagenesis. Introducing N125D, D127E, D222G and R223Q substitutions into the HA protein of A/Slovenia/2903/2015 (A/SLOV15) enhanced replicative fitness in primary human nasal epithelial cells (hNECs). This allowed A/SLOV15 to overcome inter-strain competition in QLAIV, resulting in improved VE.

During strain development for the 2021–22 QLAIV formulation, A/H1N1pdm09 LAIV viruses containing wild-type (WT) HA and neuraminidase (NA) sequences were found to replicate poorly in embryonated eggs and hNECs. We aimed to enhance replicative fitness via the HA mutagenesis approach that was performed previously for A/SLOV15. Therefore, combinations of these four mutations were introduced into the HA protein of representative 6B.1A-5a.2 viruses, A/Victoria/2570/2019 and A/Victoria/1/2020 (A/VIC1). Replicative fitness of A/VIC1 V7 was improved ~30-fold in eggs and ~300-fold in hNECs relative to its parent, without compromising other critical LAIV characteristics.

Haemagglutinin substitutions N125D, D127E, D222G and R223Q improve replicative fitness and vaccine effectiveness of an A/H1N1pdm09 live attenuated influenza vaccine virus by enhancing α-2,6 receptor binding

During 2013–14 and 2015–16, A/H1N1pdm09 live attenuated influenza vaccine (LAIV) viruses replicated inefficiently in primary human nasal epithelial cells (hNEC). This led to reduced vaccine effectiveness (VE) in quadrivalent formulations, mediated by inter-strain competition. By mutating the haemagglutinin (HA) protein, we aimed to enhance hNEC replication of a novel A/H1N1pdm09 vaccine strain to overcome competition and improve VE. Combinations of N125D, D127E, D222G and R223Q substitutions were introduced to the HA protein of A/Slovenia/2903/2015 (A/SLOV15). A/SLOV15 S13, containing all four HA substitutions, produced approximately 1000-fold more virus than parental V1 during hNEC infection. Immunogenicity in ferrets was increased by approximately 10-fold, without compromising yield in eggs or antigenic match to wild-type (wt) reference strains. Despite S13 and V1 being antigenically similar, only S13 protected ferrets from wt virus shedding and fever post-challenge. Crucially, these data suggested that enhanced fitness allowed S13 to overcome inter-strain competition in quadrivalent LAIV (QLAIV). This improved efficacy was later validated by real-world VE data. S13 displayed increased binding avidity to a mammalian-like α-2,6 receptor analogue (6-SLN), relative to V1, while maintaining avian-like 3-SLN avidity. In silico modelling of the HA receptor binding site revealed additional interactions in the S13:6-SLN binding network and a mild increase in 6-SLN binding energy, indicating a possible mechanism for increased α-2,6 receptor-binding avidity. These data confirm that rational HA mutagenesis can be used to optimise hNEC replication and VE for A/H1N1pdm09 LAIV viruses.

Sequential delivery of LAIV and SARS-CoV-2 in the ferret model can reduce SARS-CoV-2 shedding and does not result in enhanced lung pathology

Co-circulation of SARS-CoV-2 and influenza viruses could pose unpredictable risks to health systems globally, with recent studies suggesting more severe disease outcomes in co-infected patients. The initial lack of a readily available COVID-19 vaccine has reinforced the importance of influenza vaccine programmes during the COVID-19 pandemic. Live Attenuated Influenza Vaccine (LAIV) is an important tool in protecting against influenza, particularly in children. However, it is unknown whether LAIV administration influences the outcomes of acute SARS-CoV-2 infection or disease. To investigate this, quadrivalent LAIV was administered to ferrets 3 days pre- or post-SARS-CoV-2 infection. LAIV administration did not exacerbate SARS-CoV-2 disease course or lung pathology with either regimen. Additionally, LAIV administered prior to SARS-CoV-2 infection significantly reduced SARS-CoV-2 replication and shedding in the upper respiratory tract. This study demonstrated that LAIV administration in close proximity to SARS-CoV-2 infection does not exacerbate mild disease and can reduce SARS-CoV-2 shedding.

Approaches in broadening the neutralizing antibody response of the influenza vaccine

INTRODUCTION

Influenza vaccine is the mainstay for influenza prevention and elicits immune response and antigen-specific neutralizing antibodies against influenza virus. However, antigenic drift and shift can confer influenza virus to escape from the immune response induced by vaccine, and then reduce the vaccine effectiveness.

AREAS COVERED

To improve effect and neutralizing antibody response of vaccine for heterologous influenza virus, a literature review of preclinical and clinical studies published before August 2021 and searched in PubMed, which evaluated vaccine effectiveness improved by adjuvants and administration route.

EXPERT OPINION

The review showed that adjuvant, including imiquimod, GLA, MF59, and AS03, can improve the effectiveness of influenza vaccines by regulating immune system. Subjects receiving influenza vaccine combined with these adjuvants showed enhanced antibody response against homologous and heterologous virus strains compared to those vaccinated without adjuvant. This review also discussed the role of intradermal vaccination. In contrast to intramuscular vaccination, intradermal vaccination elicited a robust and prolonged antibody response against vaccine strains and drifted virus than intramuscular vaccination.

Live Attenuated Cold-Adapted Influenza Vaccines

Live attenuated, cold-adapted influenza vaccines exhibit several desirable characteristics, including the induction of systemic, mucosal, and cell-mediated immunity resulting in breadth of protection, ease of administration, and yield. Seasonal live attenuated influenza vaccines (LAIVs) were developed in the United States and Russia and have been used in several countries. In the last decade, following the incorporation of the 2009 pandemic H1N1 strain, the performance of both LAIVs has been variable and the U.S.-backbone LAIV was less effective than the corresponding inactivated influenza vaccines. The cause appears to be reduced replicative fitness of some H1N1pdm09 viruses, indicating a need for careful selection of strains included in multivalent LAIV formulations. Assays are now being implemented to select optimal strains. An improved understanding of the determinants of replicative fitness of vaccine strains and of vaccine effectiveness of LAIVs is needed for public health systems to take full advantage of these valuable vaccines.

A New Master Donor Virus for the Development of Live-Attenuated Influenza B Virus Vaccines

Influenza B viruses (IBV) circulate annually, with young children, the elderly and immunocompromised individuals being at high risk. Yearly vaccinations are recommended to protect against seasonally influenza viruses, including IBV. Live attenuated influenza vaccines (LAIV) provide the unique opportunity for direct exposure to the antigenically variable surface glycoproteins as well as the more conserved internal components. Ideally, LAIV Master Donor Viruses (MDV) should accurately reflect seasonal influenza strains. Unfortunately, the continuous evolution of IBV have led to significant changes in conserved epitopes compared to the IBV MDV based on B/Ann Arbor/1/1966 strain. Here, we propose a recent influenza B/Brisbane/60/2008 as an efficacious MDV alternative, as its internal viral proteins more accurately reflect those of circulating IBV strains. We introduced the mutations responsible for the temperature sensitive (ts), cold adapted (ca) and attenuated (att) phenotype of B/Ann Arbor/1/1966 MDV LAIV into B/Brisbane/60/2008 to generate a new MDV LAIV. In vitro and in vivo analysis demonstrated that the mutations responsible of the ts, ca, and att phenotype of B/Ann Arbor/1/1966 MDV LAIV were able to infer the same phenotype to B/Brisbane/60/2008, demonstrating its potential as a new MDV for the development of LAIV to protect against contemporary IBV strains.

Defining the root cause of reduced H1N1 live attenuated influenza vaccine effectiveness: low viral fitness leads to inter-strain competition

In the 2013-14 and 2015-16 influenza seasons, reduced vaccine effectiveness (VE) was observed for the H1N1 component of the FluMist quadrivalent live attenuated influenza vaccine (QLAIV) in the USA, leading to loss of Advisory Committee on Immunization Practices recommendation. Here we demonstrate in ferrets that 2015-16A/H1N1pdm09 vaccine strain A/Bolivia/559/2013 (A/BOL13) is outcompeted in trivalent (TLAIV) and QLAIV formulations, leading to reduced protection from wild-type challenge. While monovalent (MLAIV) A/BOL13 provided significant protection from wild-type virus shedding and fever at doses as low as 3.0 log₁₀ fluorescent focus units (FFU), it failed to provide a similar level of protection in TLAIV or QLAIV formulation, even at a 6.0 log₁₀ FFU dose. Conversely, clinically effective H1N1 strain A/New Caledonia/20/1999 provided significant protection in MLAIV, TLAIV, and QLAIV formulations. In conclusion, reduced A/BOL13 replicative fitness rendered it susceptible to inter-strain competition in QLAIV, contributing to its reduced VE in the 2015-16 season.

Reverse genetics for influenza B viruses and recent advances in vaccine development

Influenza B virus is a respiratory pathogen that affects more severely the pediatric and elderly populations. There are two lineages of influenza B virus that seem to have differential predilection for age groups. Both lineages can co-circulate during the influenza season however one is usually more prominent than the other depending on the season. There are no defined indicators to predict which lineage will dominate in any given season. In recent years, the addition of viruses from both lineages to the seasonal influenza vaccine formulation has improved vaccine protection, although quadrivalent vaccines are not available worldwide. Reverse genetics has facilitated advancements in the field of vaccine development against influenza B virus. Different strategies have been explored showing promising results that could potentially lead to the development broadly protective influenza B virus vaccines.

Genetic Basis of Attenuation of Cold-Adapted Influenza Strain B/Leningrad/14/17/55 - Backup Master Donor Virus for Influenza Type B Live Attenuated Vaccines

The reassortant vaccine strain of live attenuated influenza vaccine inherits temperature sensitivity and areactogenicity from cold-adapted attenuated master donor virus. In Russia, B/ USSR/60/69 master donor virus (B60) is currently in use for the preparation of live attenuated type B influenza vaccine candidates. Trivalent live attenuated influenza vaccine based on A/ Leningrad/134/17/57 and B60 are licensed for the use in Russia for single dose vaccination of adults and children over 3 years. B/Leningrad/14/17/55 (B14) cold-adapted virus is a backup master donor virus for live attenuated type B influenza vaccine. According to our preliminary estimates, it is more attenuated than B60, which can allow expanding applicability of this vaccine for children under 3 years of age. In this paper, the role of B14 genes in its attenuation was assessed. Representative collection of reassortants of B14 with epidemic influenza B viruses was obtained, a phenotypic analysis of reassortants was performed, and their pathogenicity for animals was assessed. The leading role of PB2 and PA genes in attenuation of B14 master donor virus was proven.

Development of Universal Influenza Vaccines Targeting Conserved Viral Proteins

Vaccination is still the most efficient way to prevent an infection with influenza viruses. Nevertheless, existing commercial vaccines face serious limitations such as availability during epidemic outbreaks and their efficacy. Existing seasonal influenza vaccines mostly induce antibody responses to the surface proteins of influenza viruses, which frequently change due to antigenic shift and or drift, thus allowing influenza viruses to avoid neutralizing antibodies. Hence, influenza vaccines need a yearly formulation to protect against new seasonal viruses. A broadly protective or universal influenza vaccine must induce effective humoral as well as cellular immunity against conserved influenza antigens, offer good protection against influenza pandemics, be safe, and have a fast production platform. Nanotechnology has great potential to improve vaccine delivery, immunogenicity, and host immune responses. As new strains of human epidemic influenza virus strains could originate from poultry and swine viruses, development of a new universal influenza vaccine will require the immune responses to be directed against viruses from different hosts. This review discusses how the new vaccine platforms and nanoparticles can be beneficial in the development of a broadly protective, universal influenza vaccine.

Haemagglutinin stability was not the primary cause of the reduced effectiveness of live attenuated influenza vaccine against A/H1N1pdm09 viruses in the 2013-2014 and 2015-2016 seasons

During the 2013-2014 influenza season, the quadrivalent live attenuated influenza vaccine (QLAIV), had lower than expected vaccine effectiveness (VE) against circulating A/H1N1pdm09 viruses in the USA. The underlying reason proposed for this was that the A/H1N1pdm09 vaccine strain, A/California/07/2009 (A/CA09), had a thermally unstable haemagglutinin (HA) protein. Consequently, a new A/H1N1pdm09 candidate strain, A/Bolivia/559/2013 (A/BOL13), was developed for inclusion in the 2015-2016 QLAIV. A key parameter for selection of A/BOL13 was its more thermostable HA phenotype compared with A/CA09. During the 2015-2016 season, QLAIV containing A/BOL13 was found in some studies to have improved, but still with suboptimal, VE against circulating A/H1N1pdm09 viruses and was not recommended for use by the CDC in the US market in the 2016-2017 influenza season. This suggested that improved HA thermostability had not entirely resolved the reduced VE observed. One hypothesis for this was that, by improving thermostability, the A/BOL13 HA protein had been over-stabilised, compromising its activation at the low endosomal pH required for successful viral entry. Here we demonstrate that, while the A/BOL13 HA protein is more stable than that of A/CA09, its thermal and pH stability were comparable with historically efficacious LAIV strains, suggesting that the HA had not been over-stabilised. Furthermore, studies simulating potential heat exposure during distribution by exposing QLAIV nasal sprayers to 33 °C for 4 h showed that, while remaining within product specification, A/CA09 viral potency was statistically decreased after 12 weeks at 2-8 °C. These data suggest that although unfavourable HA protein stability may have contributed to the reduced VE of A/CA09 in 2013-2014, it was unlikely to have affected A/BOL13 in 2015-2016. We conclude that HA stability was not the primary cause of the reduced effectiveness of LAIV against A/H1N1pdm09 viruses in the 2013-2014 and 2015-2016 seasons.

Experimental Study of Genetic Constellation of Cold-Adapted Master Donor Viruses for Live Attenuated Influenza Vaccine Type B

We studied the constellation of genes encoding polymerase complex proteins of master donor viruses for Russian live attenuated influenza vaccine type B. Reassortants of the reserve attenuation donor B/Leningrad/14/17/55 with B/USSR/60/69 master donor virus currently used for manufacturing seasonal influenza vaccine were prepared and examined. Most reassortants obtained by the classical reassortment method inherited all genes from the B/Leningrad/14/17/55 virus except the gene encoding PB1 subunit of the polymerase complex. One reassortant was selected for further evaluation of the role of PB1 gene. Greater attenuation of the strain for experimental animals (mice) in comparison with the original strains was demonstrated. This indicates high degree of constellation of genes of cold-adapted master donor viruses and the important compensating role of amino acid substitutions in the PB1 protein of B/Leningrad/14/17/55 donor virus in preventing viral hyperattenuation.

Current and future influenza vaccines

Although antiviral drugs and vaccines have reduced the economic and healthcare burdens of influenza, influenza epidemics continue to take a toll. Over the past decade, research on influenza viruses has revealed a potential path to improvement. The clues have come from accumulated discoveries from basic and clinical studies. Now, virus surveillance allows researchers to monitor influenza virus epidemic trends and to accumulate virus sequences in public databases, which leads to better selection of candidate viruses for vaccines and early detection of drug-resistant viruses. Here we provide an overview of current vaccine options and describe efforts directed toward the development of next-generation vaccines. Finally, we propose a plan for the development of an optimal influenza vaccine.

Development of a Universal Influenza Vaccine

The severity of the 2017-18 influenza season, combined with the low efficacy for some vaccine components, highlights the need to improve our current seasonal influenza vaccine. Thus, the National Institute of Allergy and Infectious Diseases recently announced a strategic plan to improve current influenza vaccines and eventually develop a "universal" influenza vaccine. This review will highlight the many different strategies being undertaken in pursuit of this goal and the exciting advances made by the influenza community. There is no doubt that an improved influenza vaccine is on the horizon.

Temperature Sensitive Mutations in Influenza A Viral Ribonucleoprotein Complex Responsible for the Attenuation of the Live Attenuated Influenza Vaccine

Live attenuated influenza vaccines (LAIV) have prevented morbidity and mortality associated with influenza viral infections for many years and represent the best therapeutic option to protect against influenza viral infections in humans. However, the development of LAIV has traditionally relied on empirical methods, such as the adaptation of viruses to replicate at low temperatures. These approaches require an extensive investment of time and resources before identifying potential vaccine candidates that can be safely implemented as LAIV to protect humans. In addition, the mechanism of attenuation of these vaccines is poorly understood in some cases. Importantly, LAIV are more efficacious than inactivated vaccines because their ability to mount efficient innate and adaptive humoral and cellular immune responses. Therefore, the design of potential LAIV based on known properties of viral proteins appears to be a highly appropriate option for the treatment of influenza viral infections. For that, the viral RNA synthesis machinery has been a research focus to identify key amino acid substitutions that can lead to viral attenuation and their use in safe, immunogenic, and protective LAIV. In this review, we discuss the potential to manipulate the influenza viral RNA-dependent RNA polymerase (RdRp) complex to generate attenuated forms of the virus that can be used as LAIV for the treatment of influenza viral infections, one of the current and most effective prophylactic options for the control of influenza in humans.

Resident memory CD8+ T cells in the upper respiratory tract prevent pulmonary influenza virus infection

Nasal epithelial tissue of the upper respiratory tract is the first site of contact by inhaled pathogens such as influenza virus. We show that this region is key to limiting viral spread to the lower respiratory tract and associated disease pathology. Immunization of the upper respiratory tract leads to the formation of local tissue-resident memory CD8⁺ T cells (Trm cells). Unlike Trm cells in the lung, these cells develop independently of local cognate antigen recognition and transforming growth factor-β signaling and persist with minimal decay, representing a long-term protective population. Repertoire characterization revealed unexpected differences between lung and nasal tissue Trm cells, the composition of which was shaped by the developmental need for lung, but not nasal, Trm cells to recognize antigen within their local tissue. We show that influenza-specific Trm cells in the nasal epithelia can block the transmission of influenza virus from the upper respiratory tract to the lung and, in doing so, prevent the development of severe pulmonary disease. Our findings reveal the protective capacity and longevity of upper respiratory tract Trm cells and highlight the potential of targeting these cells to augment protective responses induced to respiratory viral vaccines.

Development of an Alternative Modified Live Influenza B Virus Vaccine

Influenza B virus (IBV) is considered a major human pathogen, responsible for seasonal epidemics of acute respiratory illness. Two antigenically distinct IBV hemagglutinin (HA) lineages cocirculate worldwide with little cross-reactivity. Live attenuated influenza virus (LAIV) vaccines have been shown to provide better cross-protective immune responses than inactivated vaccines by eliciting local mucosal immunity and systemic B cell- and T cell-mediated memory responses. We have shown previously that incorporation of temperature-sensitive (ts) mutations into the PB1 and PB2 subunits along with a modified HA epitope tag in the C terminus of PB1 resulted in influenza A viruses (IAV) that are safe and effective as modified live attenuated (att) virus vaccines (IAV att). We explored whether analogous mutations in the IBV polymerase subunits would result in a stable virus with an att phenotype. The PB1 subunit of the influenza B/Brisbane/60/2008 strain was used to incorporate ts mutations and a C-terminal HA tag. Such modifications resulted in a B/Bris att strain with ts characteristics in vitro and an att phenotype in vivo Vaccination studies in mice showed that a single dose of the B/Bris att candidate stimulated sterilizing immunity against lethal homologous challenge and complete protection against heterologous challenge. These studies show the potential of an alternative LAIV platform for the development of IBV vaccines.IMPORTANCE A number of issues with regard to the effectiveness of the LAIV vaccine licensed in the United States (FluMist) have arisen over the past three seasons (2013-2014, 2014-2015, and 2015-2016). While the reasons for the limited robustness of the vaccine-elicited immune response remain controversial, this problem highlights the critical importance of continued investment in LAIV development and creates an opportunity to improve current strategies so as to develop more efficacious vaccines. Our laboratory has developed an alternative strategy, the incorporation of 2 amino acid mutations and a modified HA tag at the C terminus of PB1, which is sufficient to attenuate the IBV. As a LAIV, this novel vaccine provides complete protection against IBV strains. The availability of attenuated IAV and IBV backbones based on contemporary strains offers alternative platforms for the development of LAIVs that may overcome current limitations.

Nasal-associated lymphoid tissues (NALTs) support the recall but not priming of influenza virus-specific cytotoxic T cells

The lymphoid tissue that drains the upper respiratory tract represents an important induction site for cytotoxic T lymphocyte (CTL) immunity to airborne pathogens and intranasal vaccines. Here, we investigated the role of the nasal-associated lymphoid tissues (NALTs), which are mucosal-associated lymphoid organs embedded in the submucosa of the nasal passage, in the initial priming and recall expansion of CD8⁺ T cells following an upper respiratory tract infection with a pathogenic influenza virus and immunization with a live attenuated influenza virus vaccine. Whereas NALTs served as the induction site for the recall expansion of memory CD8⁺ T cells following influenza virus infection or vaccination, they failed to support activation of naïve CD8⁺ T cells. Strikingly, NALTs, unlike other lymphoid tissues, were not routinely surveyed during the steady state by circulating T cells. The selective recruitment of memory T cells into these lymphoid structures occurred in response to infection-induced elevation of the chemokine CXCL10, which attracted CXCR3⁺ memory CD8⁺ T cells. These results have significant implications for intranasal vaccines, which deliver antigen to mucosal-associated lymphoid tissue and aim to elicit protective CTL-mediated immunity.

Reverse Genetics Approaches for the Development of Influenza Vaccines

Influenza viruses cause annual seasonal epidemics and occasional pandemics of human respiratory disease. Influenza virus infections represent a serious public health and economic problem, which are most effectively prevented through vaccination. However, influenza viruses undergo continual antigenic variation, which requires either the annual reformulation of seasonal influenza vaccines or the rapid generation of vaccines against potential pandemic virus strains. The segmented nature of influenza virus allows for the reassortment between two or more viruses within a co-infected cell, and this characteristic has also been harnessed in the laboratory to generate reassortant viruses for their use as either inactivated or live-attenuated influenza vaccines. With the implementation of plasmid-based reverse genetics techniques, it is now possible to engineer recombinant influenza viruses entirely from full-length complementary DNA copies of the viral genome by transfection of susceptible cells. These reverse genetics systems have provided investigators with novel and powerful approaches to answer important questions about the biology of influenza viruses, including the function of viral proteins, their interaction with cellular host factors and the mechanisms of influenza virus transmission and pathogenesis. In addition, reverse genetics techniques have allowed the generation of recombinant influenza viruses, providing a powerful technology to develop both inactivated and live-attenuated influenza vaccines. In this review, we will summarize the current knowledge of state-of-the-art, plasmid-based, influenza reverse genetics approaches and their implementation to provide rapid, convenient, safe and more effective influenza inactivated or live-attenuated vaccines.

Genetic analysis of attenuation markers of cold-adapted X-31 influenza live vaccine donor strain

Cold-adapted live attenuated influenza vaccines (CAIVs) have been considered as a safe prophylactic measure to prevent influenza virus infections. The safety of a CAIV depends largely on genetic markers that confer specific attenuation phenotypes. Previous studies with other CAIVs reported that polymerase genes were primarily responsible for the attenuation. Here, we analyzed the genetic mutations and their phenotypic contribution in the X-31 ca strain, a recently developed alternative CAIV donor strain. During the cold-adaptation of its parental X-31 virus, various numbers of sequence changes were accumulated in all six internal genes. Phenotypic analysis with single-gene and multiple-gene reassortant viruses suggests that NP gene makes the largest contribution to the cold-adapted (ca) and temperature-sensitive (ts) characters, while the remaining other internal genes also impart attenuation characters with varying degrees. A balanced contribution of all internal genes to the attenuation suggests that X-31 ca could serve as an ideal master donor strain for CAIVs preventing influenza epidemics and pandemics.

Temperature-sensitive mutations for live-attenuated Rift Valley fever vaccines: implications from other RNA viruses

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease endemic to the African continent. RVF is characterized by high rate of abortions in ruminants and hemorrhagic fever, encephalitis, or blindness in humans. RVF is caused by the Rift Valley fever virus (RVFV: genus Phlebovirus, family Bunyaviridae). Vaccination is the only known effective strategy to prevent the disease, but there are no licensed RVF vaccines available for humans. A live-attenuated vaccine candidate derived from the wild-type pathogenic Egyptian ZH548 strain, MP-12, has been conditionally licensed for veterinary use in the U.S. MP-12 displays a temperature-sensitive (ts) phenotype and does not replicate at 41°C. The ts mutation limits viral replication at a specific body temperature and may lead to an attenuation of the virus. Here we will review well-characterized ts mutations for RNA viruses, and further discuss the potential in designing novel live-attenuated vaccines for RVF.

Influenza Vaccination Strategies: Comparing Inactivated and Live Attenuated Influenza Vaccines

Influenza is a major respiratory pathogen causing annual outbreaks and occasional pandemics. Influenza vaccination is the major method of prophylaxis. Currently annual influenza vaccination is recommended for groups at high risk of complications from influenza infection such as pregnant women, young children, people with underlying disease and the elderly, along with occupational groups such a healthcare workers and farm workers. There are two main types of vaccines available: the parenteral inactivated influenza vaccine and the intranasal live attenuated influenza vaccine. The inactivated vaccines are licensed from 6 months of age and have been used for more than 50 years with a good safety profile. Inactivated vaccines are standardized according to the presence of the viral major surface glycoprotein hemagglutinin and protection is mediated by the induction of vaccine strain specific antibody responses. In contrast, the live attenuated vaccines are licensed in Europe for children from 2-17 years of age and provide a multifaceted immune response with local and systemic antibody and T cell responses but with no clear correlate of protection. Here we discuss the immunological immune responses elicited by the two vaccines and discuss future work to better define correlates of protection.

Safety and immunogenicity in man of a cell culture derived trivalent live attenuated seasonal influenza vaccine: a Phase I dose escalating study in healthy volunteers

Live attenuated influenza vaccine (LAIV) offers the promise of inducing a variety of immune responses thereby conferring protection to circulating field strains. LAIVs are based on cold adapted and temperature sensitive phenotypes of master donor viruses (MDVs) containing the surface glycoprotein genes of seasonal influenza strains. Two types of MDV lineages have been described, the Ann Arbor lineages and the A/Leningrad/17 and B/USSR/60 lineages. Here the safety and immunogenicity of a Madin Darby Canine Kidney - cell culture based, intranasal LAIV derived from A/Leningrad/17 and B/USSR, was evaluated in healthy influenza non-naive volunteers 18-50 years of age. In a double-blind, randomized, placebo-controlled design, single escalating doses of 1×10(5), 1×10(6), or 1×10(7) tissue culture infectious dose 50% (TCID50) of vaccine containing each of the three influenza virus re-assortants recommended by the World Health Organization for the 2008-2009 season were administered intranasally. A statistically significant geometric mean increase in hemagglutination inhibition titer was reached for influenza strain A/H3N2 after immunization with all doses of LAIV. For the A/H1N1 and B strains, the GMI in HI titer did not increase for any of the doses. Virus neutralization antibody titers showed a similar response pattern. A dose-response effect could not be demonstrated for any of the strains, neither for the HI antibody nor for the VN antibody responses. No influenza like symptoms, no nasal congestions, no rhinorrhea, or other influenza related upper respiratory tract symptoms were observed. In addition, no difference in the incidence or nature of adverse events was found between vaccine and placebo treated subjects. Overall, the results indicated that the LAIV for nasal administration is immunogenic (i.e. able to provoke an immune response) and safe both from the perspective of the attenuated virus and the MDCK cell line from which it was derived, and it warrants further development.

Production of live attenuated influenza vaccines against seasonal and potential pandemic influenza viruses

Vaccination remains the most effective means to prevent morbidity and mortality caused by influenza epidemics and pandemics. Live attenuated influenza vaccine (LAIV) has been proven to be effective in preventing influenza with broad cross reactivity to drifted strains. Owing to the sophisticated nature of the influenza vaccine production process, the time needed to develop high yield LAIV strains for vaccine production and product release remains a constant challenge. This review summarizes LAIV production process with highlights on the experiences gained during the past decade generating seasonal and pandemic LAIV seeds by reverse genetics strategy.

A replication-incompetent virus possessing an uncleavable hemagglutinin as an influenza vaccine

Vaccination is one of the most effective measures to protect against influenza virus infection. Inactivated and live-attenuated influenza vaccines are available; however, their efficacy is suboptimal. To develop a safe and more immunogenic vaccine, we produced a novel replication-incompetent influenza virus that possesses uncleavable hemagglutinin (HA) and tested its vaccine potential. The uncleavable HA was engineered by substituting the arginine at the C-terminus of HA1 with threonine, which prevents cleavage of HA into its HA1 and HA2 subunits, preventing fusion between the host and viral membranes. Although this fusion-deficient HA influenza virus that possesses uncleavable HA (uncleavable HA virus) could undergo multiple cycles of replication in only wild-type HA-expressing cells, it could infect normal cells and express viral proteins in infected cells, but could not generate infectious virus from infected cells due to the uncleavable HA. When C57BL/6 mice were intranasally immunized with the uncleavable HA virus, influenza-specific IgG and IgA antibodies were detected in nasal wash and bronchoalveolar lavage samples and in serum. In addition, influenza-specific CD8(+) T cells accumulated in the lungs of these mice. Moreover, mice immunized with the uncleavable HA virus were protected against a challenge of lethal doses of influenza virus, unlike mice immunized with a formalin-inactivated virus. These findings demonstrate that this fusion-deficient virus, which possesses uncleavable HA, is a suitable influenza vaccine candidate.

Immunogenicity and safety of a quadrivalent live attenuated influenza vaccine in children

BACKGROUND

Influenza B viruses from 2 lineages cocirculate annually. Because the single B strain contained in trivalent vaccines may not match the major circulating strain, adding a second B virus could enhance protection. This study compared the safety and immunogenicity of an investigational quadrivalent Ann Arbor strain live attenuated influenza vaccine (Q/LAIV) with that of 2 trivalent vaccines (T/LAIV), each containing a B strain from a different lineage.

METHODS

This randomized, double-blind study was designed to demonstrate the immunologic noninferiority of Q/LAIV compared with T/LAIV in children 2-17 years of age by comparing postdose geometric mean titers of hemagglutination inhibition antibodies. Children were randomized 3:1:1 to receive Q/LAIV or 1 of 2 T/LAIV vaccines. Those subjects who were 9-17 years of age received 1 dose, and those 2-8 years of age received 2 doses 1 month apart. Serum immune responses were evaluated 1 month after dose 1 (dose 2 for influenza vaccine-naive subjects aged 2-8 years).

RESULTS

Q/LAIV was noninferior to T/LAIV: upper bounds for all four 95% confidence intervals for the postdose geometric mean titer ratios (T/LAIV divided by Q/LAIV) were ≤1.5, the predefined noninferiority margin. The overall seroresponse rates (4-fold rise) were comparable between treatment groups. Safety events were comparable, except that fever was more common after dose 1 in Q/LAIV subjects (5.1%) than in T/LAIV subjects (3.1%) 2-8 years of age.

CONCLUSIONS

The immunogenicity of Q/LAIV was noninferior to that of T/LAIV in children aged 2-17 years; safety was also comparable. Q/LAIV may broaden the protection against influenza B strains provided by current trivalent influenza vaccines.

Genetic engineering of live attenuated influenza viruses

The first live attenuated influenza vaccine (LAIV) was licensed in the USA in 2003; it is a trivalent vaccine composed of two type A (H1N1 and H3N2) and one type B influenza virus each at 10(7) fluorescent focus units (FFU). Each influenza vaccine strain is a reassortant virus that contains the hemagglutinin (HA) and neuraminidase (NA) gene segments from a wild-type influenza virus and the six internal protein gene segments from a master donor virus (MDV) of either cold-adapted A/Ann Arbor/6/60 or B/Ann Arbor/1/66. MDV confers the cold-adapted, temperature-sensitive, and attenuation phenotypes to the vaccine strains. The reassortant vaccine seeds are currently produced by reverse genetics and amplified in specific pathogen-free (SPF) 9-11 days old embryonated chicken eggs for manufacture. In addition, MDCK cell culture manufacture processes have been developed to produce LAIV for research use and with modifications for clinical and/or commercial grade material production.

Comparison of a live attenuated 2009 H1N1 vaccine with seasonal influenza vaccines against 2009 pandemic H1N1 virus infection in mice and ferrets

The role of seasonal influenza vaccination in pandemic influenza A H1N1 disease is important to address, because a large segment of the population is vaccinated annually. We administered 1 or 2 doses of pandemic H1N1 vaccine (CA/7 ca), a seasonal trivalent inactivated (s-TIV), or live attenuated influenza vaccine (s-LAIV) to mice and ferrets and subsequently challenged them with a pandemic H1N1 virus. In both species, CA/7 ca was immunogenic and conferred complete protection against challenge. s-TIV did not confer protection in either animal model, and s-LAIV did not confer any protection in ferrets. In mice, 2 doses of s-LAIV led to complete protection in the upper respiratory tract and partial protection in the lungs. Our data indicate that vaccination with the seasonal influenza vaccines did not confer complete protection in the lower respiratory tract in either animal model, whereas the CA/7 ca vaccine conferred complete protection in both animal models.

Molecular studies of influenza B virus in the reverse genetics era

Recovery of an infectious virus of defined genetic structure entirely from cDNA and the deduction of information about the virus resulting from phenotypic characterization of the mutant is the process of reverse genetics. This approach has been possible for a number of negative-strand RNA viruses since the recovery of rabies virus in 1994. However, the recovery of recombinant orthomyxoviruses posed a greater challenge due to the segmented nature of the genome. It was not until 1999 that such a system was reported for influenza A viruses, but since that time our knowledge of influenza A virus biology has grown dramatically. Annual influenza epidemics are caused not only by influenza A viruses but also by influenza B viruses. In 2002, two groups reported the successful recovery of influenza B virus entirely from cDNA. This has allowed greater depth of study into the biology of these viruses. This review will highlight the advances made in various areas of influenza B virus biology as a result of the development of reverse genetics techniques for these viruses, including (i) the importance of the non-coding regions of the influenza B virus genome; (ii) the generation of novel vaccine strains; (iii) studies into the mechanisms of drug resistance; (iv) the function(s) of viral proteins, both those analogous to influenza A virus proteins and those unique to influenza B viruses. The information generated by the application of influenza B virus reverse genetics systems will continue to contribute to our improved surveillance and control of human influenza.

Adjuvant effect of Bacillus firmus on the expression of cytokines and toll-like receptors in mouse nasopharynx-associated lymphoid tissue (NALT) after intranasal immunization with inactivated influenza virus type A

Due to the persisting threat of development of new highly pathogenic influenza A subtypes, a mucosal vaccination which would induce a potent and cross-protective reaction is desirable. We succeeded in mucosal immunization of mice with an inactivated influenza A virus by using delipidated Bacillus firmus (DBF) as adjuvant. The mechanism of adjuvant effect was followed in NALT by comparing the response after intranasal immunization by inactivated influenza virus type A (H1N1) alone, adjuvant alone (DBF), or by a mixture of virus+DBF. Expression of selected gene groups was tested via qPCR at 7 different time-points: cytokines (IL-2, IFN-γ, IL-4, IL-6, and IL-10), type I interferons (IFN-α4, IFN-α11, IFN-α12, and IFN-β), toll-like receptors (TLR2, TLR3, TLR7, and TLR9), iNOS and CCR7. Intranasally administered DBF and the mixture of virus+DBF induced an elevated expression of IFN-γ, IL-6 and IL-10 cytokines, type I interferons, iNOS, and pDC markers in NALT. Multimarker qPCR data was analyzed by relative quantification and by principal component analysis. DBF has been shown to be a very efficient adjuvant for the stimulation of innate immunity after IN immunization. DBF accelerated, increased, and prolonged the antiviral response.

Identification of RNA regions that determine temperature sensitivities in betanodaviruses

Betanodaviruses, the causative agents of viral nervous necrosis in marine fish, have bipartite positive-sense RNA genomes. The larger genomic segment, RNA1 (~3.1 kb), encodes an RNA-dependent RNA polymerase (protein A), and the smaller genomic segment RNA2 (~1.4 kb) codes for the coat protein. These viruses can be classified into four genotypes, designated striped jack nervous necrosis virus (SJNNV), redspotted grouper nervous necrosis virus (RGNNV), tiger puffer nervous necrosis virus (TPNNV), and barfin flounder nervous necrosis virus (BFNNV), based on similarities in their partial RNA2 sequences. The optimal temperatures for the growth of these viruses are 20-25°C (SJNNV), 25-30°C (RGNNV), 20°C (TPNNV), and 15-20°C (BFNNV). However, little is known about the mechanisms underlying the temperature sensitivity of these viruses. We first constructed two reassortants between SJNNV and RGNNV to test their temperature sensitivity. The levels of viral growth and RNA replication of these reassortants and parental viruses in cultured fish cells were similar at 25°C. However, the levels of all of the viruses but RGNNV were markedly reduced at 30°C. These results indicate that both RNA1 and RNA2 control the temperature sensitivity of betanodaviruses by modulating RNA replication or earlier viral growth processes. We then constructed ten mutated RGNNVs, the RNA1 segments of which were chimeric between SJNNV and RGNNV, and showed that only chimeric viruses bearing the RGNNV RNA1 region, encoding amino acid residues 1-445, grew similarly to the parental RGNNV at 30°C. This portion of protein A is known to serve as a mitochondrial-targeting signal rather than functioning as an enzymatic domain.

PB2 and PA genes control the expression of the temperature-sensitive phenotype of cold-adapted B/USSR/60/69 influenza master donor virus

The cold-adapted (ca) and temperature-sensitive (ts) influenza master donor virus (MDV) B/USSR/60/69 was derived from its wild-type parental virus after successive passages in eggs at 32 degrees C and 25 degrees C. This strain is currently in use for preparing reassortant influenza B vaccine viruses which are used in the Russian trivalent live attenuated influenza vaccine. Vaccine viruses are obtained by classical reassortment of MDV and a currently circulating wild-type virus. The phenotypic properties cold adaptation and temperature sensitivity are inherited from the six genes encoding the internal proteins of the MDV. However, the role of the individual gene segments in temperature sensitivity and thus attenuation is not known. In this study, 35 reassortant viruses of B/USSR/60/69 MDV with current wild-type non-ts influenza B viruses were generated in eggs or MDCK cells and studied in order to identify the genes responsible for their ts phenotype. For each virus the exact genome composition was determined as well as its ts phenotype. The results demonstrated that the polymerase PB2 and PA gene segments of B/USSR/60/69 MDV independently controlled expression of the ts phenotype of B/USSR/60/69 MDV-based reassortant viruses. The other genes coding for internal proteins played no role in this respect. This suggests that mutations in the polymerase genes PB2 and PA play an essential role in attenuation of B/USSR/60/69 MDV-based reassortant influenza B vaccine viruses.

Current status of live attenuated influenza vaccine in the United States for seasonal and pandemic influenza

Abstract  A live attenuated influenza vaccine (LAIV) is currently approved in the United States for the prevention of influenza in individuals 2–49 years of age. This article summarizes the available data describing the safety and efficacy of LAIV for the prevention of influenza in both children and adults. LAIV is administered as an intranasal spray and has been shown to provide high levels of efficacy against influenza illness caused by both matched and mismatched strains in children and adults. In studies comparing LAIV and inactivated influenza vaccine in children, LAIV recipients experienced 35–53% fewer cases of culture‐confirmed influenza illness caused by antigenically matched strains. Protection through a second influenza season against antigenically matched strains has also been seen in children. In adults, definitive comparative studies of LAIV and inactivated vaccine have not been conducted and no statistically significant differences in efficacy have been demonstrated. The most common adverse reactions with LAIV include runny nose/nasal congestion in all age groups, fever >100°F in children, and sore throat in adults. Formulations of LAIV against pandemic influenza strains, including H5N1, H9N2, and H7N3, are currently being tested in preclinical and phase I clinical studies.

Emerging influenza virus: a global threat

Since 1918, influenza virus has been one of the major causes of morbidity and mortality, especially among young children. Though the commonly circulating strain of the virus is not virulent enough to cause mortality, the ability of the virus genome to mutate at a very high rate may lead to the emergence of a highly virulent strain that may become the cause of the next pandemic. Apart from the influenza virus strain circulating in humans (H1N1 and H3N2), the avian influenza H5N1 H7 and H9 virus strains have also been reported to have caused human infections, H5N1 H7 and H9 have shown their ability to cross the species barrier from birds to humans and further replicate in humans. This review addresses the biological and epidemiological aspects of influenza virus and efforts to have a control on the virus globally.

Peptide mimics of a conserved H5N1 avian influenza virus neutralization site

A panel of 52 murine monoclonal antibodies was found to recognize antigenic determinants that had been conserved among all major genetic subgroups of the H5N1 avian influenza virus prevalent since 1997. We screened a phage display library for peptides recognized by one such antibody (8H5). We analysed the specificity of 8H5 for reactive peptides presented as fusion proteins of HBc (hepatitis B core protein) and HEV (hepatitis E virus) structural protein, p239. This was then related to the specificity of the native HA (haemagglutinin) molecule by virtue of the capacity of fusion proteins to compete for 8H5 binding with different strains of H5N1 virus and the reactivity of antisera generated against fusion proteins to bind native HA molecules, and to inhibit haemagglutination and arrest infection by the virus. Nine reactive peptides of different amino acid sequences were identified, six of which were also reactive with the antibody in association with HBc and four were in association with p239. Binding occurred with the dimeric form of the four p239-fusion proteins and one of the HBc-fusion proteins, but not with the monomeric form. The HBc-fusion proteins blocked 8H5 binding with four strains of H5N1 influenza virus. Mouse antisera generated against fusion proteins bound to HA molecules, but did not inhibit haemagglutination or arrest H5N1 infection. Our findings indicate that 8H5 recognizes discontinuous sites presented by secondary and possibly higher structural orders of the peptides in spatially favourable positions for binding with the antibody, and that the peptides partially mimic the native 8H5 epitopes on the H5N1 virus.

H5N1 vaccine-specific B cell responses in ferrets primed with live attenuated seasonal influenza vaccines

Background

Live attenuated influenza H5N1 vaccines have been produced and evaluated in mice and ferrets that were never exposed to influenza A virus infection (Suguitan et al., Plos Medicine, e360:1541, 2006). However, the preexisting influenza heterosubtypic immunity on live attenuated H5N1 vaccine induced immune response has not been evaluated.

Methodology and Principal Findings

Primary and recall B cell responses to live attenuated H5N1 vaccine viruses were examined using a sensitive antigen-specific B cell ELISpot assay to investigate the effect of preexisting heterosubtypic influenza immunity on the development of H5N1-specific B cell immune responses in ferrets. Live attenuated H5N1 A/Hong Kong/213/03 and A/Vietnam/1203/04 vaccine viruses induced measurable H5-specific IgM and IgG secreting B cells after intranasal vaccination. However, H5-specific IgG secreting cells were detected significantly earlier and at a greater frequency after H5N1 inoculation in ferrets previously primed with trivalent live attenuated influenza (H1N1, H3N2 and B) vaccine. Priming studies further revealed that the more rapid B cell responses to H5 resulted from cross-reactive B cell immunity to the hemagglutinin H1 protein. Moreover, vaccination with the H1N1 vaccine virus was able to induce protective responses capable of limiting replication of the H5N1 vaccine virus to a level comparable with prior vaccination with the H5N1 vaccine virus without affecting H5N1 vaccine virus induced antibody response.

Conclusion

The findings indicate that previous vaccination with seasonal influenza vaccine may accelerate onset of immunity by an H5N1 ca vaccine and the heterosubtypic immunity may be beneficial for pandemic preparedness.

Live attenuated vaccines for pandemic influenza

In this chapter, we will review the development of and clinical experience with the currently licensed seasonal live attenuated influenza vaccines (LAIV) and preclinical studies of H5, H7, and H9 live attenuated pandemic influenza vaccine candidates. Vectored vaccine approaches will not be reviewed in this chapter. Experience with seasonal influenza vaccination has demonstrated the safety and efficacy of LAIV in both children and adults; moreover, cross-protection among antigenically distinct viruses within the same subtype may be induced by LAIV. While clinical studies and further characterization of the immunologic response to avian influenza viruses are still needed, the experience with seasonal LAIV underscores the potential of live attenuated vaccines to play an important role in the event of a pandemic.

Evaluation of live attenuated influenza a virus h6 vaccines in mice and ferrets

Avian influenza A virus A/teal/HK/W312/97 (H6N1) possesses seven gene segments that are highly homologous to those of highly pathogenic human influenza H5N1 viruses, suggesting that a W312-like H6N1 virus might have been involved in the generation of the A/HK/97 H5N1 viruses. The continuous circulation and reassortment of influenza H6 subtype viruses in birds highlight the need to develop an H6 vaccine to prevent potential influenza pandemics caused by the H6 viruses. Based on the serum antibody cross-reactivity data obtained from 14 different H6 viruses from Eurasian and North American lineages, A/duck/HK/182/77, A/teal/HK/W312/97, and A/mallard/Alberta/89/85 were selected to produce live attenuated H6 candidate vaccines. Each of the H6 vaccine strains is a 6:2 reassortant ca virus containing HA and NA gene segments from an H6 virus and the six internal gene segments from cold-adapted A/Ann Arbor/6/60 (AA ca), the master donor virus that is used to make live attenuated influenza virus FluMist (intranasal) vaccine. All three H6 vaccine candidates exhibited phenotypic properties of temperature sensitivity (ts), ca, and attenuation (att) conferred by the internal gene segments from AA ca. Intranasal administration of a single dose of the three H6 ca vaccine viruses induced neutralizing antibodies in mice and ferrets and fully protected mice and ferrets from homologous wild-type (wt) virus challenge. Among the three H6 vaccine candidates, the A/teal/HK/W312/97 ca virus provided the broadest cross-protection against challenge with three antigenically distinct H6 wt viruses. These data support the rationale for further evaluating the A/teal/HK/W312/97 ca vaccine in humans.

Molecular studies of temperature-sensitive replication of the cold-adapted B/Ann Arbor/1/66, the master donor virus for live attenuated influenza FluMist vaccines

Cold-adapted (ca) B/Ann Arbor/1/66 is the master donor virus for influenza B (MDV-B) vaccine component of live attenuated influenza FluMist vaccine. The six internal protein gene segments of MDV-B confer the characteristic cold-adapted (ca), temperature-sensitive (ts) and attenuated (att) phenotypes to the reassortant vaccine strains that contain the HA and NA RNA segments from the circulating wild type strains. Previously, we have mapped the loci in the NP, PA and M genes that determine the ca, ts and att phenotypes of MDV-B. In this report, the ts mechanism of MDV-B was described by comparing replication of MDV-B with its wild type counterpart at permissive and restricted temperatures. We showed that the PA and NP proteins of MDV-B are defective in RNA polymerase function at the restricted temperature of 37 degrees C resulting in greatly reduced viral RNA and protein synthesis. In addition, the two M1 residues, Q159 and V183 that are unique to MDV-B, contribute to reduced virus replication at temperatures greater than 33 degrees C, possibly due to the reduced M1 membrane association and its reduced virion M1 incorporation. Thus, the previously identified MDV-B loci not only reduce viral polymerase function at the restricted temperature but also affect virus assembly and release.

Avian influenza h6 viruses productively infect and cause illness in mice and ferrets

Influenza pandemic preparedness has focused on influenza virus H5 and H7 subtypes. However, it is not possible to predict with certainty which subtype of avian influenza virus will cause the next pandemic, and it is prudent to include other avian influenza virus subtypes in pandemic preparedness efforts. An H6 influenza virus was identified as a potential progenitor of the H5N1 viruses that emerged in Hong Kong in 1997. This virus continues to circulate in the bird population in Asia, and other H6 viruses are prevalent in birds in North America and Asia. The high rate of reassortment observed in influenza viruses and the prevalence of H6 viruses in birds suggest that this subtype may pose a pandemic risk. Very little is known about the replicative capacity, immunogenicity, and correlates of protective immunity for low-pathogenicity H6 influenza viruses in mammals. We evaluated the antigenic and genetic relatedness of 14 H6 influenza viruses and their abilities to replicate and induce a cross-reactive immune response in two animal models: mice and ferrets. The different H6 viruses replicated to different levels in the respiratory tracts of mice and ferrets, causing varied degrees of morbidity and mortality in these two models. H6 virus infection induced similar patterns of neutralizing antibody responses in mice and ferrets; however, species-specific differences in the cross-reactivity of the antibody responses were observed. Overall, cross-reactivity of neutralizing antibodies in H6 virus-infected mice did not correlate well with protection against heterologous wild-type H6 viruses. However, we have identified an H6 virus that induces protective immunity against viruses in the North American and Eurasian lineages.

Live, attenuated influenza virus (LAIV) vehicles are strong inducers of immunity toward influenza B virus

Historically, vaccines developed toward influenza viruses of the B type using methodologies developed for influenza A viruses as a blueprint have not been equally efficacious or effective. Because most influenza research and public attention concerns influenza A viruses, these shortcomings have not been adequately addressed. In this manuscript, we utilized different influenza vaccine vehicles to compare immunogenicity and protection in mice and ferrets after vaccination against an influenza B virus. We report that plasmid DNA vaccines demonstrate low immunogenicity profiles and poor protection compared to either whole, inactivated influenza virus (IIV) or, live, attenuated influenza virus (LAIV) vaccines. When mixed prime:boost regimens using LAIV and IIV were studied, we observed a boosting effect in mice after priming with LAIV that was not seen when IIV was used as the prime. In ferrets LAIV induced high antibody titers after a single dose and provided a boost in IIV-primed animals. Regimens including LAIV as a prime demonstrated enhanced protection, and adjuvantation was required for efficacy using the IIV preparation. Our results differ from generally accepted influenza A virus vaccine models, and argue that strategies for control of influenza B virus should be considered separately from those for influenza A virus.