Studies on the usefulness of intranasal inactivated influenza vaccines

Subcutaneous immunisation with zymosan generates mucosal IgA-eliciting memory and protects mice from heterologous influenza virus infection

Immunoglobulin A (IgA) is the most abundant isotype of antibodies and provides a first line of defense at the mucosa against pathogens invading the host. It has been widely accepted that the mucosal IgA response provided by vaccination requires mucosal inoculation, and intranasal inoculation has been proposed for vaccines against influenza virus. Considering the difficulty of intranasal vaccination in infants or elderly people, however, parenteral vaccination that provides the mucosal IgA response is desirable. Here, we demonstrate that subcutaneous immunisation with zymosan, a yeast cell wall constituent known to be recognised by Dectin-1 and TLR2, potentiates the production of antigen-specific IgA antibodies in the sera and airway mucosa upon intranasal antigen challenge. We confirmed that the antigen-specific IgA-secreting cells accumulated in the lung and nasal-associated lymphoid tissues after the antigen challenge. Such an adjuvant effect of zymosan in the primary immunisation for the IgA response depended on Dectin-1 signalling, but not on TLR2. The IgA response to the antigen challenge required both antigen-specific memory B and T cells, and the generation of memory T cells, but not memory B cells, depended on zymosan as an adjuvant. Finally, we demonstrated that subcutaneous inoculation of inactivated influenza virus with zymosan, but not with alum, mostly protected the mice from infection with a lethal dose of a heterologous virus strain. These data suggest that zymosan is a possible adjuvant for parenteral immunisation that generates memory IgA responses to respiratory viruses such as influenza virus.

Isolated specific IgA against respiratory viruses, Influenza or SARS-CoV-2, present in the saliva of a fraction of healthy and asymptomatic volunteers

OBJECTIVES

Defense against respiratory viruses depends on an immune response present in the mucosa, as saliva IgA secretes antibodies. During the pandemic, such as influenza or SARS-CoV-2, most infected patients are asymptomatic but retain specific antibodies post-infection. The authors evaluated IgG and IgA antibodies against SARS-CoV-2 and influenza in the saliva of asymptomatic volunteers, validated with controls or vaccinated individuals.

METHODS

The authors detected specific antibodies by validated conventional ELISA using natural SARS-CoV-2 antigens from infected Vero cells or capture-ELISA for influenza using natural antigens of the influenza vaccine.

RESULTS

Saliva from influenza-vaccinated individuals had more IgA than paired serum, contrary to the findings for specific IgG. In COVID-19-vaccinated samples, specific IgA in saliva increased after vaccination, but IgG levels were high after the first dose. In saliva from the asymptomatic population (226), anti-Influenza IgG was found in 57.5% (130) of samples, higher than IgA, found in 35% (79) of samples. IgA results were similar for SARS-CoV-2, with IgA present in 30% (68) of samples, while IgG was less present, in 44.2% (100) of samples. The proportion of influenza IgG responders was higher than that for SARS-CoV-2 IgG, but both populations presented similar proportions of IgA responders, possibly due to variable memory B cell survival. For both viruses, the authors found an important proportion (> 10%) of IgA+IgG- samples, suggesting the occurrence of humoral immunity directed to the mucosa.

CONCLUSION

Specific antibodies for respiratory viruses in saliva are found in either infection or vaccination and are a convenient and sensitive diagnostic tool for host immune response.

How to assess the effectiveness of nasal influenza vaccines? Role and measurement of sIgA in mucosal secretions

Secretory IgAs (sIgA) constitute the principal isotype of antibodies present in nasal and mucosal secretions. They are secreted by plasma cells adjacent to the mucosal epithelial cells, the site where infection occurs, and are the main humoral mediator of mucosal immunity. Mucosally delivered vaccines, such as live attenuated influenza vaccine (LAIV), are able to mimic natural infection without causing disease or virus transmission and mainly elicit a local immune response. The measurement of sIgA concentrations in nasal swab/wash and saliva samples is therefore a valuable tool for evaluating their role in the effectiveness of such vaccines. Here, we describe two standardized assays (enzyme‐linked immunosorbent assay and microneutralization) available for the quantification of sIgA and discuss the advantages and limitations of their use.

Cross-Protective Potential and Protection-Relevant Immune Mechanisms of Whole Inactivated Influenza Virus Vaccines Are Determined by Adjuvants and Route of Immunization

Adjuvanted whole inactivated virus (WIV) influenza vaccines show promise as broadly protective influenza vaccine candidates. Using WIV as basis we assessed the relative efficacy of different adjuvants by carrying out a head-to-head comparison of the liposome-based adjuvants CAF01 and CAF09 and the protein-based adjuvants CTA1-DD and CTA1-3M2e-DD and evaluated whether one or more of the adjuvants could induce broadly protective immunity. Mice were immunized with WIV prepared from A/Puerto Rico/8/34 (H1N1) virus intramuscularly with or without CAF01 or intranasally with or without CAF09, CTA1-DD, or CTA1-3M2e-DD, followed by challenge with homologous, heterologous or heterosubtypic virus. In general, intranasal immunizations were significantly more effective than intramuscular immunizations in inducing virus-specific serum-IgG, mucosal-IgA, and splenic IFNγ-producing CD4 T cells. Intranasal immunizations with adjuvanted vaccines afforded strong cross-protection with milder clinical symptoms and better control of virus load in lungs. Mechanistic studies indicated that non-neutralizing IgG antibodies and CD4 T cells were responsible for the improved cross-protection while IgA antibodies were dispensable. The role of CD4 T cells was particularly pronounced for CTA1-3M2e-DD adjuvanted vaccine as evidenced by CD4 T cell-dependent reduction of lung virus titers and clinical symptoms. Thus, intranasally administered WIV in combination with effective mucosal adjuvants appears to be a promising broadly protective influenza vaccine candidate.

Intranasal inactivated influenza vaccines for the prevention of seasonal influenza epidemics

INTRODUCTION

Intranasal influenza vaccines are expected to confer protection among vaccine recipients by successful induction of mucosal immune response in the upper respiratory tract. Though only live attenuated influenza virus vaccines (LAIVs) are licensed and available for intranasal use in humans today, intranasal inactivated influenza vaccines (IIVs) are currently under reconsideration as a promising intranasal influenza vaccine.

AREAS COVERED

This review addresses the history of intranasal IIV research and development, along with a summary of the studies done so far to address the mechanism of action of intranasal IIVs.

EXPERT COMMENTARY

From numerous in vitro and in vivo studies, it has been shown that intranasal IIVs can protect hosts from a broad spectrum of influenza virus strains. In-depth studies of the mucosal antibody response following intranasal IIV administration have also elucidated the detailed functions of secretory IgA (immunoglobulin A) antibodies which are responsible for the mechanism of action of intranasal vaccines. Safe and effective intranasal IIVs are expected to be an important tool to combat seasonal influenza.

Functional and structural characteristics of secretory IgA antibodies elicited by mucosal vaccines against influenza virus

Mucosal tissues are major targets for pathogens. The secretions covering mucosal surfaces contain several types of molecules that protect the host from infection. Among these, mucosal immunoglobulins, including secretory IgA (S-IgA) antibodies, are the major contributor to pathogen-specific immune responses. IgA is the primary antibody class found in many external secretions and has unique structural and functional features not observed in other antibody classes. Recently, extensive efforts have been made to develop novel vaccines that induce immunity via the mucosal route. S-IgA is a key molecule that underpins the mechanism of action of these mucosal vaccines. Thus, precise characterization of S-IgA induced by mucosal vaccines is important, if the latter are to be used successfully in a clinical setting. Intensive studies identified the fundamental characteristics of S-IgA, which was first discovered almost half a century ago. However, S-IgA itself has not gained much attention of late, despite its importance to mucosal immunity; therefore, some important questions remain. This review summarizes the current understanding of the molecular characteristics of S-IgA and its role in intranasal mucosal vaccines against influenza virus infection.

Intranasal Inactivated Influenza Vaccines: a Reasonable Approach to Improve the Efficacy of Influenza Vaccine?

Influenza is a contagious, acute respiratory disease caused by the influenza virus. The mucosal lining in the host respiratory tract is not only the site of virus infection, but also the site of defense; it is at this site that the host immune response targets the virus and protects against reinfection. One of the most effective methods to prevent influenza is to induce specific antibody (Ab) responses in the respiratory tract by vaccination. Two types of influenza vaccines, intranasal live attenuated influenza virus (LAIV) vaccines and parenteral (injectable) inactivated vaccines, are currently used worldwide. These vaccines are approved by the European Medicines Agency (EMA) and the US Food and Drug Administration. Live attenuated vaccines induce both secretory IgA (S-IgA) and serum IgG antibodies (Abs), whereas parenteral vaccines induce only serum IgG Abs. However, intranasal administration of inactivated vaccines together with an appropriate adjuvant induces both S-IgA and IgG Abs. Several preclinical studies on adjuvant-combined, nasal-inactivated vaccines revealed that nasal S-IgA Abs, a major immune component in the upper respiratory tract, reacted with homologous virus hemagglutinin (HA) and were highly cross-reactive with viral HA variants, resulting in protection and cross-protection against infection by both homologous and variant viruses, respectively. Serum-derived IgG Abs, which are present mainly in the lower respiratory tract, are less cross-reactive and cross-protective. In addition, our own clinical trials have shown that nasal-inactivated whole virus vaccines, including a built-in adjuvant (single-stranded RNA), induced serum hemagglutination inhibition (HI) Ab titers that fulfilled the EMA criteria for vaccine efficacy. The nasal-inactivated whole virus vaccines also induced high levels of nasal HI and neutralizing Ab titers, although we have not yet evaluated the nasal HI titers due to the lack of official criteria to establish efficacy based on this parameter. Data suggest that adjuvant-combined nasal-inactivated vaccines have advantages over the current injectable vaccine because the former induce both S-IgA and serum IgG Abs. In addition, nasal-inactivated vaccines seem to be superior to the LAIV vaccines, because non-infectious preparations could be used in high-risk groups. Thus, the development of intranasal inactivated vaccines is recommended, because such vaccines are expected to improve the efficacy of influenza vaccines.

Intranasal Immunization Using Mannatide as a Novel Adjuvant for an Inactivated Influenza Vaccine and Its Adjuvant Effect Compared with MF59

Intranasal vaccination is more potent than parenteral injection for the prevention of influenza. However, because the poor efficiency of antigen uptake across the nasal mucosa is a key issue, immunostimulatory adjuvants are essential for intranasal vaccines. The immunomodulator mannatide or polyactin (PA) has been used for the clinical treatment of impaired immunity in China, but its adjuvant effect on an inactivated trivalent influenza vaccine (ITIV) via intranasal vaccination is unclear. To explore the adjuvant effect of PA, an inactivated trivalent influenza virus with or without PA or MF59 was instilled intranasally once a week in BALB/c mice. Humoral immunity was assessed by both the ELISA and hemagglutination inhibition (HI) methods using antigen-specific antibodies. Splenic lymphocyte proliferation and the IFN-γ level were measured to evaluate cell-mediated immunity. The post-vaccination serum HI antibody geometric mean titers (GMTs) for the H1N1 and H3N2 strains, antigen-specific serum IgG and IgA GMTs, mucosal SIgA GMT, splenic lymphocyte proliferation, and IFN-γ were significantly increased in the high-dose PA-adjuvanted vaccine group. The seroconversion rate and the mucosal response for the H3N2 strain were significantly elevated after high-dose PA administration. These adjuvant effects of high-dose PA for the influenza vaccine were comparable with those of the MF59 adjuvant, and abnormal signs or pathological changes were not found in the evaluated organs. In conclusion, PA is a novel mucosal adjuvant for intranasal vaccination with the ITIV that has safe and effective mucosal adjuvanticity in mice and successfully induces both serum and mucosal antibody responses and a cell-mediated response.

Intranasal Immunization of Mice to Avoid Interference of Maternal Antibody against H5N1 Infection

Maternally-derived antibodies (MDAs) can protect offspring against influenza virus infection but may also inhibit active immune responses. To overcome MDA- mediated inhibition, active immunization of offspring with an inactivated H5N1 whole-virion vaccine under the influence of MDAs was explored in mice. Female mice were vaccinated twice via the intraperitoneal (IP) or intranasal (IN) route with the vaccine prior to mating. One week after birth, the offspring were immunized twice via the IP or IN route with the same vaccine and then challenged with a lethal dose of a highly homologous virus strain. The results showed that, no matter which immunization route (IP or IN) was used for mothers, the presence of MDAs severely interfered with the active immune response of the offspring when the offspring were immunized via the IP route. Only via the IN immunization route did the offspring overcome the MDA interference. These results suggest that intranasal immunization could be a suitable inoculation route for offspring to overcome MDA interference in the defense against highly pathogenic H5N1 virus infection. This study may provide references for human and animal vaccination to overcome MDA-induced inhibition.

The effect of mucoadhesive excipient on the nasal retention time of and the antibody responses induced by an intranasal influenza vaccine

INTRODUCTION

Recently, we reported that intranasal vaccination of humans with whole inactivated influenza vaccine in the absence of mucosal adjuvant induced neutralizing antibody responses in the serum and nasal mucus. The mucoadhesive excipient carboxy-vinyl polymer (CVP) increases the viscosity and therefore mucoadhesiveness of intranasal medicaments and is an authorized excipient in Japan. In the present study, we analyzed the effect of adding CVP on intranasal whole inactivated influenza vaccine antigen dynamics and antibody responses.

METHODS

Mice and nonhuman primates (NHPs) were intranasally administered the [(18)F]-radiolabeled vaccine and subjected to positron emission tomography analysis for 6h. Dendritic cells were stimulated in vitro with the vaccine mixed with or without a mucosal adjuvant (Ampligen) and/or CVP, after which the tumor necrosis factor (TNF)-α and interferon (IFN)-β levels in the supernatants were measured. Cynomolgus monkeys were immunized intranasally with the vaccine mixed with Ampligen and/or CVP and their vaccine-specific serum IgG and IgA titers were measured on days 0 and 33.

RESULTS

The vaccine was retained significantly longer in the nasal cavity of both mice and NHPs when it was delivered with CVP rather than PBS. Accumulation of the radiolabeled vaccine in the central nervous system was not detected in either model regardless of whether CVP was used. CVP only very weakly increased the TNF-α production of vaccine-stimulated dendritic cells. IFN-β production was not observed regardless of the presence or absence of CVP. CVP increased the vaccine-specific IgA antibody responses of the intranasally vaccinated cynomolgus macaques.

CONCLUSION

CVP increased intranasal retention of whole inactivated influenza vaccine, did not promote antigen redirection to the central nervous system, and improved mucosal antibody responses. The mechanism probably relates to its mucoadhesive properties rather than its ability to directly stimulate the immune system. Intranasal vaccines with CVP may be a promising candidate vaccine formulation for humans.

Mucosal vaccines: novel strategies and applications for the control of pathogens and tumors at mucosal sites

The mucosal immune system displays several adaptations reflecting the exposure to the external environment. The efficient induction of mucosal immune responses also requires specific approaches, such as the use of appropriate administration routes and specific adjuvants and/or delivery systems. In contrast to vaccines delivered via parenteral routes, experimental, and clinical evidences demonstrated that mucosal vaccines can efficiently induce local immune responses to pathogens or tumors located at mucosal sites as well as systemic response. At least in part, such features can be explained by the compartmentalization of mucosal B and T cell populations that play important roles in the modulation of local immune responses. In the present review, we discuss molecular and cellular features of the mucosal immune system as well as novel immunization approaches that may lead to the development of innovative and efficient vaccines targeting pathogens and tumors at different mucosal sites.

Relationship of the quaternary structure of human secretory IgA to neutralization of influenza virus

Secretory IgA (S-IgA) antibodies, the major contributors to humoral mucosal immunity to influenza virus infection, are polymeric Igs present in many external secretions. In the present study, the quaternary structures of human S-IgA induced in nasal mucosa after administration of intranasal inactivated influenza vaccines were characterized in relation to neutralization potency against influenza A viruses. Human nasal IgA antibodies have been shown to contain at least five quaternary structures. Direct and real-time visualization of S-IgA using high-speed atomic force microscopy (AFM) demonstrated that trimeric and tetrameric S-IgA had six and eight antigen-binding sites, respectively, and that these structures exhibited large-scale asynchronous conformational changes while capturing influenza HA antigens in solution. Furthermore, trimeric, tetrameric, and larger polymeric structures, which are minor fractions in human nasal IgA, displayed increased neutralizing potency against influenza A viruses compared with dimeric S-IgA, suggesting that the larger polymeric than dimeric forms of S-IgA play some important roles in protection against influenza A virus infection in the human upper respiratory tract.

A lipidated form of the extracellular domain of influenza M2 protein as a self-adjuvanting vaccine candidate

The highly conserved extracellular domain of Matrix protein 2 (M2e) of influenza A virus has been previously investigated as a potential target for an universal influenza vaccine. In this study we prepared four lipopeptide influenza vaccine candidates in which the TLR2 agonist S-[2,3-bis(palmitoyloxy)propyl] cysteine, (Pam2Cys) was attached to either the N- or C-terminus of the M2e consensus sequence SLLTEVETPIRNEWGCRCNDSSDP and its analogue sequence with the two cysteine residues replaced with serine residues. The results of animal study show that each of these lipopeptides induced strong M2e-specific antibody responses in the absence of extraneous T helper cell epitope(s) which are normally incorporated in the previous studies or addition of extraneous adjuvant and that these antibodies are protective against lethal challenge with influenza virus. Comparison of different routes of inoculation demonstrated that intranasal administration of M2e lipopeptide induced higher titers of IgA and IgG2b antibodies in the bronchoalveolar lavage than did subcutaneous vaccination and was better at mitigating the severity of viral challenge. Finally, we show that anti-M2e antibody specificities absent from the antibody repertoire elicited by a commercially available influenza vaccine and by virus infection can be introduced by immunization with M2e-lipopeptide and boosted by viral challenge. Immunization with this lipidated form of the M2e epitope therefore offers a means of using a widely conserved epitope to generate protective antibodies which are not otherwise induced.

Choice and Design of Adjuvants for Parenteral and Mucosal Vaccines

The existence of pathogens that escape recognition by specific vaccines, the need to improve existing vaccines and the increased availability of therapeutic (non-infectious disease) vaccines necessitate the rational development of novel vaccine concepts based on the induction of protective cell-mediated immune responses. For naive T-cell activation, several signals resulting from innate and adaptive interactions need to be integrated, and adjuvants may interfere with some or all of these signals. Adjuvants, for example, are used to promote the immunogenicity of antigens in vaccines, by inducing a pro-inflammatory environment that enables the recruitment and promotion of the infiltration of phagocytic cells, particularly antigen-presenting cells (APC), to the injection site. Adjuvants can enhance antigen presentation, induce cytokine expression, activate APC and modulate more downstream adaptive immune reactions (vaccine delivery systems, facilitating immune Signal 1). In addition, adjuvants can act as immunopotentiators (facilitating Signals 2 and 3) exhibiting immune stimulatory effects during antigen presentation by inducing the expression of co-stimulatory molecules on APC. Together, these signals determine the strength of activation of specific T-cells, thereby also influencing the quality of the downstream T helper cytokine profiles and the differentiation of antigen-specific T helper populations (Signal 3). New adjuvants should also target specific (innate) immune cells in order to facilitate proper activation of downstream adaptive immune responses and homing (Signal 4). It is desirable that these adjuvants should be able to exert such responses in the context of mucosal administered vaccines. This review focuses on the understanding of the potential working mechanisms of the most well-known classes of adjuvants to be used effectively in vaccines.

Endocine™, N3OA and N3OASq; three mucosal adjuvants that enhance the immune response to nasal influenza vaccination

Annual outbreaks of seasonal influenza are controlled or prevented through vaccination in many countries. The seasonal vaccines used are either inactivated, currently administered parenterally, or live-attenuated given intranasally. In this study three mucosal adjuvants were examined for the influence on the humoral (mucosal and systemic) and cellular influenza A-specific immune responses induced by a nasally administered vaccine. We investigated in detail how the anionic Endocine™ and the cationic adjuvants N3OA and N3OASq mixed with a split inactivated influenza vaccine induced influenza A-specific immune responses as compared to the vaccine alone after intranasal immunization. The study showed that nasal administration of a split virus vaccine together with Endocine™ or N3OA induced significantly higher humoral and cell-mediated immune responses than the non-adjuvanted vaccine. N3OASq only significantly increased the cell-mediated immune response. Furthermore, nasal administration of the influenza vaccine in combination with any of the adjuvants; Endocine™, N3OA or N3OASq, significantly enhanced the mucosal immunity against influenza HA protein. Thus the addition of these mucosal adjuvants leads to enhanced immunity in the most relevant tissues, the upper respiratory tract and the systemic circulation. Nasal influenza vaccination with an inactivated split vaccine can therefore provide an important mucosal immune response, which is often low or absent after traditional parenteral vaccination.

Intranasal vaccination with an inactivated whole influenza virus vaccine induces strong antibody responses in serum and nasal mucus of healthy adults

Haemagglutination inhibition (HI) and neutralization (NT) titers as well as haemagglutinin (HA) specific antibody responses were examined in 50 healthy adults aged between 22 and 69 y old after two intranasal administrations of an inactivated whole virus vaccine derived from A/Victoria/210/2009 virus (45 μg HA per dose) at 3 week intervals. Serum HI titers after two-doses of the nasal vaccine showed >2.5-fold rise in the ratio of geometric mean titer upon vaccination, >40% of subjects with a ≥4-fold increase in titer and >70% of subjects with a titer of ≥1:40, all parameters associated with an effective outcome of vaccination in the criteria defined by the European Medicines Agency. Serum neutralizing antibody responses correlated with HI antibody responses, although NT titers were about 2-fold higher than HI titers. These high levels of serum responses were accompanied by high levels of HI and neutralizing antibody responses in nasal mucus as measured in concentrated nasal wash samples that were about 10 times diluted compared with natural nasal mucus. Serum and nasal HI and neutralizing antibody responses consisted of HA-specific IgG and IgA antibody responses, with IgG and IgA antibodies being dominant in serum and nasal responses, respectively.

Evaluation of the effectiveness and safety of chitosan derivatives as adjuvants for intranasal vaccines

Intranasal immunization is currently used to deliver live virus vaccines such as influenza. However, to develop an intranasal vaccine to deliver inactivated virus, a safe and effective adjuvant is necessary to enhance the mucosal immune response. Here, we demonstrate the effectiveness of a chitosan microparticle (1-20 μm, 50 kDa, degree of deacetylation=85%) and a cationized chitosan (1000 kDa, degree of deacetylation=85%) derived from natural crab shells as adjuvants for an intranasal vaccine candidate. We examined the effectiveness of chitosan derivatives as an adjuvant by co-administering them with ovalbumin (OVA) intranasally in BALB/c mice, polymeric Ig receptor knockout (pIgR-KO) mice, and cynomolgus monkeys (Macaca fascicularis). pIgR-KO mice were used to evaluate S-IgA production on the mucosal surface without nasal swab collection. Administration of OVA with chitosan microparticles or cationized chitosan induced a high OVA-specific IgA response in the serum of pIgR-KO mice and a high IgG response in the serum of BALB/c mice and cynomolgus monkeys. We also found that administration of chitosan derivatives did not have a detrimental effect on cynomolgus monkeys as determined by complete blood count, blood chemistries, and gross pathology results. These results suggest that chitosan derivatives are safe and effective mucosal adjuvants for intranasal vaccination.

[Study of next generation influenza vaccine focused on "cross-protection by mucosal immunization" and "seed virus strains"]

Endemic infection by seasonal influenza virus usually occurs every winter season. Inside the host, human influenza viruses frequently undergo various point mutations at antigenic regions, in response to antibody pressure. Furthermore, the influenza virus has undergone frequent antigenic shifts for at least 100 years, some of which have caused influenza pandemics. In Japan, intramuscular immunization with influenza split-virion vaccines has been used to prevent seasonal influenza virus infections. Unfortunately, the efficacy of the current influenza vaccine immunization method is limited, even against viruses belonging to the same clade. Furthermore, the current vaccines are not expected to be protective against antigenically shifted viruses. Therefore, new approaches to vaccine development are needed to protect human populations against a potential pandemic virus. We are studying novel influenza vaccine designs to resolve the above weaknesses of the current influenza vaccines. I will describe our vaccine studies, "Cross-protection by mucosal immunization," and, "Preparation of seed virus strains to produce vaccines for possible pandemic influenza," in this symposium.

Intranasal immunization with a formalin-inactivated human influenza A virus whole-virion vaccine alone and intranasal immunization with a split-virion vaccine with mucosal adjuvants show similar levels of cross-protection

The antigenicity of seasonal human influenza virus changes continuously; thus, a cross-protective influenza vaccine design needs to be established. Intranasal immunization with an influenza split-virion (SV) vaccine and a mucosal adjuvant induces cross-protection; however, no mucosal adjuvant has been assessed clinically. Formalin-inactivated intact human and avian viruses alone (without adjuvant) induce cross-protection against the highly pathogenic H5N1 avian influenza virus. However, it is unknown whether seasonal human influenza formalin-inactivated whole-virion (WV) vaccine alone induces cross-protection against strains within a subtype or in a different subtype of human influenza virus. Furthermore, there are few reports comparing the cross-protective efficacy of the WV vaccine and SV vaccine-mucosal adjuvant mixtures. Here, we found that the intranasal human influenza WV vaccine alone induced both the innate immune response and acquired immune response, resulting in cross-protection against drift variants within a subtype of human influenza virus. The cross-protective efficacy conferred by the WV vaccine in intranasally immunized mice was almost the same as that conferred by a mixture of SV vaccine and adjuvants. The level of cross-protective efficacy was correlated with the cross-reactive neutralizing antibody titer in the nasal wash and bronchoalveolar fluids. However, neither the SV vaccine with adjuvant nor the WV vaccine induced cross-reactive virus-specific cytotoxic T-lymphocyte activity. These results suggest that the intranasal human WV vaccine injection alone is effective against variants within a virus subtype, mainly through a humoral immune response, and that the cross-protection elicited by the WV vaccine and the SV vaccine plus mucosal adjuvants is similar.