An MF59-adjuvanted inactivated influenza vaccine containing A/Panama/1999 (H3N2) induced broader serological protection against heterovariant influenza virus strain A/Fujian/2002 than a subunit and a split influenza vaccine

Immunogenicity of Enhanced Influenza Vaccines Against Mismatched Influenza Strains in Older Adults: A Review of Randomized Controlled Trials

Antigenic drift is a major driver of viral evolution and a primary reason why influenza vaccines must be reformulated annually. Mismatch between vaccine and circulating viral strains negatively affects vaccine effectiveness and often contributes to higher rates of influenza-related hospitalizations and deaths, particularly in years dominated by A(H3N2). Several countries recommend enhanced influenza vaccines for older adults, who are at the highest risk of severe influenza complications and mortality. The immunogenicity of enhanced vaccines against heterologous A(H3N2) strains has been examined in nine studies to date. In six studies, an enhanced, licensed MF59-adjuvanted trivalent inactivated influenza vaccine (aIIV3) consistently increased heterologous antibody titers relative to standard influenza vaccine, with evidence of a broad heterologous immune response across multiple genetic clades. In one study, licensed high-dose trivalent inactivated influenza vaccine (HD-IIV3) also induced higher heterologous antibody titers than standard influenza vaccine. In a study comparing a higher dose licensed quadrivalent recombinant influenza vaccine (RIV4) with HD-IIV3 and aIIV3, no significant differences in antibody titers against a heterologous strain were observed, although seroconversion rates were higher with RIV4 versus comparators. With the unmet medical need for improved influenza vaccines, the paucity of studies especially with enhanced vaccines covering mismatched strains highlights a need for further investigation of cross-protection in older adults.

Immunosenescence and Inflamm-Aging: Clinical Interventions and the Potential for Reversal of Aging

Inflammation is often associated with the impairment of the ability to sustain the consequences of the physical, chemical, nutritional, and antigenic triggers of inflammation. The process of immunosenescence may only partially be explained by the senescence of cells, tissues, or the organism, and, hence, the hallmarks of immunosenescence may be markedly and differentially affected by the history of an individual's pathogenic encounter. Inflammation is a key component of immunosenescence, which itself is a direct consequence of aging. This review article highlights the therapeutic interventions for slowing the processes of inflamm-aging and immunosenescence and the possible reversal of aging and includes domains of immunomodulatory interventions, vaccination strategies, nutritional interventions, stem cell therapies, personalized medicine, microbiome interventions, and the positive effects of physical activity and exercise.

Vaccine Effectiveness of non-adjuvanted and adjuvanted trivalent inactivated influenza vaccines in the prevention of influenza-related hospitalization in older adults: A pooled analysis from the Serious Outcomes Surveillance (SOS) Network of the Canadian Immunization Research Network (CIRN)

BACKGROUND

Influenza vaccines prevent influenza-related morbidity and mortality; however, suboptimal vaccine effectiveness (VE) of non-adjuvanted trivalent inactivated influenza vaccine (naTIV) or quadrivalent formulations in older adults prompted the use of enhanced products such as adjuvanted TIV (aTIV). Here, the VE of aTIV is compared to naTIV for preventing influenza-associated hospitalization among older adults.

METHODS

A test-negative design study was used with pooled data from the 2012 to 2015 influenza seasons. An inverse probability of treatment (IPT)-weighted logistic regression estimated the Odds Ratio (OR) for laboratory-confirmed influenza-associated hospitalization. VE was calculated as (1-OR)*100% with accompanying 95% confidence intervals (CI).

RESULTS

Of 7,101 adults aged ≥ 65, 3,364 received naTIV and 526 received aTIV. The overall VE against influenza hospitalization was 45.9% (95% CI: 40.2%-51.1%) for naTIV and 53.5% (42.8%-62.3%) for aTIV. No statistically significant differences in VE were found between aTIV and naTIV by age group or influenza season, though a trend favoring aTIV over naTIV was noted. Frailty may have impacted VE in aTIV recipients compared to those receiving naTIV, according to an exploratory analysis; VE adjusted by frailty was 59.1% (49.6%-66.8%) for aTIV and 44.8% (39.1%-50.0%) for naTIV. The overall relative VE of aTIV to naTIV against laboratory-confirmed influenza hospital admission was 25% (OR 0.75; 0.61-0.92), demonstrating statistically significant benefit favoring aTIV.

CONCLUSIONS

Adjusting for frailty, aTIV showed statistically significantly better protection than naTIV against influenza-associated hospitalizations in older adults. In future studies, it is important to consider frailty as a significant confounder of VE.

Immunogenicity and safety of an egg culture-based quadrivalent inactivated non-adjuvanted subunit influenza vaccine in subjects ≥3 years: A randomized, multicenter, double-blind, active-controlled phase III, non-inferiority trial

Subunit influenza vaccine only formulated with surface antigen proteins has better safety profiles relative to split-virion influenza vaccine. Compared to the traditional quadrivalent split-virion influenza vaccine, a novel quadrivalent subunit influenza vaccine is urgently needed in China. We completed a phase 3, randomized, double-blind, active-controlled, non-inferiority clinical study at two sites in Henan Province, China. Eligible volunteers were split into four age cohorts (3-8 years, 9-17 years, 18-64 years, and ≥ 65 years, based on their dates of birth) and randomly assigned (1:1) to the subunit and the split-virion ecNAIIV4 groups. All volunteers were intramuscularly administered a single vaccine dose at baseline, and children aged 3-8 years received a boosting dose at day 28. And the immune response was evaluated by measuring hemagglutinin-inhibition antibody titers against the four vaccine strains in blood samples. Safety profiles had nonsignificant differences between the study groups in ≥ 3 years cohort. Most adverse reactions post-vaccination, both local and systemic, were mild to moderate and resolved within 3 days. And no serious adverse events occurred. The immunogenicity of the trial vaccine was non-inferior to the comparator. Further, a two-dose vaccine series can provide better seroprotection than that of a one-dose series in children aged 3-8 years, with clinically acceptable safety profiles. Clinical Trials Registration. ChiCTR2100049934.

Resolving adjuvant mode of action to enhance vaccine efficacy

Adjuvants are a miscellaneous range of molecules and materials that can enhance the magnitude, functionality, breadth and durability of immune responses. Despite the multiplicity of compounds with adjuvant properties, less than a dozen are in clinical use in vaccines against infectious diseases. While many factors have contributed to their slow development, among the major challenges are the high safety and efficacy standards set by current adjuvants in human vaccines and our limited understanding of how adjuvants mediate their effects. This review outlines why it is so difficult to elucidate their mechanism of action, highlights areas that require in-depth research and discusses recent advancements that are revitalising adjuvant development. It is hoped that a fuller understanding of adjuvant sensing, signalling and function will facilitate the design of vaccines that promote sustained protective immunity against challenging bacterial and viral pathogens.

Phase 3 Randomized, Multicenter, Placebo-Controlled Study to Evaluate Safety, Immunogenicity, and Lot-to-Lot Consistency of an Adjuvanted Cell Culture-Derived, H5N1 Subunit Influenza Virus Vaccine in Healthy Adult Subjects

A cell-based process may be better suited for vaccine production during a highly pathogenic avian influenza (HPAI) pandemic. This was a phase 3, randomized, controlled, observer-blind, multicenter study evaluated safety, immunogenicity, and lot-to-lot consistency of two doses of a MF59-adjuvanted, H5N1 influenza pandemic vaccine manufactured on a cell culture platform (aH5N1c) in 3196 healthy adult subjects, stratified into two age groups: 18 to <65 and ≥65 years. Immunogenicity was measured using hemagglutination inhibition (HI) titers. HI antibody responses increased after the first aH5N1c vaccine dose, and 3 weeks after the second vaccination (Day 43), age-appropriate US Center for Biologics Evaluation and Research (CBER) and former European Medicines Authority Committee for Medicinal Products for Human Use (EMA CHMP) immunogenicity criteria were met. Six months after the first vaccination, HI titers were above baseline but no longer met CBER and CHMP criteria. No relevant changes over time were seen in placebo subjects. Solicited AEs were more frequent in the active treatment than the placebo group, primarily due to injection site pain. No serious adverse events (SAEs) related to aH5N1c- were reported. aH5N1c influenza vaccine elicited high levels of antibodies following two vaccinations administered 21 days apart and met both CBER and former CHMP immunogenicity criteria at Day 43 among both younger and older adults with a clinically acceptable safety profile. Consistency of the three consecutive aH5N1c vaccine lots was demonstrated (NCT02839330).

Opposing Effects of Prior Infection versus Prior Vaccination on Vaccine Immunogenicity against Influenza A(H3N2) Viruses

Prior vaccination can alternately enhance or attenuate influenza vaccine immunogenicity and effectiveness. Analogously, we found that vaccine immunogenicity was enhanced by prior A(H3N2) virus infection among participants of the Ha Nam Cohort, Viet Nam, but was attenuated by prior vaccination among Australian Health Care Workers (HCWs) vaccinated in the same year. Here, we combined these studies to directly compare antibody titers against 35 A(H3N2) viruses spanning 1968–2018. Participants received licensed inactivated vaccines containing A/HongKong/4801/2014 (H3N2). The analysis was limited to participants aged 18–65 Y, and compared those exposed to A(H3N2) viruses circulating since 2009 by infection (Ha Nam) or vaccination (HCWs) to a reference group who had no recent A(H3N2) infection or vaccination (Ha Nam). Antibody responses were compared by fitting titer/titer-rise landscapes across strains, and by estimating titer ratios to the reference group of 2009–2018 viruses. Pre-vaccination, titers were lowest against 2009–2014 viruses among the reference (no recent exposure) group. Post-vaccination, titers were, on average, two-fold higher among participants with prior infection and two-fold lower among participants with 3–5 prior vaccinations compared to the reference group. Titer rise was negligible among participants with 3–5 prior vaccinations, poor among participants with 1–2 prior vaccinations, and equivalent or better among those with prior infection compared to the reference group. The enhancing effect of prior infection versus the incrementally attenuating effect of prior vaccinations suggests that these exposures may alternately promote and constrain the generation of memory that can be recalled by a new vaccine strain.

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.

Administration of Multivalent Influenza Virus Recombinant Hemagglutinin Vaccine in Combination-Adjuvant Elicits Broad Reactivity Beyond the Vaccine Components

Combining variant antigens into a multivalent vaccine is a traditional approach used to provide broad coverage against antigenically variable pathogens, such as polio, human papilloma and influenza viruses. However, strategies for increasing the breadth of antibody coverage beyond the vaccine are not well understood, but may provide more anticipatory protection. Influenza virus hemagglutinin (HA) is a prototypic variant antigen. Vaccines that induce HA-specific neutralizing antibodies lose efficacy as amino acid substitutions accumulate in neutralizing epitopes during influenza virus evolution. Here we studied the effect of a potent combination adjuvant (CpG/MPLA/squalene-in-water emulsion) on the breadth and maturation of the antibody response to a representative variant of HA subtypes H1, H5 and H7. Using HA protein microarrays and antigen-specific B cell labelling, we show when administered individually, each HA elicits a cross-reactive antibody profile for multiple variants within the same subtype and other closely-related subtypes (homosubtypic and heterosubtypic cross-reactivity, respectively). Despite a capacity for each subtype to induce heterosubtypic cross-reactivity, broader coverage was elicited by simply combining the subtypes into a multivalent vaccine. Importantly, multiplexing did not compromise antibody avidity or affinity maturation to the individual HA constituents. The use of adjuvants to increase the breadth of antibody coverage beyond the vaccine antigens may help future-proof vaccines against newly-emerging variants.

Engineered Nanoparticle Applications for Recombinant Influenza Vaccines

Influenza viruses cause seasonal epidemics and represent a pandemic risk. With current vaccine methods struggling to protect populations against emerging strains, there is a demand for a next-generation flu vaccine capable of providing broad protection. Recombinant biotechnology, combined with nanomedicine techniques, could address this demand by increasing immunogenicity and directing immune responses toward conserved antigenic targets on the virus. Various nanoparticle candidates have been tested for use in vaccines, including virus-like particles, protein and carbohydrate nanoconstructs, antigen-carrying lipid particles, and synthetic and inorganic particles modified for antigen presentation. These methods have yielded some promising results, including protection in animal models against antigenically distinct influenza strains, production of antibodies with broad reactivity, and activation of potent T cell responses. Based on the evidence of current research, it is feasible that the next generation of influenza vaccines will combine recombinant antigens with nanoparticle carriers.

Self-Assembled Nanoparticles: Exciting Platforms for Vaccination

Vaccination is successfully advanced to control several fatal diseases and improve human life expectancy. However, additional innovations are required in this field because there are no effective vaccines to prevent some infectious diseases. The shift from the attenuated or inactivated pathogens to safer but less immunogenic protein or peptide antigens has led to a search for effective antigen delivery carriers that can function as both antigen vehicles and intrinsic adjuvants. Among these carriers, self-assembled nanoparticles (SANPs) have shown great potential to be the best representative. For the nanoscale and multiple presentation of antigens, with accurate control over size, geometry, and functionality, these nanoparticles are assembled spontaneously and mimic pathogens, resulting in enhanced antigen presentation and increased cellular and humoral immunity responses. In addition, they may be applied through needle-free routes due to their adhesive ability, which gives them a great future in vaccination applications. This review provides an overview of various SANPs and their applications in prophylactic vaccines.

A Lipid/DNA Adjuvant-Inactivated Influenza Virus Vaccine Protects Rhesus Macaques From Uncontrolled Virus Replication After Heterosubtypic Influenza A Virus Challenge

Background

Influenza A virus (IAV) vaccines offer little protection from mismatched viruses with antigenically distant hemagglutinin (HA) glycoproteins. We sought to determine if a cationic lipid/DNA complex (CLDC) adjuvant could induce heterosubtypic protection if added to a whole inactivated IAV vaccine (WIV).

Methods

Adult rhesus macaques (RMs) were vaccinated and at 2 weeks boosted with either an H1N1-WIV or an H3N2-WIV, with and without CLDC adjuvant. Four weeks postboost, animals were challenged with an H1N1 IAV matched to the H1N1-WIV vaccine.

Results

After challenge, viral RNA (vRNA) levels in the trachea of control RMs and RMs vaccinated with the unadjuvanted H1 or H3 WIV vaccines were similar. However, vRNA levels in the trachea of both the H1-WIV/CLDC- and the H3-WIV/CLDC-vaccinated RMs (P < 0.01 and P < 0.05, respectively) were significantly lower than in unvaccinated control RMs. Heterosubtypic protection in H3-WIV/CLDC RMs was associated with significantly higher levels of nucleoprotein (NP) and matrix-1-specific immunoglobulin G antibodies (P < 0.05) and NP-specific nonneutralizing antibody-dependent natural killer cell activation (P < 0.01) compared with unprotected H3-WIV RMs.

Conclusions

Addition of the CLDC adjuvant to a simple WIV elicited immunity to conserved virus structural proteins in RMs that correlate with protection from uncontrolled virus replication after heterosubtypic influenza virus challenge.

Extending the Breadth of Influenza Vaccines: Status and Prospects for a Universal Vaccine

Despite the widespread use of seasonal influenza vaccines, there is urgent need for a universal influenza vaccine to provide broad, long-term protection. A number of factors underpin this urgency, including threats posed by zoonotic and pandemic influenza A viruses, suboptimal effectiveness of seasonal influenza vaccines, and concerns surrounding the effects of annual vaccination. In this article, we discuss approaches that are being investigated to increase influenza vaccine breadth, which are near-term, readily achievable approaches to increase the range of strains recognized within a subtype, or longer-term more challenging approaches to produce a truly universal influenza vaccine. Adjuvanted and neuraminidase-optimized vaccines are emerging as the most feasible and promising approaches to extend protection to cover a broader range of strains within a subtype. The goal of developing a universal vaccine has also been advanced with the design of immunogenic influenza HA-stem constructs that induce broadly neutralizing antibodies. However, these constructs are not yet sufficiently immunogenic to induce lasting universal immunity in humans. Advances in understanding how T cells mediate protection, and how viruses are packaged, have facilitated the rationale design and delivery of replication-incompetent virus vaccines that induce broad protection mediated by lung-resident memory T cells. While the lack of clear mechanistic correlates of protection, other than haemagglutination-inhibiting antibodies, remains an impediment to further advancing novel influenza vaccines, the pressing need for such a vaccine is supporting development of highly innovative and effective strategies.

Immunosenescence: A systems-level overview of immune cell biology and strategies for improving vaccine responses

Immunosenescence contributes to a decreased capacity of the immune system to respond effectively to infections or vaccines in the elderly. The full extent of the biological changes that lead to immunosenescence are unknown, but numerous cell types involved in innate and adaptive immunity exhibit altered phenotypes and function as a result of aging. These manifestations of immunosenescence at the cellular level are mediated by dysregulation at the genetic level, and changes throughout the immune system are, in turn, propagated by numerous cellular interactions. Environmental factors, such as nutrition, also exert significant influence on the immune system during aging. While the mechanisms that govern the onset of immunosenescence are complex, systems biology approaches allow for the identification of individual contributions from each component within the system as a whole. Although there is still much to learn regarding immunosenescence, systems-level studies of vaccine responses have been highly informative and will guide the development of new vaccine candidates, novel adjuvant formulations, and immunotherapeutic drugs to improve vaccine responses among the aging population.

Summary of the NACI literature review on the comparative effectiveness of subunit and split virus inactivated influenza vaccines in older adults

Background

Subunit and split virus inactivated influenza vaccines (IIV) are two commonly used types of seasonal influenza vaccines in Canada. The comparative effectiveness of these two formulations is particularly relevant for older adults, as older adults have reduced influenza vaccine effectiveness and experience more severe influenza than younger adults.

Objective

To compare the vaccine effectiveness and immunogenicity of unadjuvanted, standard-dose subunit IIVs versus unadjuvanted, standard-dose split virus IIVs in adults 65 years of age and older.

Methods

An a priori written protocol based on rapid review methods was developed that included studies published in 2007 or later in the EMBASE, MEDLINE and ClinicalTrials.gov databases with terms used in the objective. Due to the small number of records returned, hand searches of reference lists were completed, the publication date limit was removed, three additional databases (the Cochrane Central Register of Controlled Trials, Scopus and Web of Science) were searched, and studies including adults 60 years of age and older were included. Data from included studies were extracted into evidence tables and quality assessments were completed. The results were synthesized narratively.

Results

Eight eligible studies were identified. In the three studies that assessed vaccine effectiveness of subunit and split virus IIVs, there were no statistically significant differences in vaccine effectiveness in adults 65 years of age and older against laboratory-confirmed infection with any influenza virus strain, or against laboratory-confirmed infection with influenza A(H1N1), A(H3N2) or B virus, specifically. In the five studies that assessed immunogenicity, the findings were not consistent and the overall quality of immunogenicity evidence was weak.

Conclusion

The National Advisory Committee on Immunization (NACI) concludes that there is insufficient evidence to determine significant differences in the vaccine effectiveness or immunogenicity of unadjuvanted, standard-dose subunit and split virus IIVs in adults 65 years of age and older (Grade I evidence).

The appropriateness of the use of influenza vaccines: Recommendations from the latest seasons in Italy

Influenza is one of the major infectious causes of excess mortality, hospitalization, and an increase in healthcare expenditure in all countries. In an increasingly ageing population, many members are exposed to flu-related complications. Vaccination coverage rates for the elderly in most European countries, such as Italy, are not satisfactory, and have been decreasing with time due to a sense of skepticism toward vaccination. Nowadays, many types of vaccines are available on the Italian market to prevent influenza illness. Many studies have proven their effectiveness in preventing influenza-related complications in specific risk groups. Any vaccine can be crucial to avoid complications, hospitalizations and death, but use of the most appropriate vaccine could optimize the result at a very modest cost. General practitioners (GPs) should encourage their patients to take the influenza vaccination to prevent complications or deaths. Health authorities should give GPs the opportunity to choose the appropriate vaccines tailored to specific patients.

Vaccines for the elderly: current use and future challenges

Age-related changes of the immune system contribute to increased incidence and severity of infections in the elderly. Vaccination is the most effective measure to prevent infections and vaccination recommendations in most countries include specific guidelines for the elderly. Vaccination against influenza and Streptococcus pneumoniae is usually recommended for persons with underlying diseases and for the elderly with heterogeneous age limits between ≥ 50 years and ≥ 65 years. Some countries also recommend vaccination against herpes zoster. Several vaccines are recommended for all adults, such as regular booster shots against tetanus/diphtheria/pertussis/polio, or for specific groups, e.g. vaccination against tick-borne encephalitis in endemic areas or travel vaccines. These are also relevant for the elderly. Most currently used vaccines are less immunogenic and effective in the elderly compared to younger adults. Potential strategies to improve their immunogenicity include higher antigen dose, alternative routes of administration, and the use of adjuvants, which were all implemented for influenza vaccines, and induce moderately higher antibody concentrations. Research on universal vaccines against influenza and S. pneumoniae is ongoing in order to overcome the limitations of the current strain-specific vaccines. Respiratory syncytial virus causes significant morbidity in the elderly. Novel vaccines against this and other pathogens, for instance bacterial nosocomial infections, have tremendous potential impact on health in old age and are intensively studied by many academic and commercial organizations. In addition to novel vaccine developments, it is crucial to increase awareness for the importance of vaccination beyond the pediatric setting, as vaccination coverage is still far from optimal for the older population.

AS03- and MF59-Adjuvanted Influenza Vaccines in Children

Influenza is a major cause of respiratory disease leading to hospitalization in young children. However, seasonal trivalent influenza vaccines (TIVs) have been shown to be ineffective and poorly immunogenic in this population. The development of live-attenuated influenza vaccines and adjuvanted vaccines are important advances in the prevention of influenza in young children. The oil-in-water emulsions MF59 and adjuvant systems 03 (AS03) have been used as adjuvants in both seasonal adjuvanted trivalent influenza vaccines (ATIVs) and pandemic monovalent influenza vaccines. Compared with non-adjuvanted vaccine responses, these vaccines induce a more robust and persistent antibody response for both homologous and heterologous influenza strains in infants and young children. Evidence of a significant improvement in vaccine efficacy with these adjuvanted vaccines resulted in the use of the monovalent (A/H1N1) AS03-adjuvanted vaccine in children in the 2009 influenza pandemic and the licensure of the seasonal MF59 ATIV for children aged 6 months to 2 years in Canada. The mechanism of action of MF59 and AS03 remains unclear. Adjuvants such as MF59 induce proinflammatory cytokines and chemokines, including CXCL10, but independently of type-1 interferon. This proinflammatory response is associated with improved recruitment, activation and maturation of antigen presenting cells at the injection site. In young children MF59 ATIV produced more homogenous and robust transcriptional responses, more similar to adult-like patterns, than did TIV. Early gene signatures characteristic of the innate immune response, which correlated with antibody titers were also identified. Differences were detected when comparing child and adult responses including opposite trends in gene set enrichment at day 3 postvaccination and, unlike adult data, a lack of correlation between magnitude of plasmablast response at day 7 and antibody titers at day 28 in children. These insights show the utility of novel approaches in understanding new adjuvants and their importance for developing improved influenza vaccines for children.

Universal influenza vaccines: Shifting to better vaccines

•

Current influenza vaccines are not broadly protective.

•

Developing countries suffer the highest morbidity and mortality due to influenza.

•

Correlates of protection and a regulatory pathway must be identified.

•

Over 40 universal influenza vaccine candidates are currently in development.

Influenza virus causes acute upper and lower respiratory infections and is the most likely, among known pathogens, to cause a large epidemic in humans. Influenza virus mutates rapidly, enabling it to evade natural and vaccine-induced immunity. Furthermore, influenza viruses can cross from animals to humans, generating novel, potentially pandemic strains. Currently available influenza vaccines induce a strain specific response and may be ineffective against new influenza viruses. The difficulty in predicting circulating strains has frequently resulted in mismatch between the annual vaccine and circulating viruses. Low-resource countries remain mostly unprotected against seasonal influenza and are particularly vulnerable to future pandemics, in part, because investments in vaccine manufacturing and stockpiling are concentrated in high-resource countries. Antibodies that target conserved sites in the hemagglutinin stalk have been isolated from humans and shown to confer protection in animal models, suggesting that broadly protective immunity may be possible. Several innovative influenza vaccine candidates are currently in preclinical or early clinical development. New technologies include adjuvants, synthetic peptides, virus-like particles (VLPs), DNA vectors, messenger RNA, viral vectors, and attenuated or inactivated influenza viruses. Other approaches target the conserved exposed epitope of the surface exposed membrane matrix protein M2e. Well-conserved influenza proteins, such as nucleoprotein and matrix protein, are mainly targeted for developing strong cross-protective T cell responses. With multiple vaccine candidates moving along the testing and development pipeline, the field is steadily moving toward a product that is more potent, durable, and broadly protective than previously licensed vaccines.

Effect of vaccine administration modality on immunogenicity and efficacy

The many factors impacting the efficacy of a vaccine can be broadly divided into three categories: features of the vaccine itself, including immunogen design, vaccine type, formulation, adjuvant and dosing; individual variations among vaccine recipients and vaccine administration-related parameters. While much literature exists related to vaccines, and recently systems biology has started to dissect the impact of individual subject variation on vaccine efficacy, few studies have focused on the role of vaccine administration-related parameters on vaccine efficacy. Parenteral and mucosal vaccinations are traditional approaches for licensed vaccines; novel vaccine delivery approaches, including needless injection and adjuvant formulations, are being developed to further improve vaccine safety and efficacy. This review provides a brief summary of vaccine administration-related factors, including vaccination approach, delivery route and method of administration, to gain a better understanding of their potential impact on the safety and immunogenicity of candidate vaccines.

Advances in aluminum hydroxide-based adjuvant research and its mechanism

In the past few decades, hundreds of materials have been tried as adjuvant; however, only aluminum-based adjuvants continue to be used widely in the world. Aluminum hydroxide, aluminum phosphate and alum constitute the main forms of aluminum used as adjuvants. Among these, aluminum hydroxide is the most commonly used chemical as adjuvant. In spite of its wide spread use, surprisingly, the mechanism of how aluminum hydroxide-based adjuvants exert their beneficial effects is still not fully understood. Current explanations for the mode of action of aluminum hydroxide-based adjuvants include, among others, the repository effect, pro-phagocytic effect, and activation of the pro-inflammatory NLRP3 pathway. These collectively galvanize innate as well as acquired immune responses and activate the complement system. Factors that have a profound influence on responses evoked by aluminum hydroxide-based adjuvant applications include adsorption rate, strength of the adsorption, size and uniformity of aluminum hydroxide particles, dosage of adjuvant, and the nature of antigens. Although vaccines containing aluminum hydroxide-based adjuvants are beneficial, sometimes they cause adverse reactions. Further, these vaccines cannot be stored frozen. Until recently, aluminum hydroxide-based adjuvants were known to preferentially prime Th2-type immune responses. However, results of more recent studies show that depending on the vaccination route, aluminum hydroxide-based adjuvants can enhance both Th1 as well as Th2 cellular responses. Advances in systems biology have opened up new avenues for studying mechanisms of aluminum hydroxide-based adjuvants. These will assist in scaling new frontiers in aluminum hydroxide-based adjuvant research that include improvement of formulations, use of nanoparticles of aluminum hydroxide and development of composite adjuvants.

Immunogenicity of intramuscular MF59-adjuvanted and intradermal administered influenza enhanced vaccines in subjects aged over 60: A literature review

Because of the age-related immune system decline, 2 potentiated influenza vaccines were specifically licensed for the elderly: Fluad(®), an MF59-adjuvanted vaccine administered intramuscularly (IM-MF59), and Intanza 15 mcg(®), a non adjuvanted vaccine administered intradermally (ID). The objective of this paper was to conduct a systematic review of studies that evaluated antibody responses in the elderly following immunization with IM-MF59 or ID vaccines. The two potentiated vaccines induced immune responses satisfying, in most instances, the European Medicine Agency immunogenicity criteria, both against vaccine antigens and heterovariant drifted strains. Considering pooled data reported in the articles analyzed and papers directly comparing the 2 vaccines, the antibody responses elicited by IM-MF59 and ID were found to be generally comparable. The use of IM-MF59 and ID vaccines can be proposed as an appropriate strategy for elderly seasonal influenza vaccination although further studies are required for a more complete characterization of the 2 vaccines.

The safety and immunogenicity of a MF59-adjuvanted H5N1 prepandemic influenza vaccine in healthy adults primed with homologous or heterologous H5N1 vaccines: an observational study

Background

World Health Organization (WHO) has recommended individuals with increased risk of contracting influenza A H5N1 infection to be immunized against the virus during the inter-pandemic period. Safety and immunogenicity of H5N1 vaccine among participants primed with homologous or heterologous H5N1 vaccines produced by diverse manufactures have not been reported.

Methods

Healthy individuals aged 20 to 60 years old were recruited and stratified into three groups: participants without priming (control group), participants primed with A/Indonesia/05/2005 vaccine, participants primed with A/Vietnam/1194/2004 vaccine and A/Indonesia/05/2005 vaccine. Enrolled participants received two doses of MF59-adjuvanted A/Vietnam/1194/2004 vaccine (study vaccine). Solicited reactions were recorded by vaccine recipients. Blood samples were obtained for hemagglutination inhibition test.

Results

A total of 131 participants were enrolled. No significant adverse events were recorded. Tenderness, fatigue and general muscle ache were the most common solicited reactions which alleviated within one week of immunization. Three weeks after two doses of the study vaccine, 63%, 68% and 88% were in seroprotective status in the control group, A/Indonesia/05/2005 primed group and A/Vietnam/1194/2004 and A/Indonesia/05/2005 primed group, respectively. Participants primed with A/Vietnam/1194/2004 and A/Indonesia/05/2005 showed high immune response after booster with one dose of the study vaccine.

Conclusion

The study vaccine did not cause severe adverse events. It elicited mostly mild to moderate reactions among participants. Participants primed with A/Vietnam/1194/2004 and A/Indonesia/05/2005 vaccine showed higher immune response than those without priming or primed with A/Indonesia/05/2005 vaccine. The report suggested those with an increased risk of influenza A H5N1 virus exposure may benefit from receiving influenza A H5N1 priming during the inter-pandemic period if the antigenicity of the pandemic influenza strain is similar to that of the priming strain.

Electronic supplementary material

The online version of this article (doi:10.1186/s12879-014-0587-z) contains supplementary material, which is available to authorized users.

Comparison of the safety and immunogenicity of an MF59®-adjuvanted with a non-adjuvanted seasonal influenza vaccine in elderly subjects

AIM

Adjuvanted influenza vaccines can overcome the poor antibody response of conventional non-adjuvanted vaccines in the elderly. We evaluated the immunogenicity, safety and clinical effectiveness of an MF59(®)-adjuvanted trivalent influenza vaccine (aTIV) compared with a non-adjuvanted vaccine (TIV) in subjects ≥65 years old, with or without co-morbidities.

METHODS

In 2010-2011, subjects (N=7082) were randomized to receive one dose of aTIV or TIV. Co-primary objectives were to assess lot-to-lot consistency of aTIV, non-inferiority, superiority and immunogenicity 22 days after vaccination. Clinical effectiveness, reactogenicity and serious adverse events were monitored up to Day 366.

RESULTS

The immunological equivalence of three lots of aTIV was demonstrated. aTIV was not only non-inferior to TIV but also elicited significantly higher antibody responses at Day 22 than TIV against all homologous and heterologous strains, even in subjects with co-morbidities. Superiority was not established. Reactogenicity was higher in the aTIV group, but reactions were mild to moderate and transient.

CONCLUSIONS

aTIV elicited a significantly higher antibody response than TIV, especially against A/H3N2 strains, although superiority by pre-defined criteria was not formally met. The study demonstrates potential immunological benefits of MF59-adjuvanted influenza vaccines for the elderly. This trial was registered with www.clinicaltrials.gov (NCT01162122).

Long-term and cross-reactive immunogenicity of inactivated trivalent influenza vaccine in the elderly: MF59-adjuvanted vaccine versus unadjuvanted vaccine

Elderly people are at great risk for influenza-related serious complications. However, influenza vaccine-induced antibodies are believed to decline more rapidly in the elderly. This study was designed to evaluate the long-term and cross-reactive immunogenicity among those aged ≥65 years for two seasonal trivalent influenza vaccines during the 2009-2010 influenza season. One vaccine had the MF59 adjuvant, while the other did not contain an adjuvant. Serum hemagglutinin inhibition (HI) titers were determined pre-vaccination and at 1 and 6 months post-vaccination. Of the 100 subjects, 95 (95%) were followed-up for 1 month after vaccination, and 76 (76%) were followed-up for 6 months after vaccination. Both vaccines met the European Medicines Agency (EMA) criteria 1 month after vaccination. However, seroprotection for influenza B was not satisfactory, with a rate of 55.3% for the MF59 adjuvant vaccine and 47.9% for the vaccine without adjuvant. At 6 months post-vaccination, the MF59-adjuvanted vaccine showed a higher seroprotection rate than the unadjuvanted vaccine. At this point, the MF59-adjuvanated vaccine still met the criteria of EMA for A/H1N1 (62.5% vs. 55.5%, P = 0.64) and A/H3N2 (72.5% vs. 47.2%, P = 0.04). Both vaccines showed excellent cross-reactive immunogenicity for influenza A/Solomon Island/3/2006 (H1N1) and A/Wisconsin/67/2005 (H3N2), without significant differences. In comparison, cross-reactive immunogenicity was not remarkable for the A/California/7/2009 (H1N1) and A/New Caledonia/20/1999 (H1N1) strains, which have a greater antigenic distance. In conclusion, the MF59-adjuvanted influenza vaccine showed superior long-term immunogenicity in the elderly compared to the unadjuvanted vaccine. However, cross-reactive immunogenicity was not remarkably enhanced with the MF59 adjuvant.

CD4 T cell defects in the aged: causes, consequences and strategies to circumvent

Aging leads to reduced immunity, especially adaptive responses. A key deficiency is the poor ability to mount robust antibody response. Although intrinsic alterations in B cells with age are in part responsible, impaired CD4 T cell help makes a major contribution to the poor antibody response. Other CD4 effector responses and memory generation are also impaired. We find delayed and reduced development of CD4 T follicular help (Tfh) cells in aged mice in response to influenza infection with reduction of long-lived plasma cells. When we examine CD4 subsets we also find a shift towards Th1 and cytotoxic CD4 (ThCTL) responses. We summarize strategies to circumvent the CD4 T cell defect in aged, including adjuvants and proinflammatory cytokines. We find that we can strongly enhance responses of aged naïve CD4 T cells by using Toll-like receptor (TLR) activated dendritic cells (DC) as APC in vivo and that this leads to improved germinal center B cells and IgG antibody responses. The enhanced response of aged naïve CD4 T cells is dependent on IL-6 produced by the DC.

Inactivated and adjuvanted influenza vaccines

Inactivated influenza vaccines are produced every year to fight against the seasonal epidemics of influenza. Despite the nonoptimal coverage, even in subjects at risk like the elderly, pregnant women, etc., these vaccines significantly reduce the burden of mortality and morbidity linked to the influenza infection. Importantly, these vaccines have also contributed to reduce the impact of the last pandemics. Nevertheless, the performance of these vaccines can be improved mainly in those age groups, like children and the elderly, in which their efficacy is suboptimal. The use of adjuvants has proven effective to this scope. Oil-in-water adjuvants like MF59 and AS03 have been licensed and widely used, and shown efficacious in preventing influenza infection in the last pandemic. MF59-adjuvanted inactivated vaccine was more efficacious than non-adjuvanted vaccine in preventing influenza infection in young children and in reducing hospitalization due to the influenza infection in the elderly. Other adjuvants are now at different stages of development and some are being tested in clinical trials. The perspective remains to improve the way inactivated vaccines are prepared and to accelerate their availability, mainly in the case of influenza pandemics, and to enhance their efficacy/effectiveness for a more successful impact at the public health level.

Influenza vaccines: unmet needs and recent developments

Influenza is a worldwide public health concern. Since the introduction of trivalent influenza vaccine in 1978, vaccination has been the primary means of prevention and control of influenza. Current influenza vaccines have moderate efficacy, good safety, and acceptable tolerability; however, they have unsatisfactory efficacy in older adults, are dependent on egg supply for production, and are time-consuming to manufacture. This review outlines the unmet medical needs of current influenza vaccines. Recent developments in influenza vaccines are also described.

Targeted vaccine selection in influenza vaccination

BACKGROUND

The main target groups for influenza vaccination are the elderly, the chronically ill, infants, and toddlers. Influenza vaccines are needed that suit the immunological particularities of each of these age and risk groups. Recent years have seen the approval of influenza vaccines that are more immunogenic than before, but whose use in Germany is limited by the restriction of reimbursement to a small number of vaccines.

METHODS

The Medline database was selectively searched for pertinent literature.

RESULTS

The suboptimal immunogenicity of conventional influenza vaccines that contain inactivated viral cleavage products and subunits can be markedly improved by the use of squalene-based adjuvant systems, by the integration of viral antigens in virosomal particles, or by intradermal administration. The vaccination of elderly persons with a vaccine containing the adjuvant MF59 was found to lower the risk of hospitalization for influenza or pneumonia by 25% compared to vaccination with a trivalent inactivated vaccine (TIV). On the other hand, the adjuvant ASO3 was found to be associated with an up to 17-fold increase in the frequency of narcolepsy among 4- to 18-year-olds. In a prospective study, a virosomal vaccine lowered the frequency of laboratory-confirmed influenza in vaccinated children by 88% compared to unvaccinated children (2 versus 18 cases per 1000 individuals). A live, attenuated influenza vaccine lowered the rate of disease in children up to age 7 by 48% compared to a TIV (4.2% versus 8.1%).

CONCLUSION

The newer vaccines possess improved efficacy when used for primary and booster immunization in certain age and risk groups, and they are superior in this respect to conventional vaccines based on viral cleavage products and subunits. The risk/benefit profiles of all currently available vaccines vary depending on the age group or risk group in which they are used.

The history of MF59(®) adjuvant: a phoenix that arose from the ashes

The first clinical trial of an MF59(®)-adjuvanted influenza vaccine (Novartis) was conducted 20 years ago in 1992. The product that emerged (Fluad(®), Novartis) was licensed first in Italy in 1997 and is now licensed worldwide in 30 countries. US licensure is expected in the coming years. By contrast, many alternative adjuvanted vaccines have failed to progress. The key decisions that allowed MF59 to succeed in such a challenging environment are highlighted here and the lessons that were learned along the way are discussed. MF59 was connected to vaccines that did not succeed and was perceived as a 'failure' before it was a success. Importantly, it never failed for safety reasons and was always well tolerated. Even when safety issues have emerged for alternative adjuvants, careful analysis of the substantial safety database for MF59 have shown that there are no significant concerns with widespread use, even in more 'sensitive' populations.

Cross-protection against drifted influenza viruses: options offered by adjuvanted and intradermal vaccines

Antigenic drift, the evolutionary mechanism of influenza viruses, results in an increased susceptibility of vaccinated subjects against circulating viruses. New vaccines able to grant a broader and cross-reactive immune response against drifted influenza variants are needed. Several strategies were explored to enhance the immunogenicity of plain vaccines: adjuvants, carriers and intradermal administration of influenza vaccine emerge as a promising options. To evaluate the ability of a MF59-adjuvanted and intradermal influenza vaccine to elicit an effective antibody response against circulating viruses presenting antigenic patterns different from those of the vaccine strains, we compared antibody responses elicited by "implemented" vaccines and conventional intramuscular trivalent inactivated vaccine against heterologous circulating influenza A viruses. Different studies, simulating different epidemiological pictures produced by the natural antigenic drift of seasonal influenza viruses, highlighted the superior cross-reactivity of the antibodies elicited by MF59 and intradermal vaccines, compared with subunit or split vaccine against heterologous viruses.

Decline in miR-181a expression with age impairs T cell receptor sensitivity by increasing DUSP6 activity

The ability of the human immune system to respond to vaccination declines with age. We identified an age-associated defect in T cell receptor (TCR)-induced extracellular signal-regulated kinase (ERK) phosphorylation in naive CD4(+) T cells, whereas other signals, such as ζ chain-associated protein kinase 70 (ZAP70) and phospholipase C-γ1 phosphorylation, were not impaired. The defective ERK signaling was caused by the dual specific phosphatase 6 (DUSP6), whose protein expression increased with age due to a decline in repression by miR-181a. Reconstitution of miR-181a lowered DUSP6 expression in naive CD4(+) T cells in elderly individuals. DUSP6 repression using miR-181a or specific siRNA and DUSP6 inhibition by the allosteric inhibitor (E)-2-benzylidene-3-(cyclohexylamino)-2,3-dihydro-1H-inden-1-one improved CD4(+) T cell responses, as seen by increased expression of activation markers, improved proliferation and supported preferential T helper type 1 cell differentiation. DUSP6 is a potential intervention target for restoring T cell responses in the elderly, which may augment the effectiveness of vaccination.

Signal inhibition by the dual-specific phosphatase 4 impairs T cell-dependent B-cell responses with age

T cell-dependent B-cell responses decline with age, suggesting defective CD4 T-cell function. CD4 memory T cells from individuals older than 65 y displayed increased and sustained transcription of the dual-specific phosphatase 4 (DUSP4) that shortened expression of CD40-ligand (CD40L) and inducible T-cell costimulator (ICOS) (both P < 0.001) and decreased production of IL-4, IL-17A, and IL-21 (all P < 0.001) after in vitro activation. In vivo after influenza vaccination, activated CD4 T cells from elderly individuals had increased DUSP4 transcription (P = 0.002), which inversely correlated with the expression of CD40L (r = 0.65, P = 0.002), ICOS (r = 0.57, P = 0.008), and IL-4 (r = 0.66, P = 0.001). In CD4 KO mice reconstituted with DUSP4 OT-II T cells, DUSP4 had a negative effect on the expansion of antigen-specific B cells (P = 0.003) and the production of ova-specific antibodies (P = 0.03) after immunization. Silencing of DUSP4 in memory CD4 T cells improved CD40L (P < 0.001), IL-4 (P = 0.007), and IL-21 (P = 0.04) expression significantly more in the elderly than young adults. Consequently, the ability of CD4 memory T cells to support B-cell differentiation that was impaired in the elderly (P = 0.004) was restored. Our data suggest that increased DUSP4 expression in activated T cells in the elderly in part accounts for defective adaptive immune responses.

Comparison of the long-term immunogenicity of two pandemic influenza A/H1N1 2009 vaccines, the MF59-adjuvanted and unadjuvanted vaccines, in adults

Since the first reports of the A/H1N1 virus in April 2009, the pandemic influenza virus spread globally and circulated for a long time. The primary method for the control of influenza is vaccination, but levels of influenza vaccine-induced antibody are known to decline rapidly during a 6-month period. In adults aged 18 to 64 years, we compared the long-term immunogenicity of two of the influenza A/H1N1 2009 monovalent vaccines, 3.75-μg MF59-adjuvanted vaccine and 15-μg unadjuvanted vaccine. The serum hemagglutinin inhibition (HI) titers were determined prevaccination and at 1, 6, and 10 months after vaccination. One hundred six (88.3%) of the 120 subjects were monitored for the entire 10-month period after receiving the influenza A/H1N1 2009 monovalent vaccine. There were 60 patients who received the unadjuvanted vaccine and 46 patients who received the MF59-adjuvanted vaccine. The seroprotection rates, seroconversion rates, and the geometric mean titer (GMT) folds fulfilled the criteria of the European Medicines Agency (EMA) for influenza A/California/7/2009 (H1N1) at 1 month after vaccination irrespective of the vaccine composition. Although the GMTs at 1 month postvaccination were somewhat higher in the unadjuvanted vaccine recipients than in the MF59-adjuvanted vaccine recipients, the difference was not significant (P = 0.29). The seroprotection rates at 6 and 10 months postvaccination were preserved above 70% but only in the MF59-adjuvanted vaccine recipients. In conclusion, low-dose MF59-adjuvanted influenza vaccine, even with 3.75 μg hemagglutinin antigen, might induce excellent long-term immunity that is comparable to the conventional dose of unadjuvanted vaccine among healthy adults aged 18 to 64 years.

Intanza(®) 15 mcg intradermal influenza vaccine elicits cross-reactive antibody responses against heterologous A(H3N2) influenza viruses

The aim of the present study was to explore the ability of Intanza(®) 15 μg, the intradermal (ID) trivalent inactivated split-virion influenza vaccine containing 15 μg hemagglutinin per strain, to enhance the antibody responses against heterologous circulating H3N2 strains in adults 60 years and older. During the 2006-2007 influenza season, subjects aged 60 years or older were randomly assigned to receive one dose of ID or an intramuscular (IM, Vaxigrip(®)) influenza vaccine, which contained the reassortant A/Wisconsin/67/05(H3N2) strain as the H3N2 component. Antibody responses were assessed against the homologous vaccine strain, against the A/Brisbane/10/07(H3N2) reassortant strain and against four heterologous H3N2 field isolates (A/Genoa/62/05(H3N2), A/Genoa/3/07(H3N2), A/Genoa/2/07(H3N2), A/Genoa/3/06(H3N2)). The viruses tested belonged to three different clades that were closely related antigenically to A/California/7/04(H3N2), A/Nepal/921/06(H3N2) and A/Brisbane/10/07(H3N2). Antibody responses to these viruses were measured in 25 subjects per group using both haemagglutination inhibition (HI) and neutralization (NT) assays. At least one Committee for Medicinal Products for Human Use (CHMP) immunogenicity criteria for vaccine approval in the elderly was reached by both vaccines against all the viruses used in the study. All three CHMP criteria were reached against A/California/7/04(H3N2)-like, A/Nepal/921/06(H3N2)-like and A/Brisbane/10/07(H3N2)-like viruses by Intanza(®) 15 μg ID vaccine, while IM vaccination did not meet seroprotection criteria against circulating A/Nepal/921/06(H3N2)-like and A/Brisbane/10/07(H3N2)-like viruses or seroconversion criteria against A/Brisbane/10/07(H3N2)-like viruses. Post-vaccination HI titer, seroconversion, and seroprotection rates were higher against all viruses in subjects who received Intanza(®) 15 μg. The superiority of the seroprotection rate against the A/Nepal/921/06(H3N2)-like strain attained statistical significance despite the small sample size. Upon Beyer correction for pre-vaccination status, post-immunization HI titers against A/California/7/04(H3N2)-like and A/Brisbane/10/07(H3N2)-like strains and NT post-immunization titers against A/Wisconsin/67/05(H3N2), A/California/7/04(H3N2)-like, A/Brisbane/10/07(H3N2)-like strains were significantly higher in subjects immunized with Intanza(®) 15 μg than in individuals receiving IM vaccine. This study, although limited in the size of study population, demonstrated the broader immune response elicited by an ID influenza vaccine vs. a standard IM influenza vaccine against heterologous viruses including field isolates.

Current evidence on intradermal influenza vaccines administered by Soluvia™ licensed micro injection system

Among the several strategies explored for (1) the enhancement of the immune response to influenza immunization, (2) the improvement of the vaccine acceptability and (3) the overcoming of the egg-dependency for vaccine production, intradermal administration of influenza vaccine emerges as a promising alternative to conventional intramuscular route, thanks to the recent availability of new delivery devices and the perception of advantages in terms of immunogenicity, safety, reduction of antigen content and acceptability.

 

Data from clinical trials performed in children, adults <60 y and elderly people and post-marketing surveillance demonstrate that actually, licensed intradermal influenza vaccines, Intanza™ 9 and 15 µg and Fluzone™ Intradermal, administered by the microinjection system Soluvia™, show an excellent acceptability, tolerability and safety profile. Formulations containing 9 and 15 μg per strain demonstrate, respectively, comparable and superior immunogenicity than conventional intramuscular vaccines. Licensed intradermal influenza vaccines can be considered a valid alternative to standard intramuscular vaccination offering significant advantages in low-responder populations and helping to increase influenza vaccination coverage rates especially in people with fear of needles or high apprehension associated with annual vaccination.

Quantitative review of antibody response to inactivated seasonal influenza vaccines

BACKGROUND

Seasonal influenza epidemics are associated with significant morbidity and mortality each year, particularly amongst young children and the elderly. Seasonal influenza vaccines have been available for decades, yet influenza remains a major public health threat in the US, sparking interest in studies evaluating the effectiveness of vaccination.

OBJECTIVES

We sought to identify determinants of serological responses to inactivated seasonal influenza vaccines including number of doses, adjuvant, and subject characteristics.

METHODS

We reviewed 60 articles published between 1987 and 2006. We used weighted multiple logistic regression and random-effects models to evaluate how seroconversion and seroprotection rates varied with host and vaccine factors.

RESULTS

Both children and seniors tended to have poorer immune responses compared to adults whereas use of adjuvant and a second vaccine dose tended to improve immune response. Pre-vaccination serological status had a large impact on the immune response to vaccination. We found substantial heterogeneity among studies, even with similar population settings and vaccination regimen.

CONCLUSIONS

Future studies should stratify their results by pre-vaccination serological status in an effort to produce more precise summary estimates of vaccine response.

MF59 adjuvant: the best insurance against influenza strain diversity

MF59 is a well-established, safe and potent vaccine adjuvant that has been licensed for more than 13 years for use in an influenza vaccine focused on elderly subjects (Fluad®), Novartis, Cambridge, MA, USA). Recently, MF59 was shown to be safe in a seasonal influenza vaccine for young children and was able to increase vaccine efficacy from 43 to 89%. A key and consistent feature of MF59 is the ability of the emulsion to induce fast priming of influenza antigen-specific CD4(+) T-cell responses, to induce strong and long-lasting memory T- and B-cell responses and to broaden the immune response beyond the influenza strains actually included in the vaccine. The enhanced breadth of response is valuable in the seasonal setting, but is particularly valuable in a (pre-) pandemic setting, when it is difficult to predict which strain will emerge to cause the pandemic. We have shown that the ability of MF59 to increase the breadth of immune response against influenza vaccines is mainly due to the spreading of the repertoire of the B-cell epitopes recognized on the hemagglutinin and neuraminidase of the influenza virus.

Influenza vaccine immunology

Studying the spread of influenza in human populations and protection by influenza vaccines provides important insights into immunity against influenza. The 2009 H1N1 pandemic has taught the most recent lessons. Neutralizing and receptor-blocking antibodies against hemagglutinin are the primary means of protection from the spread of pandemic and seasonal strains. Anti-neuraminidase antibodies seem to play a secondary role. More broadly cross-reactive forms of immunity may lessen disease severity but are insufficient to prevent epidemic spread. Priming by prior exposure to related influenza strains through infection or immunization permits rapid, potent antibody responses to immunization. Priming is of greater importance to the design of immunization strategies than the immunologically fascinating phenomenon of dominant recall responses to previously encountered strains (original antigenic sin). Comparisons between non-adjuvanted inactivated vaccines and live attenuated vaccines demonstrate that both can protect, with some advantage of live attenuated vaccines in children and some advantage of inactivated vaccines in those with multiple prior exposures to influenza antigens. The addition of oil-in-water emulsion adjuvants to inactivated vaccines provides enhanced functional antibody titers, greater breadth of antibody cross-reactivity, and antigen dose sparing. The MF59 adjuvant broadens the distribution of B-cell epitopes recognized on HA and NA following immunization.

Inactivated influenza vaccines: recent progress and implications for the elderly

The current public health strategy for the containment of influenza is annual vaccination, which is recommended for the elderly and for those in risk factor categories that present the highest morbidity and mortality. However, because the immune response in the elderly is known to be less vigorous than in younger adults, research in the last decade has focused on improving the immune response to vaccination and increasing the protection of aged populations. The decreased efficacy of vaccines in the elderly is due to several factors, such as a decrease in the number of Langerhans cells, the limited capacity of dendritic cells to present antigen, defects in the expression of Toll-like receptors and the reduced expression of MHC class I and II molecules. Also, production of mature naive T cells by the thymus decreases with age. Among several approaches proposed to address the need for more immunogenic vaccines compared with conventional agents, the most well proven is the use of adjuvants. The first licensed adjuvant, aluminium-based mineral salts (alum), introduced in the 1920s, remains the standard worldwide adjuvant for human use and it has been widely used for almost a century. However, the addition of alum adjuvant to a split or subunit influenza vaccine has induced only marginal improvements. Other adjuvants have been developed and approved for human use since 1997; in particular, MF59, an oil-in-water adjuvant emulsion of squalene, which is able to increase immunogenicity of seasonal, pre-pandemic and pandemic subunit vaccines while maintaining acceptable safety and tolerability profiles. More recently, another oil-in-water emulsion, AS03, has been approved as a component of pre-pandemic H5N1 and pandemic H1N1 2009 vaccines. Besides adjuvants, several other strategies have been assessed to enhance antibody response in the elderly and other less responsive subjects, such as high-dose antigen vaccines, carrier systems (liposomes/virosomes) and the intradermal route of immunization. In particular, the potential of intradermal vaccination is well documented and the recent availability of an appropriate injection system, which combines simplicity, safety and ease of use, has allowed evaluation of the tolerability, safety and immunogenicity of the intradermal influenza vaccine in large numbers of subjects. Data that emerged from large clinical trials showed an improved immunogenicity compared with that of standard vaccine. Observational studies or comparisons between adjuvanted, intradermal or high-dose versus conventional vaccines are needed to evaluate whether the greater immunogenicity observed in a number of recent studies is correlated with greater protection against influenza and influenza-related complications and death.