Influenza vaccines: where we are, where we are going

Triple tandem trimer immunogens for HIV-1 and influenza nucleic acid-based vaccines

Recombinant native-like HIV-1 envelope glycoprotein (Env) trimers are used in candidate vaccines aimed at inducing broadly neutralizing antibodies. While state-of-the-art SOSIP or single-chain Env designs can be expressed as native-like trimers, undesired monomers, dimers and malformed trimers that elicit non-neutralizing antibodies are also formed, implying that these designs could benefit from further modifications for gene-based vaccination approaches. Here, we describe the triple tandem trimer (TTT) design, in which three Env protomers are genetically linked in a single open reading frame and express as native-like trimers. Viral vectored Env TTT induced similar neutralization titers but with a higher proportion of trimer-specific responses. The TTT design was also applied to generate influenza hemagglutinin (HA) trimers without the need for trimerization domains. Additionally, we used TTT to generate well-folded chimeric Env and HA trimers that harbor protomers from three different strains. In summary, the TTT design is a useful platform for the design of HIV-1 Env and influenza HA immunogens for a multitude of vaccination strategies.

Flublok Quadrivalent Vaccine Adjuvanted with R-DOTAP Elicits a Robust and Multifunctional CD4 T Cell Response That Is of Greater Magnitude and Functional Diversity Than Conventional Adjuvant Systems

It is clear that new approaches are needed to promote broadly protective immunity to viral pathogens, particularly those that are prone to mutation and escape from antibody-mediated immunity. CD4+ T cells, known to target many viral proteins and highly conserved peptide epitopes, can contribute greatly to protective immunity through multiple mechanisms. Despite this potential, CD4+ T cells are often poorly recruited by current vaccine strategies. Here, we have analyzed a promising new adjuvant (R-DOTAP), as well as conventional adjuvant systems AddaVax with or without an added TLR9 agonist CpG, to promote CD4+ T cell responses to the licensed vaccine Flublok containing H1, H3, and HA-B proteins. Our studies, using a preclinical mouse model of vaccination, revealed that the addition of R-DOTAP to Flublok dramatically enhances the magnitude and functionality of CD4+ T cells specific for HA-derived CD4+ T cell epitopes, far outperforming conventional adjuvant systems based on cytokine EliSpot assays and multiparameter flow cytometry. The elicited CD4+ T cells specific for HA-derived epitopes produce IL-2, IFN-γ, IL-4/5, and granzyme B and have multifunctional potential. Hence, R-DOTAP, which has been verified safe by human studies, can offer exciting opportunities as an immune stimulant for next-generation prophylactic recombinant protein-based vaccines.

Proficiency tests to evaluate the impact on assay outcomes of harmonized influenza-specific Intracellular Cytokine Staining (ICS) and IFN-ɣ Enzyme-Linked ImmunoSpot (ELISpot) protocols

The magnitude and quality of cell-mediated immune responses elicited by natural infection or vaccination are commonly measured by Interferon-ɣ (IFN-ɣ) Enzyme-Linked ImmunoSpot (ELISpot) and Intracellular Cytokine Staining (ICS). To date, laboratories apply a variety of in-house procedures which leads to diverging results, complicates interlaboratory comparisons and hampers vaccine evaluations. During the FLUCOP project, efforts have been made to develop harmonized Standard Operating Procedures (SOPs) for influenza-specific IFN-ɣ ELISpot and ICS assays. Exploratory pilot studies provided information about the interlaboratory variation before harmonization efforts were initiated. Here we report the results of two proficiency tests organized to evaluate the impact of the harmonization effort on assay results and the performance of participating FLUCOP partners. The introduction of the IFN-ɣ ELISpot SOP reduced variation of both background and stimulated responses. Post-harmonization background responses were all lower than an arbitrary threshold of 50 SFU/million cells. When stimulated with A/California and B/Phuket, a statistically significant reduction in variation (p < 0.0001) was observed and CV values were strongly reduced, from 148% to 77% for A/California and from 126% to 73% for B/Phuket. The harmonizing effect of applying an ICS SOP was also confirmed by an increased homogeneity of data obtained by the individual labs. The application of acceptance criteria on cell viability and background responses further enhanced the data homogeneity. Finally, as the same set of samples was analyzed by both the IFN-ɣ ELISpot and the ICS assays, a method comparison was performed. A clear correlation between the two methods was observed, but they cannot be considered interchangeable. In conclusion, proficiency tests show that a limited harmonization effort consisting of the introduction of SOPs and the use of the same in vitro stimulating antigens leads to a reduction of the interlaboratory variation of IFN-ɣ ELISpot data and demonstrate that substantial improvements for the ICS assay are achieved as comparable laboratory datasets could be generated. Additional steps to further reduce the interlaboratory variation of ICS data can consist of standardized gating templates and detailed data reporting instructions as well as further efforts to harmonize reagent and instrument use.

Novel Influenza Vaccines: From Research and Development (R&D) Challenges to Regulatory Responses

Influenza vaccines faced significant challenges in achieving sufficient protective efficacy and production efficiency in the past. In recent decades, novel influenza vaccines, characterized by efficient and scalable production, advanced platforms, and new adjuvant technologies, have overcome some of these weaknesses and have been widely licensed. Furthermore, researchers are actively pursuing the development of next-generation and universal influenza vaccines to provide comprehensive protection against potential pandemic subtypes or strains. However, new challenges have emerged as these novel vaccines undergo evaluation and authorization. In this review, we primarily outline the critical challenges and advancements in research and development (R&D) and highlight the improvements in regulatory responses for influenza vaccines.

Trends and Changes in Influenza-associated Hospitalizations in Children During 25 Years in Finland, 1993-2018

BACKGROUND

Limited long-term data are available on potential changes in the demographics and management of children hospitalized with influenza.

METHODS

We identified all children ≤15 years old hospitalized with virologically confirmed influenza at Turku University Hospital, Finland, during the 25-year period of July 1993-June 2018. Data on clinical variables, comorbidities and management were retrieved directly from the medical records. Population-based rates of hospitalization were calculated using official annual databases of children living in the hospital catchment area.

RESULTS

Between 1993-1998 and 2013-2018, the median age of children increased from 1.3 years to 3.3 years ( P < 0.0001). The proportion of children <2 years of age decreased from 65.2% to 36.8%, whereas the proportion of children 6-15-year-old increased from 13.0% to 36.2% ( P < 0.0001 for both). The population-based rates of hospitalization decreased by 49% in children 1 year of age (incidence rate ratio, 0.51; 95% confidence interval: 0.27-0.92; P = 0.018) and increased by 194% in children 6-15 years old (incidence rate ratio, 2.94; 95% confidence interval: 1.70-5.32; P < 0.0001). The median duration of hospitalization shortened from 2.0 days (interquartile range [IQR], 1.0-4.0) to 1.0 day (IQR, 1.0-2.0; P < 0.0001).

CONCLUSIONS

During the 25 years, the median age of hospitalized children increased by 2 years, while the duration of hospitalization shortened.

R-DOTAP Cationic Lipid Nanoparticles Outperform Squalene-Based Adjuvant Systems in Elicitation of CD4 T Cells after Recombinant Influenza Hemagglutinin Vaccination

It is clear that new approaches are needed to promote broadly protective immunity to viral pathogens, particularly those that are prone to mutation and escape from antibody-mediated immunity. Prototypic pathogens of this type are influenza and SARS-CoV-2, where the receptor-binding protein exhibits extremely high variability in its receptor-binding regions. T cells, known to target many viral proteins, and within these, highly conserved peptide epitopes, can contribute greatly to protective immunity through multiple mechanisms but are often poorly recruited by current vaccine strategies. Here, we have studied a promising novel pure enantio-specific cationic lipid 1,2-dioleoyl-3-trimethylammonium-propane (R-DOTAP), which was previously recognized for its ability to generate anti-tumor immunity through the induction of potent cytotoxic CD8 T cells. Using a preclinical mouse model, we have assessed an R-DOTAP nanoparticle adjuvant system for its ability to promote CD4 T cell responses to vaccination with recombinant influenza protein. Our studies revealed that R-DOTAP consistently outperformed a squalene-based adjuvant emulsion, even when it was introduced with a potent TLR agonist CpG, in the ability to elicit peptide epitope-specific CD4 T cells when quantified by IFN-γ and IL-2 ELISpot assays. Clinical testing of R-DOTAP containing vaccines in earlier work by others has demonstrated an acceptable safety profile. Hence, R-DOTAP can offer exciting opportunities as an immune stimulant for next-generation prophylactic recombinant protein-based vaccines.

Clinical Microbiology in Detection and Identification of Emerging Microbial Pathogens: Past, Present and Future

Clinical microbiology has possessed a marvellous past, an important present and a bright future. Western medicine modernization started with the discovery of bacterial pathogens, and from then, clinical bacteriology became a cornerstone of diagnostics. Today, clinical microbiology uses standard techniques including Gram stain morphology, in vitro culture, antigen and antibody assays, and molecular biology both to establish a diagnosis and monitor the progression of microbial infections. Clinical microbiology has played a critical role in pathogen detection and characterization for emerging infectious diseases as evidenced by the ongoing COVID-19 pandemic. Revolutionary changes are on the way in clinical microbiology with the application of “-omic” techniques, including transcriptomics and metabolomics, and optimization of clinical practice configurations to improve outcomes of patients with infectious diseases.

The role of cell-mediated immunity against influenza and its implications for vaccine evaluation

Influenza vaccines remain the most effective tools to prevent flu and its complications. Trivalent or quadrivalent inactivated influenza vaccines primarily elicit antibodies towards haemagglutinin and neuraminidase. These vaccines fail to induce high protective efficacy, in particular in older adults and immunocompromised individuals and require annual updates to keep up with evolving influenza strains (antigenic drift). Vaccine efficacy declines when there is a mismatch between its content and circulating strains. Current correlates of protection are merely based on serological parameters determined by haemagglutination inhibition or single radial haemolysis assays. However, there is ample evidence showing that these serological correlates of protection can both over- or underestimate the protective efficacy of influenza vaccines. Next-generation universal influenza vaccines that induce cross-reactive cellular immune responses (CD4+ and/or CD8+ T-cell responses) against conserved epitopes may overcome some of the shortcomings of the current inactivated vaccines by eliciting broader protection that lasts for several influenza seasons and potentially enhances pandemic preparedness. Assessment of cellular immune responses in clinical trials that evaluate the immunogenicity of these new generation vaccines is thus of utmost importance. Moreover, studies are needed to examine whether these cross-reactive cellular immune responses can be considered as new or complementary correlates of protection in the evaluation of traditional and next-generation influenza vaccines. An overview of the assays that can be applied to measure cell-mediated immune responses to influenza with their strengths and weaknesses is provided here.

Outbreak of Influenza and Other Respiratory Viruses in Hospitalized Patients Alongside the SARS-CoV-2 Pandemic

Influenza A and other respiratory viruses, circulate each winter and cause respiratory illness that can lead to severe complications in hospitalized patients. During the COVID-19 pandemic, only a few cases of respiratory viruses were detected in Israel. Our study applied RT-PCR to examine 13,674 samples collected from patients hospitalized with respiratory symptoms in 2019, 2020, and 2021 and the first half of the 2022 winter. A sharp increase in influenza A(H3N2) cases was observed in winter 2021-2022 as compared to 2020, followed by a sudden decrease in influenza cases after the detection of the SARS-CoV-2 omicron variant in Israel. Comparison of the area under the curve (AUC) of influenza infection rates during 7 consecutive winter seasons found that the minimal AUC between 2015 and 2020 was 281.1, while in 2021-2022, it was significantly lower (162.6 AUC; p = 0.0017), although the percentage of positive influenza cases was similar to those of previous years. The presented findings show how the dominance of influenza A(H3N2) abruptly ended upon circulation of the SARS-CoV-2 omicron variant. However, a post-COVID-19 influenza outbreak is possible, hence the planning of the next influenza vaccine is critical to ensure lower influenza-related hospitalization rates.