Thousands of outbreaks of the highly pathogenic avian influenza A(H5N1) virus in birds and an increasing number of mammal infections are registered annually. In 2023, multiple avian influenza outbreaks were registered among wild birds, poultry and seals in Russia. The genetic characterization of seventy-seven avian viruses and three viruses from seals showed that they belonged to the 2.3.4.4b clade and represented four distinct reassortant genotypes. The majority of viruses represented genotype BB, which was widespread in Europe in 2023. Viruses from seals and four viruses from birds, isolated from outbreaks in the Far East region, belonged to the G1 (A3) genotype and had the amino acid substitution N319K in the NP protein, previously associated with an increased virulence for mammals. In addition, one virus of the G10 genotype and two viruses, representing a previously undescribed genotype (designated as Ru-23-G4) were identified. The viruses analyzed showed normal inhibition by neuraminidase inhibitors. Seven viruses had genetic markers of amantadine resistance. All the influenza A(H5N1) viruses studied showed a binding preference for α2-3-linked sialic acids, suggesting a low risk of transmission among humans. Nevertheless, monitoring of reassortment and mammalian adaptation mutations is essential for the timely identification of viruses with increased pandemic potential.

The 2023-2024 flu season in Russia was dominated by influenza A(H3N2) viruses. In this study, we isolated seasonal influenza viruses from human respiratory specimens, analyzed their genetic and antigenic properties, and assessed their susceptibility to neuraminidase inhibitors. In total, we isolated 207 influenza virus isolates. Of them, 95.2% were subtyped as A(H3N2), 1.9% as A(H1N1)pdm09, and 2.9% as B/Victoria influenza viruses. The hemagglutinin sequences of the A(H3N2) isolates showed that they belonged to several subclades within the 2a.3a.1 genetic group, with the J2 subclade being predominant. Despite their genetic diversity, all A(H3N2) strains tested using a hemagglutination inhibition assay were antigenically similar to the egg-propagated vaccine strain A/Darwin/09/2021(H3N2). The B/Victoria virus isolates belonged to the C.5 and C.5.7 subclades of the genetic group V1A.3a.2 and were antigenically similar to cell- and egg-propagated variants of the vaccine strain B/Austria/1359417/2021. All of the A(H1N1)pdm09 isolates belonged to the 6B.1A.5a.2a genetic clade and were well-recognized by a ferret antiserum raised against a cell-propagated A/Wisconsin/67/2022(H1N1pdm09)-like reference strain. All of the tested isolates were susceptible to oseltamivir and zanamivir, including two A(H1N1)pdm09 strains with an NA-S247N substitution. Seroprevalence analysis showed that 60%, 54%, and 46% of the human blood samples tested were seropositive for the A(H3N2), A(H1N1)pdm09 and B/Victoria antigens from the 2022-2023 vaccine.

Currently A(H5Nx) avian influenza viruses are globally widespread and continue to evolve. Since their emergence in 2020 novel highly pathogenic avian influenza A(H5N1) clade 2.3.4.4b reassortant viruses have become predominant in the world and caused multiple infections in mammals. It was shown that some of A(H5N1) viruses mostly isolated from mammals contain an E627K mutation in the PB2 protein which can lead to adaptation of influenza viruses to mammalian cells. In 2023 in Russia we have isolated two highly pathogenic avian influenza A(H5N1) clade 2.3.4.4b viruses from birds one of which contained an E627K mutation in the PB2 protein. This virus had increased virulence in mice. Limited airborne transmission of the virus with the PB2-E627K mutation was observed between ferrets, in which infectious virus was detected in the nasal washings of the three of the twelve recipient ferrets, and clinical symptoms of the disease were observed in one case. Both viruses showed dominant binding to avian-type sialoside receptors, which was most likely the reason for the limited transmissibility. Thus, this study indicates a possible limited increase in the pandemic potential of A(H5N1) 2.3.4.4b viruses and highlights the importance of continuous avian influenza surveillance for pandemic preparedness and response.

Population immunity is a determining factor in relation to the spread of various variants of the influenza virus, and therefore is of great importance for predicting epidemics, characterizing the epidemic process and assessing the effectiveness of vaccination campaigns. The aim of the work was to monitor markers of seasonal influenza viruses and avian influenza viruses in the blood serum of residents of the Russian Federation in 2023–2024. Materials and methods. Blood serum samples from healthy donors were collected in the Siberian Federal District of the Russian Federation in October-November 2023. In addition, blood sera from people who had had contact with sick and/or dead birds and from residents of regions located on migration routes of wild waterfowl were studied in HI-test (hemagglutination inhibition) and virus neutralization. Results and discussion. It is shown that ahead of the epidemic season of 2023/2024, population immunity to influenza in the Siberian Federal District was at the level recommended by the World Health Organization (WHO) – at least 50 % of the immune population. However, among individuals who had had contact with sick and/or dead birds, humoral immunity to seasonal influenza was significantly lower – from 5 % to 30 % seropositive, depending on the region. HI-test on avian influenza viruses A/H5Nx and A/H9N2 has revealed 0 and 3.7 % of positive samples, respectively. The risk of a pandemic influenza virus emergence can be reduced by 75–100 % vaccination against seasonal influenza and monitoring antibody levels in poultry farm workers and employees of other organizations directly involved in poultry breeding and processing.

The influenza virus is one of the most dangerous causative agents of respiratory diseases, and its study is important for epidemiological control, especially in the case of cocirculation with SARS-CoV-2. Comparative analysis of influenza viruses isolated from early and severe cases in epidemic seasons before and during the COVID-19 pandemic in Russia. The article is based on the results of monitoring the circulation of seasonal influenza viruses obtained in 2019–2023. Samples from early and severe cases of influenza were studied using real-time PCR and whole-genome sequencing. Antigenic characterization of isolated viruses was carried out, and their sensitivity to antiviral drugs was studied. The flu season of 2019–2020 in Russia was the last epidemic season before the COVID-19 pandemic with the predominant cocirculation of influenza B and influenza A/H1 N1 pdm09 viruses. After the onset of the pandemic in the 2020–2021 season, the influenza virus was practically absent in Russia and was detected sporadically. Virus circulation resumed in the 2021–2022 season with dominance of A/H3N2 (clade 3C.2a1b.2a2) and continued in the 2022–2023 season with the dominance of A/Hi N1 pdm09 (clade 6B.1A.5a.2a) and the spread of influenza B/Victoria viruses (clade V1A.3a.2), which were antigenically different from the viruses circulating before the COVID-19 pandemic. Genetic analysis of the D222G/N mutations in the hemagglutinin of the A/H1N1pdm09 viruses, which are associated with increased disease severity, revealed an approximately equivalent selection of the D222G and D222N mutations in the 2019–2020 season and increased occurrence of the D222N variant in the 2022–2023 season. Cocirculation with SARS-CoV-2, the return of influenza circulation to epidemic levels, the emergence of new antigenic variants and pathogenicity factors emphasize the need to monitor and study influenza viruses for epidemiological analysis and prognosis, as well as for the development and application of effective measures to protect the population.

Introduction: Influenza is a highly contagious disease which can cause severe complications and even death, particularly in high-risk populations. Continuous monitoring and careful testing of seasonal influenza viruses, especially for severe cases, is necessary to develop and optimize prevention and treatment. Objective: To establish genetic and antigenic characteristics of influenza viruses detected in lethal cases in the Russian Federation in the 2023–2024 respiratory virus season. Materials and methods: From September 29, 2023 to June 1, 2024, 859 specimens from influenza cases (nasopharyngeal swabs) and 101 autopsy specimens of trachea, bronchi, and lungs from lethal cases of the disease from 57 regions of the Russian Federation were analyzed by PCR. For those with a sufficient amount of genetic material, whole-genome sequencing was performed using the Illumina MiSeq platform. Results: The 2023–2024 influenza season in the Russian Federation was characterized by the dominance of A/H3N2 viruses of subclade 2a.3a.1. Influenza A/H1N1pdm09 and influenza type B viruses were detected to a lesser extent in that period. Additional diversification of A/H3N2 viruses was observed during the season with accumulation of amino acid substitutions in the hemagglutinin antigenic sites. Most lethal cases were among those at high risk and unvaccinated. No mutations associated with increased pathogenicity and virulence were identified in A/H3N2 viruses from lethal cases. No molecular markers of drug resistance to neuraminidase inhibitors and baloxavir marboxil were identified in all viruses tested in the 2023–2024 season. Conclusion: Influenza A/H3N2 viruses from the lethal cases of the diseases tested at the State Research Center for Virology and Biotechnology “Vector” in the 2023–2024 respiratory virus season were similar to those detected in recovered patients. The isolated viruses were genetically and antigenically similar to the vaccine strain and had no markers of increased pathogenicity or drug resistance.

The year 2023 saw a challenging epizootiological and epidemiological situation regarding highly pathogenic avian influenza. The virus affected 150 bird species and dozens of mammal species. More than 650 outbreaks were reported in poultry across 29 countries, resulting in the death or destruction of approximately 19 million specimens. There was a high incidence of the influenza among wild birds (approximately 3,000 outbreaks in 65 countries) and mammals (more than 16,000 cases). The majority of outbreaks in wild birds, poultry and mammals were caused by influenza A(H5N1) clade 2.3.4.4b viruses. Many countries in Europe, Asia, Africa, North and South America experienced the outbreaks throughout the year. For the first time, a polar bear death from A(H5N1) virus was documented. Moreover, molecular markers of virus adaptation to mammals were found in PB2 proteins of 50 % of influenza A(H5N1) viruses that caused the death of animals. During the year 2023, human infections with highly pathogenic avian influenza A(H5N1) viruses were reported in Cambodia, Chile, China, and the UK. In addition, human infections with A(H3N8), A(H5N6), A(H9N2) and A(H10N5) viruses were reported in China. In Russia in 2023, outbreaks among wild birds and poultry were registered in 25 regions, as well as an outbreak among fur seals in the Sakhalin Region. The stated outbreaks were caused by highly virulent influenza A(H5N1) clade 2.3.4.4b. Hemagglutinin sequences of all Russian viruses analyzed in this study in 2023 were genetically close to the WHO candidate vaccine strains A/Astrakhan/3212/2020 (H5N8), A/chicken/ Ghana/AVL-763_21VIR7050-39/2021 (H5N1) and A/American Wigeon/South Carolina/22-000345-001/2021 (H5N1). All studied A(H5N1) viruses were antigenically similar to the A/Astrakhan/3212/2020 vaccine strain.

Cell cultures are used as classic models for virology. Development of transgenesis and genome editing methods leads to a new possibility in creation of transgenic cell lines that can serve as more suitable models for virological research. This work describes the obtaining transgenic cell lines and the study of their susceptibility to viral infections.

The State Research Center of Virology and Biotechnology “VECTOR” of the Federal Service for the Oversight of Consumer Protection and Welfare (Rospotrebnadzor) has developed the peptide-based EpiVacCorona vaccine, which is the first synthetic peptide-based antiviral vaccine for mass immunization in international vaccinology. An early clinical trial (Phase I–II) demonstrated that the EpiVacCorona vaccine is a safe product. The “Multicenter double-blind, placebo-controlled, comparative, randomized trial to assess the tolerability, safety, immunogenicity and prophylactic efficacy of the EpiVacCorona COVID-19 vaccine based on peptide antigens in 3000 volunteers aged 18 years and older” was performed regarding vaccine safety. The key objectives of the study were to evaluate the safety and prophylactic efficacy of the two-dose EpiVacCorona vaccine administered via the intramuscular route. The results of the clinical study (Phase III) demonstrated the safety of the EpiVacCorona vaccine. Vaccine administration was accompanied by mild local reactions in ≤27% of cases and mild systemic reactions in ≤14% of cases. The prophylactic efficacy of the EpiVacCorona COVID-19 vaccine after the completion of the vaccination series was 82.5% (CI95 = 75.3–87.6%). The high safety and efficacy of the vaccine give grounds for recommending this vaccine for regular seasonal prevention of COVID-19 as a safe and effective medicinal product.

In the process of the development of structural biology, both the size and the complexity of the determined macromolecular structures have grown significantly. As a result, the range of application areas for the results of structural studies of biological macromolecules has expanded. Significant progress in the development of structural biology methods has been largely achieved through the use of synchrotron radiation. Modern sources of synchrotron radiation allow to conduct high-performance structural studies with high temporal and spatial resolution. Thus, modern techniques make it possible to obtain not only static structures, but also to study dynamic processes, which play a key role in understanding biological mechanisms. One of the key directions in the development of structural research is the drug design based on the structures of biomolecules. Synchrotron radiation offers insights into the three-dimensional time-resolved structure of individual viral proteins and their complexes at atomic resolution. The rapid and accurate determination of protein structures is crucial for understanding viral pathogenicity and designing targeted therapeutics. Through the application of experimental techniques, including X-ray crystallography and small-angle X-ray scattering (SAXS), it is possible to elucidate the structural details of SARS-CoV-2 virion containing 4 structural, 16 nonstructural proteins (nsp), and several accessory proteins. The most studied potential targets for vaccines and drugs are the structural spike (S) protein, which is responsible for entering the host cell, as well as nonstructural proteins essential for replication and transcription, such as main protease (M pro ), papain-like protease (PL pro ), and RNA-dependent RNA polymerase (RdRp). This article provides a brief overview of structural analysis techniques, with focus on synchrotron radiation-based methods applied to the analysis of SARS-CoV-2 proteins.

This paper describes the current situation on highly pathogenic avian influenza virus in 2022 and provides forecast of the possible further spread of avian influenza in Russia. In 2022, the circulation of a wide variety of highly pathogenic avian influenza virus subtypes, which have epizootiological and epidemiological significance, was recorded in the world. Outbreaks caused by highly pathogenic avian influenza virus were reported in over 60 countries. In addition, human infections with influenza viruses of the A(H5Nx) and A(H9N2) subtypes were registered. There was a large-scale epizootic which affected more than 10 regions of the European part of Russia and the Russian Far East in 2022. Outbreaks among wild birds and poultry were caused by the highly pathogenic influenza virus A(H5N1) of the clade 2.3.4.4b, at the same time genetic and antigenic diversity was observed among viruses circulating in Russia. Thus, an essential geographical role of the territory of Russia in the global spread of avian influenza virus has been shown once again, which highlights the importance of continuous avian influenza virus surveillance in the country.

In the pandemic context of a novel coronavirus infection SARS-CoV-2, after a year break, the seasonal circulation of influenza continues. With such cocirculation, there is a risk of a mixed infection, which can lead to a more severe course of the disease. Monitoring of respiratory infections in the epidemic season 2022-2023 showed the presence of mixed influenza viruses and SARS-CoV-2 infection cases. Most specimens were received from people at risk group. Among detected cases of coinfection, a higher incidence of cases with a fatal outcome was detected than among detected cases of influenza and COVID-19 separately. Cases of coinfection require special attention in epidemiological monitoring, diagnosis and treatment. Identified cases of coinfection are associated with higher severity and mortality than cases of influenza and COVID19 separately. Such cases require special attention in epidemiological monitoring, diagnosis and treatment.

Scientific relevance. The use of recombinant antigens in vaccine production is limited because vaccines based on such antigens tend to have low immunogenicity. However, a COVID-19 vaccine that combines recombinant SARS-CoV-2 spike glycoprotein as its antigen and virus-like immune-stimulating complexes (ISCOMs) as its adjuvant (Nuvaxovid) induces a protective virus-neutralising response. The State Research Center of Virology and Biotechnology “Vector” (hereinafter, Vector) has developed the ISCOM adjuvant Matrix-V, which plays a key role in inducing virus-neutralising antibodies. Studying Matrix-V will provide for the wide use of recombinant antigens combined with this adjuvant in the development and production of novel Russian vaccines.Aim. This study aimed to evaluate the humoral immune responses of experimental animals to intramuscular injections of a complex combining the recombinant Wuhan-type SARS-CoV-2 spike RBD antigen and the virus-like ISCOM adjuvant containing Quillaja saponaria saponins.Materials and methods. The Matrix-V ISCOM adjuvant was produced using Vector’s proprietary technology, which involves cross-flow filtration through Sartorius VivaFlow cassettes. To determine the saponin and residual detergent concentrations in Matrix-V, the authors conducted high-performance liquid chromatography. Having produced the recombinant SARS-CoV-2 RBD antigen, the authors used electron microscopy to analyse the ultrastructure of the ISCOM–antigen complex. In the study of the ISCOM–antigen complex, 25 female Balb/c mice (5 groups) and 15 male and female outbred guinea pigs (3 groups) received two intramuscular injections with a 14-day interval. Serum tests relied on virus neutralisation (VN) and enzyme-linked immunosorbent assay (ELISA) methods and used antigens of 8 SARS-CoV-2 variants (State Collection of Viruses and Rickettsia, Vector). The authors used Statistica 10 to analyse the results.Results. Two injections of the SARS-CoV-2 RBD antigen (mice: 7 μg, guinea pigs: 1 μg) alone did not induce statistically significant virus-neutralising antibody responses, as shown by the VN results. Two injections of the SARS-CoV-2 RBD antigen (mice: 7 μg, guinea pigs: 1 μg) adjuvanted with Matrix-V (25 μg) resulted in geometric mean antibody titres of 1:83–1:178 (mice) and 1:174–1:587 (guinea pigs) in the VN tests with the Wuhan variant. One injection of the antigen (1 μg or 7 μg) with Matrix-V (25 μg) induced antibodies only in individual cases, as demonstrated by the VN and/or ELISA results. The most intensive immune response was observed in ELISA tests with the Delta variant after two injections of the Ecto-S-Wuhan (1 μg) and Matrix-V (25 μg) complex. Immune responses did not differ between the group that received two injections of the Ecto-S-Wuhan antigen (1 μg) without the ISCOM adjuvant and the negative control group (titres below 1:100; p=0.95). Two injections of the SARS-CoV-2 RBD antigen (7 μg) without the ISCOM adjuvant induced antibodies in mice (titres between 1:248 and 1:1477).Conclusions. Two intramuscular injections of the complex containing the recombinant SARS-CoV-2 RBD antigen and the Matrix-V ISCOM adjuvant induce virus-neutralising antibodies. The approach proposed by the authors has the potential for use in the development of immunobiological medicinal products to prevent and treat a wide range of infectious diseases.

In Russia, during the COVID-19 pandemic, a decrease in influenza circulation was initially observed. Influenza circulation re-emerged with the dominance of new clades of A(H3N2) viruses in 2021-2022 and A(H1N1)pdm09 viruses in 2022-2023. In this study, we aimed to characterize influenza viruses during the 2022-2023 season in Russia, as well as investigate A(H1N1)pdm09 HA-D222G/N polymorphism associated with increased disease severity. PCR testing of 780 clinical specimens showed 72.2% of them to be positive for A(H1N1)pdm09, 2.8% for A(H3N2), and 25% for influenza B viruses. The majority of A(H1N1)pdm09 viruses analyzed belonged to the newly emerged 6B.1A.5a.2a clade. The intra-sample predominance of HA-D222G/N virus variants was observed in 29% of the specimens from A(H1N1)pdm09 fatal cases. The D222N polymorphic variant was registered more frequently than D222G. All the B/Victoria viruses analyzed belonged to the V1A.3a.2 clade. Several identified A(H3N2) viruses belonged to one of the four subclades (2a.1b, 2a.3a.1, 2a.3b, 2b) within the 3C.2a1b.2a.2 group. The majority of antigenically characterized viruses bore similarities to the corresponding 2022-2023 NH vaccine strains. Only one influenza A(H1N1)pdm09 virus showed reduced inhibition by neuraminidase inhibitors. None of the influenza viruses analyzed had genetic markers of reduced susceptibility to baloxavir.

The WHO has developed a tool to assess the risk of newly emerging influenza viruses with pandemic potential (TIPRA). According to TIPRA, the main parameters for assessing the risk of human-to-human transmission of a novel influenza virus are its ability to bind to human cell receptors of the upper respiratory tract (URT) and transmit in model animals. The aim of this study was to quantify airborne transmission of human and animal influenza viruses in the ferret model. The transmission of influenza viruses was studied in the ferret model in an aerobiology chamber. Airborne particles concentration and fractional composition in the aerobiology chamber were measured using an aerosol particle counter and analytical aerosol filters. Viral load in ferret nasal washings and aerosol filters was determined by titration in MDCK cells and quantitative RT-PCR. Genetic analysis of influenza viruses was performed using virus genome sequences obtained by NGS. After intranasal infection, human and animal influenza viruses replicated in the cells of nasal mucosa in ferrets. The level of virus airborne particles contamination provided by infected animals depends on the infectious dose and differs significantly between influenza virus strains. The studied avian influenza viruses show insufficient transmission in the ferret model, while human and swine influenza viruses are highly transmitted in ferrets. We propose a quantitative model of airborne transmission of influenza virus from donor to recipient ferrets. Level of influenza virus transmission in the ferret model correlates with genetic markers of virus receptor specificity and the level of virus airborne particle contamination induced by donor ferrets.

Influenza virus transmission is a crucial factor in understanding the spread of the virus within populations and developing effective control strategies. Studying the transmission patterns of influenza virus allows for better risk assessment and prediction of disease outbreaks. By monitoring the spread of the virus and identifying high-risk populations and geographic areas, it is possible to allocate resources more effectively, implement timely interventions, and provide targeted healthcare interventions to diminish the burden of influenza virus on vulnerable populations. Theoretical models of virus transmission are used to study and simulate of influenza virus spread within populations. These models aim to capture the complex dynamics of transmission, including factors such as population size, contact patterns, infectiousness, and susceptibility. Animal models serve as valuable tools for studying the dynamics of influenza virus transmission. This article presents a brief overview of existing research on the qualitative and quantitative study of influenza virus transmission in animal models. We discuss the methodologies employed, key insights gained from these studies, and their relevance.

In this study, we investigated the features of the infectious process by simulating co-infection with SARS-CoV-2 and human adenovirus type 5 (HAdV-5) or influenza A virus (IAV) in vitro and in vivo. The determination of infectious activity of viruses and digital PCR demonstrated that during simultaneous and sequential HAdV-5 followed by SARS-CoV-2 infection in vitro and in vivo, the HAdV-5 infection does not interfere with replication of SARS-CoV-2. The hamsters co-infected and mono-infected with SARS-CoV-2 exhibited nearly identical viral titers and viral loads of SARS-CoV-2 in the lungs. The hamsters and ferrets co-infected by SARS-CoV-2- and IAV demonstrated more pronounced clinical manifestations than mono-infected animals. Additionally, the lung histological data illustrate that HAdV-5 or IAV and SARS-CoV-2 co-infection induces more severe pathological changes in the lungs than mono-infection. The expression of several genes specific to interferon and cytokine signaling pathways in the lungs of co-infected hamsters was more upregulated compared to single infected with SARS-CoV-2 animals. Thus, co-infection with HAdV-5 or IAV and SARS-CoV-2 leads to more severe pulmonary disease in animals.

The purpose of the present work was to evaluate population immunity to influenza and molecular genetic analysis of influenza viruses detected in the Russian Federation over 2020-2022. In this study, 1344 samples of blood serum collected prior to the 2021-2022 flu season in Siberian, Southern, Far Eastern, Volga and Ural Federal Districts were studied. Seropositivity to the A/Victoria/2570/2019 vaccine strain (H1N1) pdm09 was detected in 25% to 31% of samples from the four federal districts, and in 8% of samples from the Far Eastern Federal District. Seropositivity to the A/Cambodia/e0826360/2020 strain (H3N2) was detected in 24% to 37% of the samples. The lowest population immunity was revealed to the influenza B/Washington/02/2019 vaccine strain (Victoria lineage), with < 10% of serum samples reactive to the studied strain. Since March 2020, the worldwide turnover of all seasonal respiratory viruses has sharply decreased, except of rhinoviruses. From March 2020 to June 2021, we have identified six B/Victoria influenza viruses from sporadic cases of influenza. From June 2021 to the end February 2022, the State Research Center “Vector” received 901 samples positive for influenza A(H3N2) virus RNA, two specimens positive for A(H1N1) pdm09 virus RNA, and 17 samples positive for influenza B. All studied A(H3N2) viruses belonged to the 3C.2a1b.2a2 subclade (Bangladesh group). The two verified A(H1N1) pdm09 influenza viruses belonged to the 6B.1A.5a clade. All studied influenza B viruses were assigned to the B/Victoria genetic lineage, and to 1A.3a2 subclade. The genomes of all identified viruses did not contain mutations of the NA gene responsible for drug resistance to neuraminidase inhibitors, or mutations in РA gene responsible for baloxavir resistance. All viruses tested by fluorescence assay were sensitive to oseltamivir and zanamivir. The worldwide frequency of influenza isolates resistant to antineuraminidase drugs does not exceed 1-2% of cases. Hence, oseltamivir and zanamivir provide effective treatment for seasonal influenza.

The COVID-19 pandemic has exacerbated the public’s need for effective vaccines. Consequently, significant financial support has been provided to developers of a number of innovative vaccines, including the vaccines with saponin-based adjuvants. In 2021, the World Health Organisation recommended Mosquirix, the first malaria vaccine, which contains a saponin adjuvant. An anti-covid vaccine by Novavax is in the approval phase. A promising approach to vaccine development is presented by the use of virus-like immune-stimulating complexes (ISCOMs) containing saponins and by the creation of combinations of ISCOMs with antigens. The aim of the study was to develop, produce and characterise virus-like immune-stimulating complexes based on saponins of Quillaja saponaria, as well as similar saponins of Russian-sourced Polemonium caeruleum. Materials and methods: The ISCOM adjuvants, Matrix-BQ and Matrix-BP, were produced using liquid chromatography and examined using electron microscopy. Balb/c mice were immunised intraperitoneally and intramuscularly with ISCOM-antigen preparations. Afterwards, the immunised animals were challenged with the influenza virus strain, A/California/4/2009(H1N1)pdm09, adapted and lethal to mice. The serum samples were examined using haemagglutination inhibition (HI) tests. Results: The authors produced the ISCOMs containing saponins of Quillaja saponaria and Polemonium caeruleum. After one intramuscular injection of either of the ISCOM-antigen preparations with 1 µg of each of A/Brisbane/02/2018 (H1N1) pdm09, A/Kansas/14/2017 (H3N2), and B/Phuket/3073/2013 haemagglutinin antigens (HAs), HI tests detected serum antibody titres to the corresponding antigens of ≥1:40. Two intramuscular injections of the ISCOM-antigen preparation containing 50 ng of each of the HAs and Matrix-BQ resulted in a protective response. In some animals, two intraperitoneal injections of ISCOM-antigen preparations resulted in the maximum antibody titre to the A/Kansas/14/2017 (H3N2) vaccine strain of 1:20,480. Two intramuscular injections of a test preparation containing 5 µg, 1 µg, 200 ng, or 50 ng of each of the HAs and Matrix-BQ or a control preparation containing 5 µg, 1 µg, or 200 ng of each of the HAs (commercially available vaccines) to the mice that were afterwards infected with the lethal influenza strain protected the experimental animals from death. Conclusions: The ISCOM-based preparations had high immunostimulatory activity in the mouse-model study. The presented results indicate the potential of further studies of ISCOM-based preparations in terms of both vaccine and immunotherapeutic development.