Prevalence and associated outcomes of coinfection between SARS-CoV-2 and influenza: a systematic review and meta-analysis

Impact of co-infection with SARS-CoV-2 and other respiratory viruses on illness: Pooled analyses of 11 COVID-19 cohorts

OBJECTIVE

This individual patient data meta-analysis investigates the impact of SARS-CoV-2 infection with or without other respiratory viruses on Acute Respiratory Illness (ARI) occurrence and severity.

METHODS

We pooled individual participant data from 11 prospective COVID-19 community and healthcare cohorts (2020-2024). A subject's first respiratory sample was tested for SARS-CoV-2 and a panel of respiratory viruses. The association of SARS-CoV-2 single versus viral co-infection with ARI occurrence and severity was analyzed using mixed effects regression. The analysis was repeated for Human Rhinovirus (HRV).

RESULTS

Of 1606 SARS-CoV-2 positive episodes (1597 subjects), 124 (7.7%) were co-infected with another respiratory virus, the majority with HRV (66.1%). SARS-CoV-2 Co-infection was associated with a lower odds of ARI than SARS-CoV-2 single infection in community cohorts (adjusted (a) OR: 0.39; 95%CI: 0.21-0.71). This association was not observed for ARI severity in healthcare cohorts (aOR: 1.76; 95%CI: 0.67-4.61). Co-infection versus single infection with HRV was associated with higher ARI occurrence and severity in both settings (community: aOR: 1.72 and healthcare: aOR: 6.04).

CONCLUSION

In community settings, SARS-CoV-2 co-infection with another virus, particularly HRV, attenuates ARI compared to SARS-CoV-2 single infection. The low number of detected co-infections with other viruses, such as influenza or RSV, limits generalizability to other combinations of co-infecting viruses.

Co-infection dynamics of SARS-CoV-2 and respiratory viruses in the 2022/2023 respiratory season in the Netherlands

OBJECTIVES

Evaluation of the presence and effect of SARS-CoV-2 co-infections on disease severity.

METHODS

We collected both symptom data and nose- and throat samples from symptomatic people during the 2022/2023 respiratory season in a large participatory surveillance study in the Netherlands, and tested these for 18 respiratory viruses, including SARS-CoV-2. We compared reported health status, symptoms and odds of having a single respiratory viral infection or co-infection with SARS-CoV-2 and another respiratory virus.

RESULTS

In total, 4655 samples were included with 22% (n=1017) testing SARS-CoV-2 positive. Of these 11% (n=116) also tested positive for a second respiratory virus. The most frequently occurring co-infections in SARS-CoV-2 positive participants were with rhinovirus (59%; n=69), seasonal coronaviruses (15%; n=17), and adenovirus (7%; n=8). Participants with a co-infection with one of these three viruses did not report more severe disease compared to those with a SARS-CoV-2 mono-infection. The odds of experiencing SARS-CoV-2 co-infection with seasonal coronavirus or rhinovirus were lower compared to the odds of the respective non-SARS-CoV-2 mono-infection (OR: 0.16, CI 95%: 0.10 - 0.24; OR: 0.21 CI 95%: 0.17 - 0.26; respectively).

CONCLUSIONS

SARS-CoV-2 co-infections with rhinovirus, seasonal coronavirus, and adenovirus are frequently observed in the general population, but are not associated with more severe disease compared to SARS-CoV-2 mono-infections. Furthermore, we found indications for inter-virus interaction with rhinovirus and seasonal coronavirus, possibly decreasing the risk of co-infection.

Viral Infections in Elderly Individuals: A Comprehensive Overview of SARS-CoV-2 and Influenza Susceptibility, Pathogenesis, and Clinical Treatment Strategies

As age increases, the immune function of elderly individuals gradually decreases, increasing their susceptibility to infectious diseases. Therefore, further research on common viral infections in the elderly population, especially severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and influenza viruses, is crucial for scientific progress. This review delves into the genetic structure, infection mechanisms, and impact of coinfections with these two viruses and provides a detailed analysis of the reasons for the increased susceptibility of elderly individuals to dual viral infections. We evaluated the clinical manifestations in elderly individuals following coinfections, including complications in the respiratory, gastrointestinal, nervous, and cardiovascular systems. Ultimately, we have summarized the current strategies for the prevention, diagnosis, and treatment of SARS-CoV-2 and influenza coinfections in older adults. Through these studies, we aim to reduce the risk of dual infections in elderly individuals and provide a scientific basis for the prevention, diagnosis, and treatment of age-related viral diseases, thereby improving their health status.

Trends of respiratory viruses and factors associated with severe acute respiratory infection in patients presenting at a university hospital: a 6-year retrospective study across the COVID-19 pandemic

Background

The COVID-19 pandemic significantly disrupted the epidemiology of respiratory viruses, altering seasonal patterns and reducing circulation. While recovery trends have been observed, factors associated with severe acute respiratory infections (SARIs) during pre- and post-pandemic periods remain underexplored in middle-income countries.

Objective

This study aimed to analyze the trends in respiratory virus circulation and identify factors associated with SARI in patients attending a tertiary care university hospital in western Mexico over a six-year period spanning the pre-pandemic, pandemic, and post-pandemic phases.

Methods

A retrospective study was conducted using data from 19,088 symptomatic patients tested for respiratory viruses between 2018 and 2024. Viral trends were analyzed through interrupted time series (ITS) modeling, incorporating locally estimated scatterplot smoothing (LOESS) and raw positivity rates. Additionally, ITS analysis was performed to evaluate temporal changes in SARI proportions across different phases of the pandemic. Multivariate logistic regression models were applied to determine independent risk factors for SARI across different time periods.

Results

During the pandemic (2020-2021), respiratory virus positivity rates significantly declined, particularly for influenza, which experienced a sharp reduction but rebounded post-2022. Respiratory syncytial virus (RSV) demonstrated a delayed resurgence, whereas other respiratory viruses exhibited heterogeneous rebound patterns. ITS modeling of SARI proportions revealed a significant pre-pandemic increasing trend, followed by a slower rise during the pandemic, and a sharp post-pandemic drop in early 2022, before resuming an upward trajectory. Among older adults (>65 years), a marked increase in SARI was observed at the beginning of the pandemic, while younger groups showed more stable patterns. Logistic regression identified advanced age, male sex, cardiovascular disease, obesity, and immunosuppression as major risk factors for SARI, while vaccination consistently showed a protective effect across all periods and subgroups.

Conclusion

The COVID-19 pandemic induced persistent shifts in respiratory virus circulation, disrupting seasonal dynamics and modifying the burden of SARI. The findings underscore the importance of continuous surveillance, targeted vaccination programs, and early diagnostics to mitigate severe outcomes. These results highlight the need for adaptive public health strategies in middle-income countries to address evolving respiratory disease threats.

A contiunous-time SIS criss-cross model of co-infection in a heterogeneous population

In this paper, we introduce and analyze a contiunous-time model of co-infection dynamics in a heterogeneous population consisting of two subpopulations that differ in the risk of getting infected by individuals with two diseases. We assume that each parameter reflecting a given process for each subpopulation has different values, which makes the population completely heterogeneous. Such complexity and the population heterogeneity make our paper unique, reflecting co-infection dynamics. Moreover, we establish an epidemic spread for each disease not only in a sole subpopulation but also with criss-cross transmission, meaning between different subpopulations. The proposed system has a disease-free stationary state and two states reflecting the presence of one disease. We indicate conditions for their existence and local stability. The conditions for the local stability for states reflecting one disease have a complicated form, so we strengthened them so that they are more transparent. Investigation on the existence of a postulated endemic state corresponding to both disease's presence leads to a complex analysis, which is why we only provide an insight on this issue. Here, we also provide the basic reproduction number of our model and investigate properties of this number. The system has a universal structure; as such, it can be applied to investigate co-infection of different infectious diseases.

Comparison of clinical characteristics and severity of COVID-19 with or without viral co-infection in hospitalized children

BACKGROUND

Co-infection with other pathogens can alter the severity and clinical outcomes of viral infections. However, the information regarding viral co-infections in pediatric coronavirus disease 2019 (COVID-19) cases is still limited.

METHODS

This is a nationwide, retrospective cohort study using the data from the COVID-19 Registry Japan. The pediatric (<18 years), laboratory confirmed, hospitalized COVID-19 patients in the Omicron variant of concern predominant period (January 2022 to January 2024) were included. Co-infection was investigated by multiplex PCR. We compared clinical characteristics, symptoms, severity, and outcomes between children with and without co-infection.

RESULTS

Among 245 hospitalized pediatric COVID-19 patients, 78 (31.8 %) had co-infections. The patient backgrounds of the "co-infection" and "SARS-CoV-2 alone" groups were similar, although age distribution was different, with a lower number of patients over 12 years in the co-infection group (n = 2, 2.6 % vs. n = 29, 17.4 %; P < 0.001). Among the patients with co-infection, the most common pathogen was enterovirus/rhinovirus (51.3 %), followed by parainfluenza virus (23.1 %) and adenovirus (12.8 %). Patients with co-infection more commonly had respiratory symptoms, including SpO2 < 96 %, shortness of breath, runny nose, and wheezing. Requirement of non-invasive oxygen support was higher in the co-infection group (n = 27, 34.6 % vs. n = 28, 16.8 %, P = 0.006). By multivariable logistic regression analysis, co-infection and presence of any comorbidity were identified as significant risk factors for necessity of oxygen therapy (odds ratio [95 % confidence interval] 2.44 [1.29-4.63] and 3.99 [2.07-7.82], respectively).

CONCLUSIONS

Viral co-infection may increase the risk of respiratory distress in pediatric COVID-19 patients.

Pathogenesis of influenza and SARS-CoV-2 co-infection at the extremes of age: decipher the ominous tales of immune vulnerability

The co-circulation of influenza and SARS-CoV-2 has led to co-infection events, primarily affecting children and older adults, who are at higher risk for severe disease. Although co-infection prevalence is relatively low, it is associated with worse outcomes compared to mono-infections. Previous studies have shown that the outcomes of co-infection depend on multiple factors, including viral interference, virus-host interaction and host response. Children and the elderly exhibit distinct patterns of antiviral response, which involve airway epithelium, mucociliary clearance, innate and adaptive immune cells, and inflammatory mediators. This review explores the pathogeneses of SARS-CoV-2 and influenza co-infection, focusing on the antiviral responses in children and the elderly. By comparing immature immunity in children and immune senescence in older adults, we aim to provide insights for the clinical management of severe co-infection cases.

Lethal Synergistic Infections by Two Concurrent Respiratory Pathogens

Lethal synergistic infections by concurrent pathogens have occurred in humans, including human immunodeficiency virus and Mycobacterium tuberculosis infections, or in animal or human models of influenza virus, or bacteria, e.g., Streptococcus pneumoniae, concurrent with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, the intracellular synergistic interaction possibilities between two respiratory viral pathogens, or between viral and fungal pathogens, merits additional examination. The requirements for synergistic concurrent pathogen infections are: a) relatively little detrimental interference between two pathogens, b) one pathogen having the capability of directly or indirectly assisting the second pathogen by direct immuno-manipulation or indirect provision of infection opportunities and/or metabolic assistance, c) substantial human or environmental prevalence, possibly including a prevalence in any type of health-care facilities or other locations having congregations of potentially infected human or animal vectors and d) substantial transmissibility of the pathogens, which would make their concurrent pathogen infections much more probable. A new definition of pathogen synergy is proposed: "pathogen synergy is an interaction of two or more pathogens during concurrent infections causing an increased infection severity compared to mono-infections by the individual pathogens." Non-respiratory pathogens can also concurrently infect organs besides the lungs. However, the air-transmissible respiratory pathogens, particularly the RNA viruses, can enable highly widespread and synergistic concurrent infections. For instance, certain strains of coronaviruses, influenza viruses and similar respiratory viruses, are highly transmissible and/or widely prevalent in various vectors for transmission to humans and have numerous capabilities for altering lung immune defenses.

Viral co-detection of influenza virus and other respiratory viruses in hospitalized Brazilian patients during the first three years of the coronavirus disease (COVID)-19 pandemic: an epidemiological profile

Introduction

In Brazil, few studies were performed regarding the co-detection of respiratory viruses in hospitalized patients. In this way, the study aimed to describe the epidemiological profile of hospitalized patients due to influenza virus infection that presented co-detection with another respiratory virus.

Methods

The epidemiological analysis was made by collecting data from Open-Data-SUS. The study comprised patients infected by the influenza A or B virus with positive co-detection of another respiratory virus, such as adenovirus, bocavirus, metapneumovirus, parainfluenza virus (types 1, 2, 3, and 4), rhinovirus, and respiratory syncytial virus (RSV). The markers [gender, age, clinical signs and symptoms, comorbidities, need for intensive care unit (ICU) treatment, and need for ventilatory support] were associated with the chance of death. The data was collected during the first three years of the coronavirus disease (COVID)-19 pandemic-from December 19, 2019, to April 06, 2023.

Results

A total of 477 patients were included, among them, the influenza A virus was detected in 400 (83.9%) cases. The co-detection occurred, respectively, for RSV (53.0%), rhinovirus (14.0%), adenovirus (13.4%), parainfluenza virus type 1 (10.7%), parainfluenza virus type 3 (5.2%), metapneumovirus (3.8%), parainfluenza virus type 2 (3.6%), bocavirus (3.4%), and parainfluenza virus type 4 (1.5%). The co-detection rate was higher in the male sex (50.7%), age between 0-12 years of age (65.8%), and white individuals (61.8%). The most common clinical symptoms were cough (90.6%), dyspnea (78.8%), and fever (78.6%). A total of 167 (35.0%) people had at least one comorbidity, mainly cardiopathy (14.3%), asthma (8.4%), and diabetes mellitus (7.3%). The need for ICU treatment occurred in 147 (30.8%) cases, with most of them needing ventilatory support (66.8%), mainly non-invasive ones (57.2%). A total of 33 (6.9%) patients died and the main predictors of death were bocavirus infection (OR = 14.78 [95%CI = 2.84-76.98]), metapneumovirus infection (OR = 8.50 [95%CI = 1.86-38.78]), race (other races vs. white people) (OR = 3.67 [95%CI = 1.39-9.74]), cardiopathy (OR = 3.48 [95%CI = 1.13-10.71]), and need for ICU treatment (OR = 7.64 [95%CI = 2.44-23.92]).

Conclusion

Co-detection between the influenza virus and other respiratory viruses occurred, mainly with RSV, rhinovirus, and adenovirus being more common in men, white people, and in the juvenile phase. Co-detection of influenza virus with bocavirus and metapneumovirus was associated with an increased chance of death. Other factors such as race, cardiopathy, and the need for an ICU were also associated with a higher chance of death.

Bacterial and Viral Co-Infections in COVID-19 Patients: Etiology and Clinical Impact

BACKGROUND

Mixed infections can worsen disease symptoms. This study investigated the impact of mixed infections with viral and bacterial pathogens in patients positive for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

METHODS

Using the in-house multiplex PCR method, we tested 337 SARS-CoV-2 positive samples for co-infections with three bacterial and 14 other viral pathogens.

RESULTS

Between August 2021 and May 2022, 8% of 337 SARS-CoV-2-positive patients had bacterial co-infections, 5.6% had viral co-infections, and 1.4% had triple mixed infections. The most common causes of mixed infections were Haemophilus influenzae (5.93%) and respiratory syncytial virus (RSV) (1.18%). Children < 5 years old had more frequent co-infections than adults < 65 years old (20.8% vs. 16.4%), while adults showed a more severe clinical picture with a higher C-reactive protein (CRP) level (78.1 vs.16.2 mg/L; p = 0.033), a lower oxygen saturation (SpO2) (89.5 vs. 93.2%), and a longer hospital stay (8.1 vs. 3.1 days; p = 0.025) (mean levels). The risk of a fatal outcome was 41% in unvaccinated patients (p = 0.713), which increased by 2.66% with co-infection with two pathogens (p = 0.342) and by 26% with three pathogens (p = 0.005). Additionally, 50% of intensive care unit (ICU) patients had a triple infection, compared with only 1.3% in the inpatient unit (p = 0.0029). The risk of death and/or ICU admission was 12 times higher (p = 0.042) with an additional pathogen and increased by 95% (p = 0.003) with a third concomitant pathogen.

CONCLUSIONS

Regular multiplex testing is important for prompt treatment and targeted antibiotic use.

Infection Rates and Symptomatic Proportion of SARS-CoV-2 and Influenza in Pediatric Population, China, 2023

We conducted a longitudinal cohort study of SARS-CoV-2 and influenza rates in childcare centers and schools in Wuxi, China, collecting 1,760 environmental samples and 9,214 throat swabs from 593 students (regardless of symptoms) in weekly collections during February-June 2023. We estimated a cumulative infection rate of 124.8 (74 episodes)/1,000 persons for SARS-CoV-2 and 128.2 (76 episodes)/1,000 persons for influenza. The highest SARS-CoV-2 infection rate was in persons 18 years of age, and for influenza, in children 4 years of age. The asymptomatic proportion of SARS-CoV-2 was 59.6% and 66.7% for influenza; SARS-CoV-2 symptomatic proportion was lower in 16-18-year-olds than in 4-6-year-olds. Only samples from frequently touched surface tested positive for SARS-CoV-2 (4/1,052) and influenza (1/1,052). We found asynchronous circulation patterns of SARS-CoV-2 and influenza, similar to trends in national sentinel surveillance. The results support vaccination among pediatric populations and other interventions, such as environmental disinfection in educational settings.

A chimeric mRNA vaccine of S-RBD with HA conferring broad protection against influenza and COVID-19 variants

Influenza and coronavirus disease 2019 (COVID-19) represent two respiratory diseases that have significantly impacted global health, resulting in substantial disease burden and mortality. An optimal solution would be a combined vaccine capable of addressing both diseases, thereby obviating the need for multiple vaccinations. Previously, we conceived a chimeric protein subunit vaccine targeting both influenza virus and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), utilizing the receptor binding domain of spike protein (S-RBD) and the stalk region of hemagglutinin protein (HA-stalk) components. By integrating the S-RBD from the SARS-CoV-2 Delta variant with the headless hemagglutinin (HA) from H1N1 influenza virus, we constructed stable trimeric structures that remain accessible to neutralizing antibodies. This vaccine has demonstrated its potential by conferring protection against a spectrum of strains in mouse models. In this study, we designed an mRNA vaccine candidate encoding the chimeric antigen. The resultant humoral and cellular immune responses were meticulously evaluated in mouse models. Furthermore, the protective efficacy of the vaccine was rigorously examined through challenges with either homologous or heterologous influenza viruses or SARS-CoV-2 strains. Our findings reveal that the mRNA vaccine exhibited robust immunogenicity, engendering high and sustained levels of neutralizing antibodies accompanied by robust and persistent cellular immunity. Notably, this vaccine effectively afforded complete protection to mice against H1N1 or heterosubtypic H5N8 subtypes, as well as the SARS-CoV-2 Delta and Omicron BA.2 variants. Additionally, our mRNA vaccine design can be easily adapted from Delta RBD to Omicron RBD antigens, providing protection against emerging variants. The development of two-in-one vaccine targeting both influenza and COVID-19, incorporating the mRNA platform, may provide a versatile approach to combating future pandemics.

Prevalence of Co-Infections in Primary Care Patients with Medically Attended Acute Respiratory Infection in the 2022/2023 Season

In the post-pandemic period, an endemic circulation of respiratory viruses has been re-established. Respiratory viruses are co-circulating with SARS-CoV-2. We performed a retrospective analysis of co-infections in primary care patients with medically attended acute respiratory infections (MAARI) who consulted from week 40/2022 to week 39/2023 and were tested for a panel of respiratory viruses. Out of 2099 samples tested, 1260 (60.0%) were positive for one virus. In 340 samples, co-infection was detected: two viruses in 281 (13.4%), three viruses in 51 (2.4%), and four viruses in eight (0.4%) samples. Respiratory viruses co-infected the patients with MAARI at very different rates. The lowest rates of co-infections were confirmed for influenza B (13.8%) and influenza A (22.9%) and the highest for human bocaviruses (84.0%) and human parechoviruses (82.1%). Co-infections were detected in 28.2% of SARS-CoV-2 positive samples. SARS-CoV-2 has never been co-infected with influenza B virus, enterovirus or adenovirus, although the latter was found as a co-infecting virus with all other respiratory viruses tested. The rate of co-infections decreased significantly with increasing age (p-value 0.000), and no difference was found regarding gender (p-value 0.672). It is important to understand the epidemiology of respiratory co-infections for prevention and management decisions in patients with MAARI.

Modulation of innate immunity related genes resulting in prophylactic antimicrobial and antiviral properties

BACKGROUND

The innate immunity acts during the early phases of infection and its failure in response to a multilayer network of co-infections is cause of immune system dysregulation. Epidemiological SARS-CoV-2 infections data, show that Influenza Virus (FLU-A-B-C) and Respiratory Syncytial Virus (RSV) are co-habiting those respiratory traits. These viruses, especially in children (mostly affected by 'multi-system inflammatory syndrome in children' [MIS-C] and the winter pandemic FLU), in the aged population, and in 'fragile' patients are causing alteration in immune response. Then, bacterial and fungal pathogens are also co-habiting the upper respiratory traits (e.g., Staphylococcus aureus and Candida albicans), thus contributing to morbidity in those COVID-19 affected patients.

METHODS

Liquid chromatography coupled with high-resolution mass spectrometry using the quadrupole orbital ion trap analyser (i.e., UHPLC-Q-Orbitrap HRMS) was adopted to measure the polyphenols content of a new nutraceutical formula (Solution-3). Viral infections with SARS-CoV-2 (EG.5), FLU-A and RSV-A viruses (as performed in BLS3 authorised laboratory) and real time RT-PCR (qPCR) assay were used to test the antiviral action of the nutraceutical formula. Dilution susceptibility tests have been used to estimate the minimum inhibitory and bactericidal concentration (MIC and MBC, respectively) of Solution-3 on a variety of microorganisms belonging to Gram positive/ negative bacteria and fungi. Transcriptomic data analyses and functional genomics (i.e., RNAseq and data mining), coupled to qPCR and ELISA assays have been used to investigate the mechanisms of action of the nutraceutical formula on those processes involved in innate immune response.

RESULTS

Here, we have tested the combination of natural products containing higher amounts of polyphenols (i.e., propolis, Verbascum thapsus L., and Thymus vulgaris L.), together with the inorganic long chain polyphosphates 'polyPs' with antiviral, antibacterial, and antifungal behaviours, against SARS-CoV-2, FLU-A, RSV-A, Gram positive/ negative bacteria and fungi (i.e., Candida albicans). These components synergistically exert an immunomodulatory action by enhancing those processes involved in innate immune response (e.g., cytokines: IFNγ, TNFα, IL-10, IL-6/12; chemokines: CXCL1; antimicrobial peptides: HBD-2, LL-37; complement system: C3).

CONCLUSION

The prophylactic antimicrobial success of this nutraceutical formula against SARS-CoV-2, FLU-A and RSV-A viruses, together with the common bacteria and fungi co-infections as present in human oral cavity, is expected to be valuable.

Towards broad-spectrum protection: the development and challenges of combined respiratory virus vaccines

In the post-COVID-19 era, the co-circulation of respiratory viruses, including influenza, SARS-CoV-2, and respiratory syncytial virus (RSV), continues to have significant health impacts and presents ongoing public health challenges. Vaccination remains the most effective measure for preventing viral infections. To address the concurrent circulation of these respiratory viruses, extensive efforts have been dedicated to the development of combined vaccines. These vaccines utilize a range of platforms, including mRNA-based vaccines, viral vector vaccines, and subunit vaccines, providing opportunities in addressing multiple pathogens at once. This review delves into the major advancements in the field of combined vaccine research, underscoring the strategic use of various platforms to tackle the simultaneous circulation of respiratory viruses effectively.

Expression of the SARS-CoV-2 receptor-binding domain by live attenuated influenza vaccine virus as a strategy for designing a bivalent vaccine against COVID-19 and influenza

Influenza and SARS-CoV-2 are two major respiratory pathogens that cocirculate in humans and cause serious illness with the potential to exacerbate disease in the event of co-infection. To develop a bivalent vaccine, capable of protecting against both infections, we inserted the receptor-binding domain (RBD) of the SARS-CoV-2 spike protein into hemagglutinin (HA) molecule or into the open reading frame of the truncated nonstructural protein 1 (NS1) of live attenuated influenza vaccine (LAIV) virus and assessed phenotypic characteristics of the rescued LAIV-RBD viruses, as well as their immunogenicity in mouse and Syrian hamster animal models. A panel of 9 recombinant LAIV-RBD viruses was rescued using the A/Leningrad/17 backbone. Notably, only two variants with RBD insertions into the HA molecule could express sufficient quantities of RBD protein in infected MDCK cells. Intranasal immunization of mice induced high levels of anti-influenza antibody responses in all chimeric LAIV-RBD viruses, which was comparable to the LAIV virus vector. The RBD-specific antibody responses were most pronounced in the variant expressing RBD194 fragment as a chimeric HA protein. This candidate was further tested in Syrian hamsters and was shown to be immunogenic and capable of protecting animals against both infections.