Efficacy of a hybrid vaccine against COVID-19 and influenza

In a recently published study in vaccinationsresearchers evaluated the efficacy of a hybrid vaccine against influenza and coronavirus disease 2019 (COVID-19) viruses.

Study: Influenza virus-like particle-based hybrid vaccine with RBD induces immunity against influenza and SARS-CoV-2 viruses. Photo credit: insta_photos/Shutterstock

People worldwide are exposed to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and the influenza virus. Therefore, the development of a two-in-one vaccine that could elicit protection against these two infections is a topic of great interest.

About the study

In the present study, researchers developed a hybrid vaccine using an influenza virus-like particulate vaccine (VLP) incorporating a glycosylphosphatidylinositol (GPI)-anchored SARS-CoV-2 receptor-binding domain (RBD) linked to a granulocyte-macrophage colony -stimulating factor (GM-CSF) fusion protein.

The team developed a fusion protein gene by fusing deoxyribonucleic acid (DNA) sequences specific for the RBD domain of the SARS-CoV-2 spike (S) protein, GPI anchor signal derived from human CD59, and mouse GM-CSF are. Flow cytometry was then performed to show expression of the fusion protein on the transfected CHO-S cells. The team also assessed whether the anti-RBD antibodies present in the recovering sera of SARS-CoV-2 infected patients identified the fusion protein’s RBD by performing an enzyme-linked immunosorbent assay (ELISA).

The team then assessed whether the fusion protein had dual functions as GM-CSF while binding to human ACE2. In addition, researchers developed the VLP hybrid vaccine by incorporating the influenza VLP with GPI-RBD-GM-CSF and GPI-Interleukin (IL)-12 via protein transfer. Flow cytometry using anti-GM-CSF antibodies and fluorochrome-conjugated anti-IL-12 was also performed to verify dual incorporation of GPI-RBD-GM-CSF and GPI-IL-12 on VLPs. The maintenance of function by VLP-incorporated GM-CSF was verified by culturing mouse bone marrow-derived dendritic cells (BDMC), followed by measuring BMDC proliferation using the XTT assay.

The team tested the induction of the antibody response by the GPI-RBD-GM-CSF fusion protein by immunizing BALB/c mice with either the fusion protein or VLPs incorporated into the fusion protein along with GPI-IL-12. As a control, VLP without cytokines or commercially available RBD was used. The team gave the mouse population a booster dose two to four weeks after the first dose. Blood samples were taken from the mice every two to four weeks to assess antibody titer, virus neutralization tests and inhibition of ACE2 binding.


The study results showed that almost 76% of the fusion protein was secreted from the cell surface of the RBD-GM-CSF fusion protein, suggesting that the fusion protein was anchored to the cell membrane by a GPI tail. The RBD in the fusion protein showed ACE2 binding activity, while RBD-specific neutralizing antibodies could bind to GPI-RBD-GM-CSF present on the CHO-S cell transfectants.

Antibodies found in the sera of COVID-19 patients could efficiently bind to GPI-RBD-GM-CSF. In addition, ELISA confirmed that RBD from the purified fusion protein retained its receptor binding activity. Furthermore, the XTT assay showed that GPI-RBD-GM-CSF incorporated into the VLP vaccine could effectively stimulate BMDC proliferation.

The team observed that the VLP hybrid vaccine elicited a significant antibody response following the administration of the GPI-RBD-GM-CSF booster dose. The antibody response elicited by the fusion protein was similar by both the intramuscular and subcutaneous routes of vaccine administration. The antibodies bound selectively to the CHO-S cells expressing the fusion protein but not to the untransfected CHO-S cells. This demonstrated the high specificity of the antibody response to the fusion protein.

The team also found that the purified GPI-RBD-GM-CSF fusion protein-induced antibody levels were comparable to those elicited by the VLP hybrid vaccine. However, the neutralizing antibody titers were significantly low in the mice immunized only with the GPI-RBD-GM-CSF protein without any VLP. This indicated that fusion protein incorporated into VLP elicited a more robust neutralizing antibody response compared to that of the soluble fusion protein.

Furthermore, the purified GPI-RBD-GM-CSF fusion protein alone or when incorporated into VLPs induced a substantial antibody response. Alone, the fusion protein induced a primarily Th2-type immunoglobulin 1 (IgG1) response, while the hybrid VLP vaccine elicited both IgG1 and IgG2a responses. Notably, incorporation of GPI-IL-12 into the hybrid VLP vaccine did not affect the IgG2a response.

The team also observed that the hybrid vaccine alone and the hybrid vaccine without IL-12 elicited robust antibody responses against the RBD as well as the influenza VLP antigens. These antibodies could sufficiently neutralize a live viral infection when evaluated using a plaque reduction neutralization titer (PRNT) assay. In addition, substantially reduced viral replication was observed in the hybrid vaccine-treated mice compared to the VLP-treated mice.

Overall, study results indicated that delivery of SARS-CoV-2 RBD protein via influenza VLPs along with cytokine adjuvants served as an effective platform for multivalent vaccine development. Researchers believe that using immobilized cytokines as an adjuvant will help elicit antiviral immunity with minimal side effects.

Magazine reference:

  • Bommireddy, R.; Stein, S.; Bhatnagar, N.; Kumari, P.; Munoz, LE; Oh, J.; Kim, K.-H.; Berry, JTL; Jacobsen, KM; Jaafar, L.; et al. (2022). Influenza virus-like particle-based hybrid vaccine with RBD induces immunity against influenza and SARS-CoV-2 viruses. vaccinations. doi: https://doi.org/10.3390/vaccines10060944 https://www.mdpi.com/2076-393X/10/6/944

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