In a recent study published in the Journal of Virology, researchers developed a recombinant bivalent vaccine against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and influenza viruses.
The coronavirus disease 2019 (COVID-19) pandemic has been a serious global public health threat. Multiple vaccines have been used to prevent COVID-19, yet, the emergence of highly transmissible SARS-CoV-2 variants of concern (VOCs) has jeopardized the efforts to contain the pandemic. SARS-CoV-2 VOCs cause vaccine-breakthrough infections, thereby challenging the efficacy of existing COVID-19 vaccines and warranting the development of improved vaccines.
Influenza is a contagious respiratory disease caused primarily by the influenza A virus (IAV). Seasonal influenza is a public health concern with more than 300,000 annual deaths. Influenza vaccines are the most effective means to prevent illness, but they are less effective (10% to 60%) due to the differences between the vaccine strain and circulating strains.
Hence, designing a universal vaccine against all influenza strains is essential. Both influenza and COVID-19 are contagious diseases transmitted during the same seasons and pose a global threat to public health; as such, it is highly beneficial to design a vaccine that concurrently protects against SARS-CoV-2 and influenza viruses.
The study and findings
In the present study, researchers constructed three recombinant vesicular-stomatitis virus (rVSV)-based bivalent vaccine candidates for COVID-19 and influenza. First, they generated coding DNAs (cDNAs) encoding the spike protein (SP) of SARS-CoV-2 Delta with a 17 amino acids (aa) deletion in the C-terminus and a point mutation (I742A) [henceforth, SPΔC1]. Additionally, cDNAs encoding the S2 domain with a 381 aa deletion were constructed (SPΔC2).
The receptor-binding domain (RBD) from the SARS-CoV-2 Wuhan-Hu-1 strain was fused with the glycoprotein (GP) of the Ebola virus to generate the ERBD cDNA construct. Each of the three cDNAs was inserted into the rVSV-EM2e vaccine vector, which contained the Ebola GP fused with four copies of influenza M2 ectodomain (M2e) polypeptide to generate V-EM2e/ SPΔC1, V-EM2e/ SPΔC2, or V-EM2e/ERBD.
Two M2e copies were derived from human influenza strains, one from the avian flu strain and one from the swine flu strain. The replication ability of the candidate vaccines was examined in multiple cell lines such as A549, MRC-5, U251MG, cluster of differentiation 4-positive (CD4+) Jurkat T cells, and monocyte-derived macrophages (MDMs) and dendritic cells (MDDCs).
Although wild-type VSV exhibited efficient replication and typical cytopathic effects (CPEs) in various cell lines, rVSV candidates failed to infect MRC-5 and CD4+ Jurkat T cells. The vaccine candidates showed positive infection in other cell types and replicated much slower with less CPE than wild-type VSV. Two doses of each candidate vaccine were administered on days 0 and 14 in BALB/c mice through intramuscular (IM) or intranasal (IN) injection.
Serum anti-SARS-CoV-2 RBD and anti-M2 antibodies were measured. They found higher levels of anti-SARS-CoV-2 IgA and IgG antibodies with V-EM2e/ SPΔC1 and V-EM2e/ SPΔC2 candidates with IM administration than IN delivery. V-EM2e/ ERBD immunization via the IM route elicited much lower anti-RBD IgG antibodies than the other two vaccine candidates.
All candidates induced similarly high anti-M2 IgG and IgA antibodies regardless of the delivery route. Next, they evaluated neutralizing potency of antibodies induced by the vaccine candidates using SPΔC pseudoviruses. V-EM2e/ SPΔC1 vaccine induced the highest titers of neutralizing antibodies (nAbs) against SpΔCwildtype and SpΔCDelta pseudoviral infections, while V-EM2e/ERBD immunization had low neutralizing activity.
Next, splenocytes from control and immunized mice were cultured without (any) peptides, with an S1 overlapping peptide pool or influenza M2e peptides. Stimulating IN-immunized mouse splenocytes with S1 or M2e peptides markedly increased the secretion of interferon-gamma (IFN-γ) and, to a lesser extent, interleukin (IL)-4 compared to controls. However, IL-5 production was not stimulated by either S1 or M2e peptides.
In IM-immunized mouse splenocytes, elevated cytokine levels were evident before and remained unchanged after stimulation. Besides, mice immunized with V-EM2e/ SPΔC1 via IM or IN route were challenged with a lethal dose of H1N1 or H3N2 influenza strain on day 28. Control mice exhibited higher morbidity than immunized mice and lost > 20% weight by five/six days.
In contrast, vaccinated mice exhibited moderate weight loss, with a 100% survival rate regardless of the vaccination route. Lastly, the authors investigated the protective effect of V-EM2e/ SPΔC1 and V-EM2e/ SPΔC2 immunization in Syrian hamsters against SARS-CoV-2 infection. The team observed that a single IM dose of either vaccine was adequate to elicit peak anti-spike IgG antibody titers.
Hamsters were challenged with SARS-CoV-2 Delta 14 days after administering the second vaccine dose. Control hamsters (non-vaccinated) showed weight loss after infection and recovered by day 12. Vaccinated animals showed a marginal weight loss and began recuperating after two days. Oral swabs collected on day 3 showed significantly reduced viral RNA levels in vaccinated animals.
In summary, of the three bivalent vaccine candidates, V-EM2e/ SPΔC1 and V-EM2e/ SPΔC2 induced robust nAbs, humoral and cellular responses, and protected mice/hamsters against influenza (H1N1 and H3N2) and SARS-CoV-2 Delta infections. Taken together, the results provided substantial evidence for the excellent efficacy of the bivalent vaccine platform that could be expedited to create vaccines against novel or resurgent SARS-CoV-2 variants and IAV infections.