There was a rise in the event of coronavirus disease 2019 (COVID-19) vaccines in several countries because of the rapid spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

The RNA vaccine platform is a next-generation platform that comprises messenger RNA (mRNA), which helps to encode the goal antigen. The primary concept of an mRNA vaccine was developed in 1990; nonetheless, its application was limited because of ineffective in vivo delivery, instability, and high innate immunogenicity of mRNA.

Study: Evaluation of vaccine-induced immune responses in line with the immunization sequences of mRNA and protein vaccines. Image Credit: Kunal Mahto/Shutterstock

Background

Technological advances in lipid nanoparticles (LNPs), in addition to the introduction of nucleoside modification by pseudouridine will help to beat such limitations. The mixing of mRNA doesn’t happen into the host genome. This enables the manufacture of mRNA in a cell-free manner, leading to cost-effective, scalable, and rapid production.

Authorization of two COVID-19 mRNA vaccines (Pfizer/BioNTech BNT162b2 and Moderna mRNA-1273) took place by the US Food and Drug Administration (FDA) for emergency use inside a yr of their development. Other COVID-19 vaccines, comparable to viral vector and inactivated vaccines, have also been approved and administered worldwide.

Recently, a conventional protein subunit type vaccine, Novavax has been approved by the FDA for emergency use. The provision of those vaccines has led to the administration of various kinds of vaccines (heterologous prime-boost vaccination) to immunize people against a single virus.

Studies have reported heterologous prime-boost strategies to induce higher T-cell responses and neutralizing antibody titers. Nevertheless, the efficacy and immunogenicity of heterologous priming-boosting using mRNA and protein vaccines are unknown. The impact of the immunization sequence on vaccine efficacy can be not reported.

The influenza virus is a crucial zoonotic virus that causes about 3 to five million severe illness cases and 290,000 to 650,000 deaths globally annually. Although current influenza vaccines are effective against the virus, the emergence of novel pandemic strains r failure to predict the vaccine strain can result in a discount of their effectiveness. Subsequently, developing an influenza vaccine that might be rapidly produced is essential.

Currently, a couple of quadrivalent and monovalent influenza vaccines that encode hemagglutinin (HA) from seasonal influenza strains are undergoing clinical trials, and lots of others are within the preclinical phase.

A latest study available on as a preprint on Research Square* and under review at npj Vaccines aimed to research whether the order of immunization of vaccine types impacted the efficacy of a heterologous prime-boost vaccination strategy.

Concerning the study

The study involved six-week-old female BALB/c mice acclimatized for one week before starting the experiment. The DNA template for the mRNA vaccine was obtained from a DNA fragment that encoded the HA protein of the influenza A virus, followed by the synthesis of mRNA-HA. Thereafter, transfection of Vero cells took place using 10 µg mRNA followed by western blot.

Formulation of LNPs took place together with their characterization. Immunization of mice took place at a 2-week interval using 5 µg mRNA-HA or 1 µg HA protein. Elisa was used to find out antibody levels, followed by an ELISpot assay.

Mice were then challenged with the virus, after which clinical illness, survival, and body weight were assessed. Real-time polymerase chain response (PCR) was carried out using total RNA from bronchoalveolar lavage fluid (BALF) and lungs. Finally, BALF collection and histopathological evaluation were done.

Study findings

The outcomes indicated that priming with led to high levels of IgG2a, whereas priming with protein-HA led to an IgG1-biased response. Homologous mRNA-HA-immunization (R-R) and heterologous mRNA-HA/protein-HA-immunization (R-P) were observed to guide to balanced IgG1/IgG2a responses.

The R-P group was observed to indicate the next hemagglutination inhibition (HI) and microneutralization (MN) as in comparison with the P-R group. No significant differences regarding the interferon-γ (IFN-γ) cytokine-producing cells in splenocytes were observed between the R-P and P-R groups.

A better frequency of antigen-specific IFN-γ producing cells in CD4 + T cells was observed within the R-P group as in comparison with the P-P and P-R groups. A better frequency of IFN-γ or TNF-α producing cells in CD8 + T cells was also observed within the R-R group, while the next frequency of interleukin-2 (IL-2)-producing cells in CD4 + T cells was observed in P-R and R-P groups. Furthermore, higher numbers of CD4 + and CD8 + T cells were reported within the R-P and P-R groups as in comparison with the P-P group.

Different gene expression patterns were observed within the P-P and R-R groups but not between R-P and P-R groups. The P-P and P-R groups were observed to indicate increased neutrophil degranulation and mast cell pathways. Furthermore, increased stimulatory C-type lectin receptor signaling pathways, cytotoxic T cell differentiation pathways, and helper T cell diapedesis were also increased within the P-R group.

The R-P group was observed to have similar enriched pathways together with increased CD8 + T-cell activation and Th2 differentiation pathways. The R-R group was observed to indicate increased regulation of the dendritic cell pathway, innate immune response signaling, Th2, and cytotoxic T-cell differentiation pathways.

Moreover, it was also observed to indicate increased expression of Bcl6, which is a transcription factor for Hmgb1 and follicular helper T-cells, together with interferon regulatory factor 1 (Irf1) and V-set immunoregulatory receptor.

Moreover, no significant difference was observed in regards to the protective efficacy of the homologous and heterologous prime-boost regimes. Mild to moderate lung changes were observed within the P-R group, while minimal to mild changes were observed within the R-P group. The viral titers within the BALF and lungs were observed to be reduced 1-week post-viral challenge within the R-P group in comparison with the P-R group. IgG2a levels were observed to be higher within the P-R group as in comparison with the R-P group.

Higher percentages of IL-2, TNF-α, and IFN-γ producing CD4 +T cells were observed within the R-P group, while higher percentages of IL-2, TNF-α, and IFN-γ producing CD8 + T cells were observed within the P-R group. Moreover, the central CD8 + T cells in addition to proliferating effector CD4 + and CD8 + T cells following viral challenge, were observed to be lowest within the R-P group.

Conclusion

Subsequently, the present study suggests that a heterologous vaccination strategy with an initial inoculation with an mRNA vaccine followed by a secondary or tertiary inoculation with a protein vaccine is likely to be probably the most effective and secure vaccination strategy against the virus.

*Vital notice

Research Square publishes preliminary scientific reports that aren’t peer-reviewed and, due to this fact, shouldn’t be thought to be conclusive, guide clinical practice/health-related behavior, or treated as established information.