In the midst of the ongoing COVID-19 coronavirus pandemic, influenza virus remains a major threat to public health due to its potential to cause epidemics and pandemics with significant human mortality. Cases of H7N9 human infections emerged in eastern China in 2013 and immediately raised pandemic concerns as historically, pandemics were caused by the introduction of new subtypes into immunologically naïve human populations. Highly pathogenic H7N9 cases with severe disease were reported recently, indicating the continuing public health threat and the need for a prophylactic vaccine. Here we review the development of recombinant influenza virus-like particles (VLPs) as vaccines against H7N9 virus. Several approaches to vaccine development are reviewed including the expression of VLPs in mammalian, plant and insect cell expression systems. Although considerable progress has been achieved, including demonstration of safety and immunogenicity of H7N9 VLPs in the human clinical trials, the remaining challenges need to be addressed. These challenges include improvements to the manufacturing processes, as well as enhancements to immunogenicity in order to elicit protective immunity to multiple variants and subtypes of influenza virus.

The safety and genetic stability of V4020, a novel Venezuelan Equine Encephalitis Virus (VEEV) vaccine based on the investigational VEEV TC-83 strain, was evaluated in mice. V4020 was generated from infectious DNA, contains a stabilizing mutation in the E2-120 glycoprotein, and includes rearrangement of structural genes. After intracranial inoculation (IC), replication of V4020 was more attenuated than TC-83, as documented by low clinical scores, inflammation, viral load in brain, and earlier viral clearance. During the first 9 days post-inoculation (DPI), genes involved in inflammation, cytokine signaling, adaptive immune responses, and apoptosis were upregulated in both groups. However, the magnitude of upregulation was greater in TC-83 than V4020 mice, and this pattern persisted till 13 DPI, while V4020 gene expression profiles declined to mock-infected levels. In addition, genetic markers of macrophages, DCs, and microglia were strongly upregulated in TC-83 mice. During five serial passages in the brain, less severe clinical manifestations and a lower viral load were observed in V4020 mice and all animals survived. In contrast, 13.3% of mice met euthanasia criteria during the passages in TC-83 group. At 2 DPI, RNA-Seq analysis of brain tissues revealed that V4020 mice had lower rates of mutations throughout five passages. A higher synonymous mutation ratio was observed in the nsP4 (RdRP) gene of TC-83 compared to V4020 mice. At 2 DPI, both viruses induced different expression profiles of host genes involved in neuro-regeneration. Taken together, these results provide evidence for the improved safety and genetic stability of the experimental V4020 VEEV vaccine in a murine model.

Highly pathogenic Avian Influenza (HPAI) viruses continue to cause severe economic losses in poultry species worldwide. HPAI virus of subtype H5N1 was reported in Egypt in 2006, and despite vaccination efforts, the virus has become endemic. The current study aims to evaluate the efficacy of a virus-like particle (VLP) based vaccine in vivo using specific pathogen-free (SPF) chickens. The vaccine was prepared from the HPAI H5N1 virus of clade 2.2.1.2 using the baculovirus expression system. The VLPs were quantitated and characterized, including electron microscopy. In addition, the protection level of the VLPs was evaluated by using two different regimens, including one dose and two-dose vaccinated groups, which gave up to 70% and 100% protection level, respectively. The results of this study emphasize the potential usefulness of the VLPs-based vaccine as an alternative vaccine candidate for the control of AIV infection in poultry.

Single-dose, immunogenic DNA (iDNA) vaccines coding for whole live-attenuated viruses are reviewed. This platform, sometimes called immunization DNA, has been used for vaccine development for flavi- and alphaviruses. An iDNA vaccine uses plasmid DNA to launch live-attenuated virus vaccines in vitro or in vivo. When iDNA is injected into mammalian cells in vitro or in vivo, the RNA genome of an attenuated virus is transcribed, which starts replication of a defined, live-attenuated vaccine virus in cell culture or the cells of a vaccine recipient. In the latter case, an immune response to the live virus vaccine is elicited, which protects against the pathogenic virus. Unlike other nucleic acid vaccines, such as mRNA and standard DNA vaccines, iDNA vaccines elicit protection with a single dose, thus providing major improvement to epidemic preparedness. Still, iDNA vaccines retain the advantages of other nucleic acid vaccines. In summary, the iDNA platform combines the advantages of reverse genetics and DNA immunization with the high immunogenicity of live-attenuated vaccines, resulting in enhanced safety and immunogenicity. This vaccine platform has expanded the field of genetic DNA and RNA vaccines with a novel type of immunogenic DNA vaccines that encode entire live-attenuated viruses.

(1) Background: Mopeia–Lassa reassortant ML29 virus is an investigational, reassortant virus vaccine for the prevention of Lassa fever caused by Lassa virus (LASV). (2) Methods: The vaccine virus ML29-SF was prepared in Vero cells using a serum-free culture medium under Good Manufacturing Practice. A 2-week repeat dose toxicity study was performed in guinea pigs under Good Laboratory Practice (GLP) regulations to assess the local and systemic toxicological effects. (3) Results: Following an intramuscular (IM) or subcutaneous (SC) injection of 10⁴ PFU of ML29-SF LASV vaccine at the start of the study, with a second dose 15 days later, no toxicological response attributable to the vaccine was observed. Vaccine-related effects were not observed in any in-life or post-mortem parameter evaluated, including clinical observations, injection site observations, body temperature, body weight, food consumption, ophthalmology, immunology, hematology, clinical chemistry, gross anatomical pathology, organ weights, and histopathology. An immunogenic response, as measured by the elicitation of IgG antibodies against major LASV immunogens, nucleocapsid and glycoprotein precursor, was observed in all vaccine-treated animals prior to the booster dose (Study Day 15) which endured through the end of the study (Study Day 42). There was no evidence of viral shedding in any vaccinated animal. (4) Conclusions: Overall, this single-dose vaccine was locally and systemically well tolerated even after a two-dose repeat administration, confirming the high level of safety of ML29-SF vaccination and supporting the future evaluation of this LASV vaccine, including in clinical trials.