The ongoing COVID-19 pandemic is causing huge impact on health, life and global economy which is characterized by rapid spreading of SARS-CoV-2, high number of confirmed cases and a fatality/case rate worldwide reported by WHO. The most effective intervention measure will be to develop safe and effective vaccines to protect the population from the disease and limit the spread of the virus. An inactivated, whole virus vaccine candidate of SARS-CoV-2 has been developed by Wuhan Institute of Biological Products and Wuhan Institute of Virology. The low toxicity, immunogenicity and immune persistence were investigated in preclinical studies using 7 different species of animals. The results showed that the vaccine candidate was well tolerated and stimulated high levels of specific IgG and neutralizing antibodies. Low or no toxicity in three species of animals was also demonstrated in preclinical study of the vaccine candidate. Biochemical analysis of structural proteins and purity analysis were performed. The inactivated, whole virion vaccine was characterized with safe double-inactivation, no use of DNases and high purity. Dosages, boosting times, adjuvants, and immunization schedules were shown to be important for stimulating a strong humoral immune response in animals tested. Preliminary observation in ongoing phase I and II clinical trials of the vaccine candidate in Wuzhi County, Henan Province, showed that the vaccine is well tolerant. The results were characterized by very low proportion and low degree of side effects, high levels of neutralizing antibodies and seroconversion. These results consistent with the results obtained from preclinical data on the safety.

Dear Editor, Since the outbreak of a novel coronavirus disease (COVID-19) in late 2019, it has spread rapidly and developed into a global pandemic. As of August 12, 2020, more than 215 countries and territories around the world have reported more than 20.5 million confirmed COVID-19 cases with over 745,693 deaths (https://www. worldometers.info/coronavirus/#countries). Such harsh conditions urged scientists across the world to gear up to develop vaccines and antiviral drugs against COVID-19, which also lead to massive requirement for experimental animals. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative pathogen of COVID-19. It has been demonstrated that SARS-CoV-2 uses angiotensin converting enzyme 2 (ACE2) as cellular receptor for entry into target cells. Mouse model is the most commonly used animal model for studying human diseases. However, SARS-CoV-2 fails to invade and replicate in this traditional animal model due to the structural differences in mouse ACE2 (mACE2) compared with human ACE2 (hACE2), 1 which has become the major hurdle for COVID-19 study. Currently, several strategies have been developed to overcome this receptor incompatibility by: (i) generating transgenic mice bearing hACE2 receptor, 2-4 (ii) establishing adenovirus hACE2 mouse model with recombinant adenovirus expressing hACE2, 5 and (iii) adapting the SARS-CoV-2 by serial passages in the respiratory tract of mice. 6-8 In this study, we used an alternative strategy to generate a SARS-CoV-2-sensitive mouse model by exogenous delivery of hACE2 with Venezuelan equine encephalitis replicon particles (VEEV-VRP-hACE2) (Supplementary information, Fig. S1a). VEEV is a positive sense, single-stranded RNA virus which belongs to the genus Alphavirus, family Togaviridae. Alphavirus replicon particles (VRPs), including VEEV-VRPs, represent efficient vectors for gene delivery and have been applied to studies of vaccine development, gene therapy and cell transduction. They contain self-replicating RNAencoding viral replicase proteins (nsP1-nsP4) and express the gene of interest in place of viral structural protein genes. 9 By providing viral structural proteins in trans, the replicon RNA is packaged into VEEV-VRPs for in vitro and in vivo gene delivery. 10 Due to their intrinsic biological properties, VEEV-VRPs offer several advantages with a broad range of susceptible host cells, high expression level of cytoplasmic proteins and easy manipulation of recombinant RNA molecules using cDNA clones. 10,11 Here, Venezuelan equine encephalitis virus (VEEV) replicon expressing hACE2 with a C-terminal Stag was packaged into VRPs using the helper RNAs encoding VEEV capsid and envelope proteins to produce VEEV-VRP-hACE2 (Supplementary information, Fig. S1). MLE-12 cells (mouse lung type II epithelial cell line) were used to evaluate the availability of VEEV-VRP-hACE2 for SARS-CoV-2-sensitive cells establishment. After confirming hACE2 expression in MLE-12 cells transduced with VEEV-VRP-hACE2 (VRP-hACE2) through indirect immunofluoresc...

In the face of COVID-19 pandemic caused by the newly emerged SARS-CoV-2, an inactivated, Vero cell-based, whole virion vaccine candidate has been developed and entered into phase III clinical trials within six months. Biochemical and immunogenic characterization of structural proteins and their post-translational modifications in virions, the end-products of the vaccine candidate, would be essential for the quality control and process development of vaccine products and for studying the immunogenicity and pathogenesis of SARS-CoV-2. By using a panel of rabbit antisera against virions and five structural proteins together with a convalescent serum, the spike (S) glycoprotein was shown to be N-linked glycosylated, PNGase F-sensitive, endoglycosidase H-resistant and cleaved by Furin-like proteases into S1 and S2 subunits. The full-length S and S1/S2 subunits could form homodimers/trimers. The 3 membrane (M) protein was partially N-linked glycosylated; the accessory protein 3a existed in three different forms, indicative of cleavage and dimerization. Furthermore, analysis of the antigenicity of these proteins and their post-translationally modified forms demonstrated that S protein induced the strongest antibody response in both convalescent and immunized animal sera. Interestingly, immunization with the inactivated vaccine did not elicit antibody response against the S2 subunit, whereas strong antibody response against both S1 and S2 subunits was detected in the convalescent serum. Moreover, vaccination stimulated stronger antibody response against S multimers than did the natural infection. This study revealed that the native S glycoprotein stimulated neutralizing antibodies, while bacterially-expressed S fragments did not. The study on S modifications would facilitate design of S-based anti-SARS-CoV-2 vaccines.

The ongoing coronavirus disease 2019 (COVID-19) pandemic caused more than 96 million infections and over 2 million deaths worldwide so far. However, there is no approved vaccine available for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the disease causative agent. Vaccine is the most effective approach to eradicate a pathogen. The tests of safety and efficacy in animals are pivotal for developing a vaccine and before the vaccine is applied to human populations. Here we evaluated the safety, immunogenicity, and efficacy of an inactivated vaccine based on the whole viral particles in human ACE2 transgenic mouse and in non-human primates. Our data showed that the inactivated vaccine successfully induced SARS-CoV-2-specific neutralizing antibodies in mice and non-human primates, and subsequently provided partial (in low dose) or full (in high dose) protection of challenge in the tested animals. In addition, passive serum transferred from vaccine-immunized mice could also provide full protection from SARS-CoV-2 infection in mice. These results warranted positive outcomes in future clinical trials in humans. Supplementary Information The online version contains supplementary material available at 10.1007/s12250-021-00376-w.