Heterologous Omicron-adapted vaccine as a secondary booster promotes neutralizing antibodies against Omicron and its sub-lineages in mice.

Over one billion people have received 2-3 dosages of an inactivated COVID-19 vaccine for basic immunization. Whether a booster dose should be delivered to protect against the Omicron variant and its sub-lineages, remains controversial. Here, we tested different vaccine platforms targeting the ancestral or Omicron strain as a secondary booster of the ancestral inactivated vaccine in mice. We found that the Omicron-adapted inactivated viral vaccine promoted a neutralizing antibody response against Omicron in mice. Furthermore, heterologous immunization with COVID-19 vaccines based on different platforms remarkably elevated the levels of cross- neutralizing antibody against Omicron and its sub-lineages. Omicron-adapted vaccines based on heterologous platforms should be prioritized in future vaccination strategies to control COVID-19.

A Short 5'triphosphate RNA nCoV-L Induces a Broad-Spectrum Antiviral Response by Activating RIG-I.

Small molecular nucleic acid drugs produce antiviral effects by activating pattern recognition receptors (PRRs). In this study, a small molecular nucleotide containing 5'triphosphoric acid (5'PPP) and possessing a double-stranded structure was designed and named nCoV-L. nCoV-L was found to specifically activate RIG-I, induce interferon responses, and inhibit duplication of four RNA viruses (Human enterovirus 71, Human poliovirus 1, Human coxsackievirus B5 and Influenza A virus) in cells. In vivo, nCoV-L quickly induced interferon responses and protected BALB/c suckling mice from a lethal dose of the enterovirus 71. Additionally, prophylactic administration of nCoV-L was found to reduce mouse death and relieve morbidity symptoms in a K18-hACE2 mouse lethal model of SARS-CoV-2. In summary, these findings indicate that nCoV-L activates RIG-I and quickly induces effective antiviral signals. Thus, it has potential as a broad-spectrum antiviral drug.

Heterologous immunization with adenovirus vectored and inactivated vaccines effectively protects against SARS-CoV-2 variants in mice and macaques.

To cope with the decline in COVID-19 vaccine-induced immunity caused by emerging SARS-CoV-2 variants, a heterologous immunization regimen using chimpanzee adenovirus vectored vaccine expressing SARS-CoV-2 spike (ChAd-S) and an inactivated vaccine (IV) was tested in mice and non-human primates (NHPs). Heterologous regimen successfully enhanced or at least maintained antibody and T cell responses and effectively protected against SARS-CoV-2 variants in mice and NHPs. An additional heterologous booster in mice further improved and prolonged the spike-specific antibody response and conferred effective neutralizing activity against the Omicron variant. Interestingly, priming with ChAd-S and boosting with IV reduced the lung injury risk caused by T cell over activation in NHPs compared to homologous ChAd-S regimen, meanwhile maintained the flexibility of antibody regulation system to react to virus invasion by upregulating or preserving antibody levels. This study demonstrated the satisfactory compatibility of ChAd-S and IV in prime-boost vaccination in animal models.

A Novel Single-Stranded RNA-Based Adjuvant Improves the Immunogenicity of the SARS-CoV-2 Recombinant Protein Vaccine.

The research and development (R&D) of novel adjuvants is an effective measure for improving the immunogenicity of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) recombinant protein vaccine. Toward this end, we designed a novel single-stranded RNA-based adjuvant, L2, from the SARS-CoV-2 prototype genome. L2 could initiate retinoic acid-inducible gene-I signaling pathways to effectively activate the innate immunity. ZF2001, an aluminum hydroxide (Al) adjuvanted SARS-CoV-2 recombinant receptor binding domain (RBD) subunit vaccine with emergency use authorization in China, was used for comparison. L2, with adjuvant compatibility with RBD, elevated the antibody response to a level more than that achieved with Al, CpG 7909, or poly(I:C) as adjuvants in mice. L2 plus Al with composite adjuvant compatibility with RBD markedly improved the immunogenicity of ZF2001; in particular, neutralizing antibody titers increased by about 44-fold for Omicron, and the combination also induced higher levels of antibodies than CpG 7909/poly(I:C) plus Al in mice. Moreover, L2 and L2 plus Al effectively improved the Th1 immune response, rather than the Th2 immune response. Taken together, L2, used as an adjuvant, enhanced the immune response of the SARS-CoV-2 recombinant RBD protein vaccine in mice. These findings should provide a basis for the R&D of novel RNA-based adjuvants.

Transcriptomic analysis of the innate immune signatures of a SARS-CoV-2 protein subunit vaccine ZF2001 and an mRNA vaccine RRV.

Analysis of large-scale gene expression post vaccination can provide an overview of immune responses. We used transcriptional approaches to comprehensively analyze the innate immune response signatures elicited by protein subunit (PS) vaccine ZF2001 and an mRNA vaccine named RRV. A fine-grained time-dependent dissection of large-scale gene expression post immunization revealed that ZF001 induced MHC class II-related genes, including cd74 and H2-Aa, more expeditiously than the RRV. Notably, the RRV induced MHC class I-related genes such as Tap1/2, B2m, and H2-D1/K1. At day 21 post immunization, the titres of binding and neutralization antibody (NAb) induced by both vaccines were comparable, which were accordant with the expression level of genes essential to BCR/TCR signalling transduction and B/T cells activation at day 7. However, compared to ZF2001, the early responses of RRV were more robust, including the activation of pattern recognition receptors (PRRs), expression of genes involved in RNA degradation, and transcription inhibition, which are directly related to anti-viral signals. This pattern also coincided with the induction of cytokines by the RRV. Generally, the transcriptomic patterns of two very different vaccines mapped here provide a framework for establishing correlates between the induction of genes and protection, which can be tailored for evoking specific and potent immune responses against SARS-CoV-2.

Advances in COVID-19 mRNA vaccine development.

To date, the coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has determined 399,600,607 cases and 5,757,562 deaths worldwide. COVID-19 is a serious threat to human health globally. The World Health Organization (WHO) has declared COVID-19 pandemic a major public health emergency. Vaccination is the most effective and economical intervention for controlling the spread of epidemics, and consequently saving lives and protecting the health of the population. Various techniques have been employed in the development of COVID-19 vaccines. Among these, the COVID-19 messenger RNA (mRNA) vaccine has been drawing increasing attention owing to its great application prospects and advantages, which include short development cycle, easy industrialization, simple production process, flexibility to respond to new variants, and the capacity to induce better immune response. This review summarizes current knowledge on the structural characteristics, antigen design strategies, delivery systems, industrialization potential, quality control, latest clinical trials and real-world data of COVID-19 mRNA vaccines as well as mRNA technology. Current challenges and future directions in the development of preventive mRNA vaccines for major infectious diseases are also discussed.

Establishment of the first Chinese national standard for protein subunit SARS-CoV-2 vaccine.

A reference standard is needed for quality control of protein subunit SARS-CoV-2 vaccines to meet urgent domestic needs. The Chinese National Institutes for Food and Drug Control (NIFDC) launched a project to establish the first reference material for the protein subunit SARS-CoV-2 vaccine to be used for calibration of antigen testing. The potency and stability of the national candidate standard (CS) were determined by collaborative calibration, and accelerated and freeze-thaw degradation studies. Moreover, a suitability study of the CS was performed. Eight laboratories in mainland China were asked to detect antigen content of CS using a common validated enzyme-linked immunosorbent assay (ELISA) kit established by NIFDC and in-house kits in the collaborative study. Six laboratories returned valid results, which established that the antigen content of the CS was 876,938 YU/mL, with good agreement across laboratories. In the suitability study, the CS exhibited excellent parallelism and a linear relationship with four samples produced by different expression systems and target proteins. In addition, good stability in the accelerated and freeze-thaw degradation study was observed. In conclusion, the CS was approved by the Biological Product Reference Standards Sub-Committee of the National Drug Reference Standards Committee as the first Chinese national standard for determining antigen content of protein subunit SARS-CoV-2 vaccines, with an assigned antigen content of 877,000 U/mL (Lot. 300050-202101). This standard will contribute to a standardized assessment of protein subunit SARS-CoV-2 vaccine in China and may provide experience for developing reference materials for antigen content detection of SARS-CoV-2 vaccine in other countries.

Boosting with heterologous vaccines effectively improves protective immune responses of the inactivated SARS-CoV-2 vaccine.

Since the outbreak of COVID-19, a variety of vaccine platforms have been developed. Amongst these, inactivated vaccines have been authorized for emergency use or conditional marketing in many countries. To further enhance the protective immune responses in populations that have completed vaccination regimen, we investigated the immunogenic characteristics of different vaccine platforms and tried homologous or heterologous boost strategy post two doses of inactivated vaccines in a mouse model. Our results showed that the humoral and cellular immune responses induced by different vaccines when administered individually differ significantly. In particular, inactivated vaccines showed relatively lower level of neutralizing antibody and T cell responses, but a higher IgG2a/IgG1 ratio compared with other vaccines. Boosting with either recombinant subunit, adenovirus vectored or mRNA vaccine after two-doses of inactivated vaccine further improved both neutralizing antibody and Spike-specific Th1-type T cell responses compared to boosting with a third dose of inactivated vaccine. Our results provide new ideas for prophylactic inoculation strategy of SARS-CoV-2 vaccines.