June 16, 2024

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First COVID-XBB Vaccine Approved for Emergency Use

First COVID-XBB Vaccine Approved for Emergency Use: Approximately 93.28% Protection Against XBB Infection 14 Days after Vaccination. 


First COVID-XBB Vaccine Approved for Emergency Use: Approximately 93.28% Protection Against XBB Infection 14 Days after Vaccination. 

On June 8th, the recombinant trivalent COVID-19 vaccine targeting XBB (XBB+BA.5+Delta variants), developed by West China School of Medicine, Sichuan University, in collaboration with WestVac BioPharma, was approved for emergency use by relevant national authorities.

This marks the first globally approved COVID-19 vaccine specifically targeting XBB and other variant strains. It is expected that the public will be able to receive this vaccine within the next 1 to 2 months.‘


First COVID-XBB Vaccine Approved for Emergency Use: Approximately 93.28% Protection Against XBB Infection 14 Days after Vaccination


The WestVac trivalent XBB vaccine, named Wiexin®, utilizes subunit vaccine antigens designed to bind to the spike protein receptor-binding domain (S-RBD) and heptad repeat (HR) of the XBB and BA.5 mutant strains of the coronavirus.

These antigens self-assemble into stable trimeric protein particles, which are then combined with an oil-in-water emulsion adjuvant based on squalene after purification.

This innovative adjuvant significantly enhances the neutralizing antibody titers of the vaccine and induces a stronger T-cell immune response.


Clinical trial data shows that the Wiexin® trivalent XBB vaccine induces high levels of neutralizing antibodies against various mutant strains such as XBB.1, XBB.1.5, XBB.1.16, XBB.1.9.1, XBB.2.3, BA.5, BF.7, BQ.1, BA.2.75. 14 days after receiving the vaccine, it demonstrates a protective efficacy of 93.28% against symptomatic COVID-19 caused by XBB.1, XBB.1.5, and XBB.1.9 variant strains, with good safety profile.


During a press conference held by the Joint Prevention and Control Mechanism of the State Council on May 8th, Chen Cao, a researcher from the Virus Disease Institute of the Chinese Center for Disease Control and Prevention, stated that the XBB variant is a recombinant variant of the Omicron strain with enhanced transmissibility and immune escape capabilities compared to earlier Omicron variants. According to current monitoring data, the proportion of XBB variant cases among imported cases in late April reached 97.5% in China.

By May 8th, XBB variants had become the predominant circulating strains of the novel coronavirus in the country

. On May 18th, the Technical Advisory Group on COVID-19 Vaccine Composition of the World Health Organization (WHO TAG-CO-VAC) recommended updating the composition of this year’s COVID-19 vaccines to induce neutralizing antibody responses specifically targeting the global dominant XBB variant strains.


WestVac BioPharma is an innovative biopharmaceutical company that specializes in vaccine research, production, and sales. It was founded by scientists from the National Key Laboratory of Biotherapy at West China Hospital, Sichuan University. In December 2022, WestVac BioPharma’s recombinant COVID-19 vaccine (Sf9 cells), named Wiexin®, was approved for emergency use by relevant national authorities and has since been successfully tendered and supplied for vaccination in more than 20 provinces across China.



What technical details are worth noting about this vaccine?

From the news reports, it is known that the WestVac vaccine, whether the original strain or the XBB trivalent, is produced using insect cell line Sf.

Sf cells are one of many systems used for producing recombinant protein vaccines and do not have any absolute special features.

GSK’s bivalent HPV vaccine is also produced using Sf, but the high efficacy of the HPV vaccine is mainly due to the fact that the recombinant proteins can self-assemble into virus-like particles, which have high immunogenicity and induce antibody levels far exceeding natural HPV infections.

Additionally, GSK also uses the highly efficient adjuvant AS04 in their HPV vaccine. There is no evidence suggesting that the Sf protein production system has a special effect on enhancing vaccine effectiveness.


Both the news release for the original strain and the XBB trivalent vaccine mention the use of the RBD portion of the S protein as the antigen, which is similar to the earliest approved domestically produced recombinant protein vaccine Zhifei. Moreover, like Zhifei, WestVac is also produced as a multimer, with Zhifei being a dimer in the case of the original strain vaccine, while WestVac XBB vaccine is mentioned as a trimer.


It is worth noting that the immunogenicity of the short antigen RBD appears to be inferior to that of the full-length S protein, which seems to be a general trend.

For example, BioNTech has used the same mRNA vaccine platform to develop both designs, and the full-length S protein outperforms the RBD in terms of safety and immunogenicity. Another notable vaccine that uses RBD instead of the full-length S protein is Clover’s mRNA vaccine, which was once touted as the leading Chinese mRNA COVID-19 vaccine seed but has not released any data following a rather average Phase I clinical trial.


However, what is even more noteworthy is the discrepancy in the adjuvant reported between the original strain version and the XBB version of the WestVac vaccine.

The adjuvant mentioned in the original strain paper for the WestVac vaccine is traditional aluminum hydroxide adjuvant, which is also used by Zhifei.

Aluminum hydroxide has lower immunostimulatory properties compared to various new types of adjuvants. Therefore, both vaccines require three doses to induce neutralizing antibodies, and this low immunogenicity performance is as expected.

However, it is puzzling that online reports now claim that the WestVac XBB trivalent vaccine uses an adjuvant based on squalene-water emulsion.

If different adjuvants are used, then it belongs to completely different vaccines, and generally, the same brand name should not be used. Furthermore, safety data and other information cannot be extrapolated from a vaccine with a completely different adjuvant.


What is squalene adjuvant?

According to the press release of the Wixen XBB trivalent vaccine, it uses a squalene-based water-in-oil emulsion adjuvant. The news report also states that the vaccine’s effectiveness has greatly increased due to this adjuvant. Adjuvants are components used in some vaccines to enhance a stronger immune response. Some vaccines with a robust immune response utilize potent adjuvants. So, what is squalene adjuvant?

Based on the description, it should be similar to GSK’s AS03 and Novartis’ MF59 adjuvants. Both of these adjuvants contain squalene. AS03 also contains polysorbate 80 (also known as Tween 80) and vitamin E, while MF59 contains polysorbate 80 and squalene trioleate (Span 85).

Water-in-oil emulsion is a common formulation for squalene adjuvants. If squalene adjuvants are not prepared using this method, it may affect the ability to stimulate the immune system, meaning it may not function as an adjuvant.

The mentioned squalene adjuvants have been around for some time. AS03 was used in GSK’s 2009 H1N1 vaccine, and a flu vaccine called Fluad (ad refers to containing an adjuvant) used MF59 in seasonal influenza vaccines in Europe and the United States.

AS03 is no longer on the market. GSK’s bivalent HPV vaccine used the next-generation adjuvant AS04, and the shingles vaccine Shingrix used AS01B. GSK’s recently approved RSV vaccine used AS01E, with the same components as AS01 but in a lower dose, likely to reduce the severity of common adverse reactions like fever and injection site pain. None of these adjuvants contain squalene; instead, they contain monophosphoryl lipid A (MPLA), which stimulates TLR-4.

AS03 and MF59 have been primarily used in influenza vaccines, so most of the related data are focused on influenza vaccines. Compared to adjuvant-free flu vaccines, the use of these adjuvants can induce higher levels of antibodies or achieve similar levels of antibodies with a smaller amount of antigen.

Regarding the squalene adjuvant used in the Wixen vaccine and its effects, there are currently no published reports or literature available. Additionally, as mentioned earlier, the Wixen original strain vaccine approved in 2022 appears to use a different adjuvant, not squalene. If the adjuvant is different, safety data from previous vaccines cannot be used as a reference for vaccines with changed antigens.


(source:internet, reference only)

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