Scientists have developed a protein-based COVID-19 vaccine candidate that mimics the shape of the virus to trigger a robust antibody response.

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In the study published in the journal ACS Central Science, the researchers immunised experimental specimens with nanoparticles that mimic SARS-CoV-2, the virus that causes COVID-19, by displaying multiple copies of the receptor binding domain (RBD) antigen. Most protein-based vaccines train the immune system to recognise the RBD, a portion of the SARS-CoV-2 spike protein, which the virus uses to enter and infect human cells. The spike protein binds to the ACE-2 receptor on host cell surfaces, which acts as a gateway for the entry of the virus.

However, not all vaccines elicit both antibody and T cell responses, both of which are thought to be important for longer-lasting immunity. The researchers from the University of Chicago, US, had previously developed a vaccine delivery tool called polymersomes -- self-assembling, spherical nanoparticles that can encapsulate antigens and adjuvants -- and then release them inside immune cells. Adjuvents are helper molecules that boost the immune response. Polymersomes trigger robust T cell immunity, the researchers said.

The team wondered if they could further improve the antibody response by engineering the nanoparticles to mimic viruses by displaying multiple copies of the RBD on their surfaces. The researchers made polymersomes that were similar in size to SARS-CoV-2 and decorated them with many RBDs. After characterising the nanoparticles in the lab, they injected them, along with separate polymersomes containing an adjuvant, in two doses that were three weeks apart.

For comparison, they immunised another group of with polymersomes that encapsulated the RBD, along with the nanoparticles containing the adjuvant. Although both groups produced high levels of RBD-specific antibodies, only the surface-decorated polymersomes generated neutralising antibodies that prevented SARS-CoV-2 infection in cells. Both the surface-decorated and encapsulated RBDs triggered robust T cell responses, the researchers said. Although the new vaccine still needs to be tested for safety and efficacy in humans, it could have advantages over mRNA vaccines with regard to widespread distribution in resource-limited areas, they said. That is because the surface-decorated polymersomes are stable and active for at least six months with refrigeration, the researchers said. In contrast, mRNA vaccines require subzero temperature storage, they added.