A more effective subunit vaccine

Subunit vaccines are those which contain only a part of a pathogen’s protein. They are amongst the safest and the most used against multiple infectious diseases (hepatitis B, diphtheria or tetanus for example). An essential component of these vaccines is the adjuvant, which helps to create a stronger, longer lasting and better quality immune response. Aluminium salts can be used, while new adjuvants have been created over recent years, notably squalene (extract of fish oil). A collaborative study, carried out by American researchers, has recently shown the effectiveness of different adjuvants in subunit vaccines in stimulating a protective immune system response against SARS-CoV-2.

These researchers developed a vaccine made up of a part of the virus’ spike (S) surface protein, the RBD (Receptor Binding Domain) subunit coupled to a nanoparticle. The RBD binds with the ACE2 cellular receptor to enable entry of the viral particle into the cell and is the principal target of neutralizing antibodies.

The researchers produced 5 versions of the vaccine with 5 different adjuvants, including aluminium salts and squalene. They tested the effectiveness of these 5 candidate-vaccines by immunising macaques, then infecting them with SARS-CoV-2 intranasally 4 weeks later. All 5 induced a strong antibody response after a second immunisation, and the strongest response was obtained with the vaccine using squalene as an adjuvant. This response was maintained for 6 months after vaccination. These vaccines also appear to be effective against the UK variant, but the antibody response is lower against the South African variant. The researchers were able to prove that these vaccines protect primates against infection.

The development of effective vaccines against COVID-19 remains our best hope for a way out of this health crisis. Even though several vaccines are already available, it could be invaluable if we were able to immunise children, for example, with a vaccine that has been proven to be safe and effective. Further evaluation studies in phase 2 and 3 clinical testing remain to be carried out.