They find that the rAd26-S and rAD5-S vaccine is safe and does not cause serious adverse effects in the healthy volunteers, with no more severe effects than mild common side effects that occur with any vaccine based on a recombinant viral vector.

After vaccination with rAd26 and rAd5, neutralising antibodies, antibodies to defend the cell from SARS-CoV-2, did form, but the levels of neutralising antibodies formed were less than that of the ChAdOx1 vaccine. However, the rAd26-S and rAD5-S vaccine did elicit the same titre of SARS-CoV-2 neutralising antibodies as did people recovered from COVID-19.

There was a strong humoral immune response in 100% of the volunteers, with equal or higher IgG antibody titres in vaccinated people than those in the convalescent plasma of those already infected with COVID-19, showing the immunogenicity of the vaccine.

There was a strong cellular (T-cell) response to the vaccine, shown from higher rates of CD8+ T cells and increased interferon-y secretion.

Using the rAd26-S for priming and rAd5-S for boosting (this heterologous prime-boost immunisation) effectively elicited an immune response to overcome the immune response that is formed to the viral vector components.

The researchers still need to expand this study to test the vaccine in different populations, including older aged groups and at-risk groups.