In a study led by Rudolf Valenta of the Medical University of Vienna, Institute of Pathophysiology and Allergy Research, an Austrian subpopulation of vaccinated and recovered individuals was examined for their antibody status and protection against the Wuhan, Delta and Omicron variants, the latter being currently prevalent in Austria. For this purpose, a test developed for the previous variants, which investigates whether the virus can bind to the receptor on human cells via its receptor binding domain (RBD), was rapidly adapted for Omicron, which, if needed, can also be done for any newly emerging variant swiftly. Accordingly, vaccinees of all vaccines and vaccine combinations currently licensed in Austria were examined.

The results showed that both COVID-19 convalescent individuals and individuals who had been vaccinated twice had developed antibody protection against Delta. However, the antibodies were not able to block receptor binding against Omicron.

Third vaccination offers best protection - but far from being optimal

Blockade of Omicron was better in those individuals who had received a third vaccination. "The third vaccination developed protective antibodies in many individuals," explains Rudolf Valenta, "however, there is also a significant proportion (20 percent) in whom no protection was established”.

The receptor binding domain (RBD) which is used by SARS-CoV-2 to enter human cells via the ACE2 receptor differed only slightly in all previously known SARS-CoV-2 variants, so that infections with these and the currently available vaccines provided protection against the previous Corona virus variants. Omicron is the first variant that differs greatly from the previous variants in RBD, consequently infections with the previous variants and currently available vaccines provide little or no protection against Omicron.

For the immunologist, the best protection under such circumstances would be to develop a broadly effective combination vaccine that protects against both the previous variants and Omicron. "Until we have such a vaccine, only repeated vaccinations with the existing vaccines will provide some protection. The protective effect achieved by vaccination can be evaluated with special tests that can be rapidly adapted to new virus variants.