Study finds limited efficacy of Covid-19 vaccines against Omicron

People who are doubly vaccinated and those who have recovered from infection by previous strains of the SARS-CoV-2 virus have no antibody protection against the Omicron variant, according to a study. The research, recently published in the journal Allergy, found that only individuals vaccinated with a third or booster dose of the Covid-19 vaccine form antibodies that can partially block Omicron.

Researchers from the Medical University of Vienna examined an Austrian subpopulation of vaccinated and recovered individuals for their antibody status and protection against the Wuhan, Delta and Omicron variants.

They adapted for Omicron 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). RBD is used by SARS-CoV-2 to enter human cells via the ACE2 receptor on cells. The researchers also examined people who had received different vaccines and combinations currently licensed in Austria.

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The findings show that both Covid-19 convalescent individuals and individuals who had been vaccinated twice developed antibody protection against Delta. However, the antibodies were not able to block the receptor-binding domain against Omicron, the researchers said. The study found that blockade of Omicron was better in those individuals who had received a third dose of vaccine.

"The third vaccination developed protective antibodies in many individuals however, there is also a significant proportion (20 per cent) in whom no protection was established," said Rudolf Valenta, who led the research team.

The RBD 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 variants, the researchers said. 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, they said.

The researchers noted that the best protection 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," Valenta said.

"The protective effect achieved by vaccination can be evaluated with special tests that can be rapidly adapted to new virus variants," the immunologist said.

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