New Delhi, Jan 22 (IANS): Millions of people in 48 countries have received two shots of Sinovac, a Chinese-manufactured Covid-19 vaccine, but those vaccinations alone are of no help against the Omicron variant, a peer-reviewed study published in Nature Medicine has said.

An analysis of blood serum from 101 individuals from the Dominican Republic showed that Omicron infection produced no neutralising antibodies among those who received the standard two-shot regimen of the Sinovac vaccine.

ntibody levels against Omicron rose among those who had also received a booster shot of the mRNA vaccine made by Pfizer-BioNTech, showed the new study by researchers at Yale University and the Dominican Republic.

When researchers compared these samples with blood serum samples stored at Yale, they found that even those who had received two Sinovac shots and a booster had antibody levels that were only about the same as those who'd received two shots of the mRNA vaccines but no booster shot.

In other studies, the two-shot mRNA regimen without a booster has been shown to offer only limited protection against Omicron.

The researchers also found that individuals who had been infected by earlier strains of the SARS-Cov-2 virus saw little immune protection against Omicron.

"An additional booster shot, and possibly two, are clearly needed in areas of the globe where the Sinovac shot has been the chief source of vaccination," said Akiko Iwasaki, the Waldemar Von Zedtwitz Professor of Immunobiology and senior author of the paper.

"Booster shots are clearly needed in this population because we know that even two doses of mRNA vaccines do not offer sufficient protection against infection with Omicron," Iwasaki added.

Omicron has proven particularly problematic to combat because it possesses 36 mutations on the spike proteins on its surface, which the virus uses to enter cells.

Existing mRNA vaccines are designed to trigger antibody response when spike proteins are recognised.