Beijing, Dec 20 (IANS): A Covid booster dose developed by China's Sinopharm has shown to generate weaker immune responses against the new highly transmissible Omicron variant, known to evade vaccine efficacy.

The hyper mutated Omicron variant of concern is spread so far to about 90 countries, raising serious concerns about the reduced vaccine efficacy and the increased risk of reinfection.

In a study, researchers from Shanghai Jiao Tong University and a Shanghai-based lab specialising in respiratory infectious diseases, compared the activity of Sinopharm's booster vaccine against an older coronavirus strain from Wuhan.

The yet-to-be peer reviewed study showed that neutralising antibody activity of a Sinopharm BBIBP-CorV booster against Omicron showed a 20.1-fold reduction, compared with its activity against a Wuhan strain.

The study included 292 healthcare workers who had administered a third homologous boosting vaccination eight to nine months after completion of the priming two-dose inactivated vaccination.

The team investigated whether the newly identified Omicron variant could escape serum antibody neutralisation elicited by the booster vaccination.

The results showed that a third booster dose with BBIBP-CorV leads to a significant rebound in neutralising immune response against SARS-CoV-2. The neutralisation titer on day 28 after the third booster dose was 6.1 times higher than on day 28 after the second dose.

However, "the Omicron variant did cause significantly lower neutralisation sensitivity compared to the wild-type strain of the booster elicited serum, with about 20.1-fold reduction", Xin-xin Zhang, from Ruijin Hospital, from the varsity's School of Medicine.

"Our study demonstrated that a third booster dose of BBIBP-CorV led to a significant rebound in neutralising immune response against SARS-CoV-2. The Omicron variant showed extensive, but incomplete escape of the booster elicited neutralisation."

A separate study, not peer-reviewed yet, showed that Sinopharm's two-dose vaccines against Covid also produce little or no antibodies against Omicron.

Led by researchers at the University of Washington and Swiss drugmaker Humabs Biomed SA, the study showed only three out of 13 people who had taken both doses of Sinopharm's shot showed neutralising antibodies against Omicron.

Meanwhile, vaccines developed by US drugmakers Pfizer, Moderna, Johnson and Johnson against Covid-19 have also proven to be less effective against the new super mutant Omicron variant that harbours up to 36 mutations in spike protein.