A Covid-19 vaccine booster shot by China’s Sinopharm showed significantly weaker neutralising activity against the Omicron variant than for an older coronavirus strain from Wuhan, Chinese researchers found. Researchers from Shanghai Jiao Tong University and the Institute of Respiratory Diseases said that they had assessed samples from 292 health care workers given a third shot eight to nine months after their second, in a study published on Saturday on preprint server medRxiv. The study, which has not been peer-reviewed, found that the neutralising antibody activity of a Sinopharm BBIBP-CorV booster against Omicron showed a 20.1-fold reduction. Given that Omicron is the most divergent so far of the coronavirus variants, there are concerns that it could evade the immunity given by existing vaccines and cause a large number of breakthrough infections. Waning immunity and viral diversification both create a potential need for further boosters, the study report said. As supporting evidence, the study found that about eight to nine months after a second dose, the neutralising activity of the Sinopharm jab “declined rapidly and could hardly be detected”, adding to the importance of third booster doses – which have been shown to be capable of enhancing antibody response. However, the authors cautioned that neutralisation was only a part of the human immune response and did not apply to all vaccines, and real-world studies regarding the protection efficacy of booster vaccination against Omicron was so far lacking. Sinopharm’s BBIBP-CorV vaccine and Sinovac Biotech’s CoronaVac are the two most used vaccines in China and are the leading Covid-19 vaccines exported by the country. The study’s authors did not immediately respond to an interview request from the South China Morning Post . Sinopharm is working on an updated Covid-19 vaccine for variants Earlier this month, a laboratory study by the University of Washington and Swiss antibody therapeutics company Humabs BioMed also found that two doses of Sinopharm’s vaccine offered weak protection against the Omicron strain. A separate study released last week by researchers from the University of Hong Kong suggested that Sinovac’s vaccine was not able to produce adequate antibodies to neutralise the Omicron strain.