A Covid-19 vaccine developed by a unit of China's Chongqing Zhifei Biological Products largely retained its neutralising effect against the Delta variant but there was a slight reduction, Chinese researchers found in a laboratory study, according to a Reuters report.

Scientists and regulators are watching closely to see whether the Delta variant, which is significantly more contagious than the original variant of the new coronavirus, will render available vaccines and treatments insufficient.

Tested against particles mimicking the real Delta variant, serum samples from people vaccinated with Zhifei's three-dose regimen showed a 1.2-fold reduction in neutralising effect compared with against particles mimicking an older variant, researchers said in a paper published on Friday without peer review.