HONG KONG, Dec 15 (Reuters) - Researchers in Hong Kong have urged people to get a third dose of COVID-19 vaccine as soon as possible, after a study showed insufficient antibodies were generated by the Sinovac (SVA.O) and BioNTech (22UAy.DE) products to fend off Omicron. Tuesday"s release of the results of a study by scientists in the microbiology department of the University of Hong Kong was the first published preliminary data on the impact of Sinovac"s vaccine against the Omicron variant of coronavirus. None of the serum of the 25 Coronavac vaccine recipients contained detectable antibodies that neutralised the new variant, according to the preprint study that has been accepted for publication in the journal Clinical Infectious Diseases, the researchers said. Just 20% to 24% of BioNTech vaccine recipients had detectable neutralising antibodies against Omicron, the study found. "The public is advised to get a third dose of the vaccine as soon as possible while waiting for the next generation of a more matched vaccine," the researchers said in a news release. The study, funded by the Hong Kong government, was carried out by microbiologists Yuen Kwok-yung, Kelvin To and Chen Honglin. Sinovac did not immediately respond to questions on the study, but a spokesperson said its own laboratory testing showed a third dose of its vaccine was effective in producing Omicron antibodies. BioNTech did not immediately respond to questions on the study. The fast-spreading Delta variant remains dominant worldwide, and it is unclear if Omicron is inherently more contagious, the World Health Organization (WHO) said in a brief on Sunday. Scientists say it is still too early to know if Omicron causes more or less severe COVID-19 than previous variants. A study of real-world data published on Tuesday showed Pfizer-BioNTech"s COVID-19 vaccine was less effective in South Africa at keeping out of hospital those infected with the virus since the Omicron variant emerged last month. read more Last week, the two firms said a three-dose course of their vaccine had neutralised Omicron in a laboratory test, an early sign that booster shots could be key for protection against it. read more