Dec 31 (Reuters) - Sinovac"s two-dose COVID-19 vaccine followed by a booster Pfizer-BioNTech shot showed a lower immune response against the Omicron variant compared with other strains, according to a study by researchers. The study, which has not been peer-reviewed yet, was conducted by researchers from Yale University, the Dominican Republic"s Ministry of Health and other institutions. The Sinovac (SVA.O) two-dose regimen along with the Pfizer (PFE.N) shot produced an antibody response similar to a two-dose mRNA vaccine, according to the study. Antibody levels against Omicron were 6.3-fold lower when compared with the ancestral variant and 2.7-fold lower when compared with Delta. Akiko Iwasaki, one of the authors of the study, said on Twitter that CoronaVac recipients may need two additional booster doses to achieve protective levels needed against Omicron. The two-dose Sinovac vaccine alone did not show any detectable neutralization against Omicron, according to the study that analysed plasma samples from 101 participants in the Dominican Republic. A study from Hong Kong last week said that even three doses of the Sinovac vaccine did not produce enough antibody response against Omicron and that it had to be boosted by a Pfizer-BioNTech shot to achieve "protective levels." read more Sinovac"s CoronaVac and state-owned Sinopharm"s BBIBP-CorV vaccine are the two most-used vaccines in China and the leading COVID-19 shots exported by the country. Hong Kong has been using the Sinovac and Pfizer-BioNTech vaccines.