Covid-19 vaccine candidates developed by a Sinopharm subsidiary and Sinovac Biotech to target the Omicron variant were approved for clinical trials in Hong Kong, the companies said on Saturday.

Scientists worldwide are racing to study upgraded injections against Omicron, as data indicated that antibodies elicited by vaccines based on older strains show weaker activity to neutralise the highly transmissible variant.

The two candidates from units of Sinopharm subsidiary China National Biotec Group (CNBG) and one from Sinovac contain inactivated or “killed” coronavirus and are similar to vaccines that the companies are supplying in China and overseas, the companies said in statements.

The Sinopharm candidates will be tested as boosters in adults who have already received two or three vaccine doses, CNBG said. It did not specify which vaccine products the trial participants would have received before taking the experimental booster, or how many subjects would be recruited.

Sinovac said it will push forward studies in its existing CoronaVac vaccine’s protection against emerging variants.

A Chinese study showed that a fourth dose of BBIBP-CorV, an existing Sinopharm Covid vaccine, did not significantly lift antibody levels against Omicron when administered six months after a third booster dose to a regular two-dose regimen.

While the fourth dose restored antibody levels to around the peaks that followed the third dose, researchers said new vaccines would offer a better alternative as future boosters.