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Two homegrown Omicron-specific COVID-19 vaccine candidates developed by Sinopharm and Sinovac Biotech have been approved for clinical trials by China's top drug regulator, the two companies said on Tuesday.

If the trial proceeds successfully, the experimental vaccine from Sinopharm will likely be used as a third or fourth booster shot for fully vaccinated people age 18 and older, the company said.

China National Biotech Group, a subsidiary of Sinopharm, said in a statement released on its website that the vaccine was already cleared for clinical trials in Hong Kong on April 13.

CNBG is the developer behind two vaccines widely used in China and worldwide. It began research into a vaccine targeting the Omicron variant in early December.

Preclinical studies and animal trials have shown the new vaccine can induce high levels of neutralizing antibodies against Omicron and several other variants, the company said.

It added that clinical trials will be rolled out among people age 18 and older who have already been vaccinated with two or three doses of COVID-19 vaccines.

Sinovac Biotech also said on Tuesday its upgraded vaccine was granted the greenlight to launch clinical trials in Hong Kong on April 14, and it got clearance on the Chinese mainland on Tuesday.

The Omicron strain first emerged in late November and quickly became the dominant strain globally. Compared with the ancestral strain of the novel coronavirus, on which existing vaccines are based, scientists have found the highly mutated variant is better at evading immunity from past infections or vaccinations.

The discovery prompted global vaccine makers to upgrade current vaccines and tailor them to tackle Omicron.

Zheng Zhongwei, head of China's vaccine development task force, said during a forum last week that research into Omicron-specific vaccines must be accelerated in response to the high incidence of breakthrough infections, which are positive cases found in fully vaccinated people.

He said three domestically developed vaccines targeting Omicron were on track to begin clinical trials. "We are confident that in the near future, such vaccines will be launched into the market," he said.

CNBG has another Omicron-targeted vaccine under development that was also cleared for human trials in Hong Kong this month.

Overseas, US pharmaceuticals Pfizer and Moderna have begun testing several Omicron-targeted vaccines and vaccines designed to ward off multiple strains. Pfizer CEO Albert Bourla said recently that the company hopes to have an updated vaccine ready by this fall, ABC News reported.