China has launched multiple research efforts to develop tailored vaccines against emerging COVID-19 variants, as its mass immunization program hit over 2.1 billion doses, health officials said on Sept 7.

Zheng Zhongwei, a National Health Commission official who heads China's COVID-19 vaccine development task force, said that although domestic vaccines have demonstrated efficacy against new virus strains, a series of research projects aimed at developing vaccines targeting virus mutations has been rolled out as a precautionary measure.

He said Chinese vaccine makers have completed preclinical studies on inactivated vaccines designed to tackle the Gamma strain first detected in Brazil and the Delta strain first documented in India.

"Some developers have already submitted applications for conducting clinical trials to drug regulators," he told a news conference.

In addition, research into recombinant subunit vaccines designed to ward off different strains — also known as broad-spectrum or multivalent vaccines — is also underway, he added.

Other types of vaccines adapted to the Beta strain first reported in the United Kingdom and the Delta strain are undergoing animal tests, Zheng said, adding that some arrangements have been made to facilitate examination and approval procedures of these new products.

"With this preparatory work in place, we believe even if some fundamental mutations occur in the future and escape vaccine-triggered immunity, we can promptly come up with new vaccines and scale up production of them," he said.

As of Sept 6, China had delivered more than 2.11 billion COVID-19 doses, covering nearly 1.1 billion people, according to the National Health Commission.

Nearly 970 million people had been fully vaccinated, said Wu Liangyou, deputy director of the commission's Disease Prevention and Control Bureau.

He added that over 162 million doses had been given to minors aged 12 to 17 since China began vaccinating children and teenagers in mid-July.

Zheng also acknowledged the potential of sequential immunization－the mixing of different COVID-19 vaccines to elicit strong immune responses. Currently, the public is not advised to receive different vaccines.

He said experts have supported the launch of research and trials testing the approach, but more data on its safety and efficacy are needed.

China has granted conditional market approval for four domestic vaccines, and emergency use authorization for three others. More than a dozen vaccines adopting different technologies are undergoing clinical trials, Zheng said.