BEIJING, June 5 (Reuters) - China has approved emergency use of Sinovac Biotech's COVID-19 vaccine in people aged between three and 17, its chairman Yin Weidong told state TV late on Friday (Jun 4).

China's mass vaccination drive, which administered 723.5 million doses of vaccines as of Jun 3, is currently only open to those aged 18 and above.

When Sinovac's vaccine will be offered to younger groups depends on health authorities formulating China's inoculation strategies, Yin told state TV in an live interview.

Yin said minors have lower priority for vaccination against the coronavirus compared with the elderly, who face higher risk of severe symptoms after infection.

Preliminary results from Phase I and II clinical trials showed the vaccine could trigger immune response in three to 17 year-old participants, and most adverse reactions were mild.

State-backed drugmaker Sinopharm, which has two shots using similar technology to Sinovac's product, is also submitting data for clearance in younger groups. A vaccine from CanSino Biologics', adopting a different technique, has entered a Phase II trial involving those aged between six and 17.

Sinovac has also completed a Phase II clinical trial where participants were injected with a third booster dose after completing two regular shots, Yin said.

Participants saw 10 fold increase in antibody levels compared with previous levels in a week, and 20 fold in half a month, Yin said.

Yin cautioned that Sinovac still needs to complete longer-term observation of antibody duration before it can make recommendation to authorities about when a third dose should be given.