South Africa on Friday joined the international Phase 3 clinical trials of China's Sinovac Biotech COVID-19 vaccine for those aged between 6 months and 17 years, with first of the 2,000 children receiving their jabs.

The identities of the boy and a girl, both aged 17 years, were kept secret to protect their privacy as they received their vaccines at the MeCRU Clinical Research Unit, based at the Sefako Makgatho Health Sciences University, north of Pretoria.

The primary objective of the global study is to evaluate the efficacy of two doses of CoronaVac against confirmed symptomatic COVID-19 cases in children and adolescents aged 6 months to 17 years. Efficacy will also be evaluated against hospitalisation and severe COVID-19 cases.

The study worldwide will enroll a total of 14,000 children and adolescents in various paediatric age groups in South Africa, Chile, the Philippines, Malaysia and Kenya.

In South Africa, Numolux has partnered with seven clinical research sites across the country to enroll 2,000 participants from 6 months to 17 years.

The trials will start with the 12 to 17 age group, before moving to younger children.

A representative of Numolux, the South African licence holder for Sinovac, said the safety of the vaccine has already been established abroad.

"The vaccine has been tested in phase three trials in adults in Brazil, in Chile, in Indonesia and in the Philippines. It has been shown to be very effective in these studies ranging from 51 per cent in a healthcare population, to 93 per cent in the Turkish and Indonesian populations and 87 per cent in Chile," said Dr Sanet Aspinal, the Numolux group virologist.

Sinovac currently has conditional approval from the South African Health Products Regulatory Authority (SAHPRA) for people aged 18 to 59 years, although the vaccine has not yet been distributed for adults.

SAHPRA has also approved this latest South African trial.

Sinovac's medical director for clinical research Gang Zeng said the vaccine has already been administered to about 40 million children aged between three and 17 in China.

"This study has public health importance globally because a vaccine that prevents COVID-19 disease and transmission in paediatrics would be a crucial tool to assist in curbing the pandemic," the university said in a statement.

"Although children and adolescents have a milder form of the disease than adults, they remain susceptible to infection and severe manifestations across all ages," it added.

Numolux Group CEO Hilton Klein said it was critical to protect children of Africa.

"(We have) joined Sinovac in the fight against this deadly monster to ensure that the children of Africa have access to a safe and effective children's vaccine to protect them against COVID-19," Klein said.