Two doses of the China-made Covid-19 vaccine, CoronaVac, are safe and produce a strong antibody response among children and adolescents aged 3-17 years, according to a study published in The Lancet Infectious Diseases journal.

The phase 1/2 trial on 550 young people found that over 96 per cent of children and adolescents who received two doses of the vaccine, manufactured by Sinovac, developed antibodies against SARS-CoV-2, the virus that causes Covid-19.

Most adverse reactions were mild or moderate, with pain at the injection site the most commonly reported symptom, the researchers said.

"Children and adolescents with Covid-19 usually have mild or asymptomatic infections compared with adults. However, a small number may still be at risk of severe illness," said Qiang Gao from Sinovac Life Sciences, China.

"They can also transmit the virus to others, making it vital to test the safety and effectiveness of Covid-19 vaccines in younger age groups," Gao said.

The researchers conducted the phase 1/2 clinical trial of CoronaVac in healthy children and adolescents aged 3-17 years in Zanhuang County, China.

Between October 31 and December 2, 2020, 72 participants were enrolled in phase 1, and 480 participants enrolled in phase 2 between December 12 and December 30, 2020.

The vaccine -- either 1.5 microgram ( g) or 3 g per dose -- or a control was given by intramuscular injection in two doses, at day 0 and day 28.

Among the 550 participants who received at least one dose of vaccine or the control, adverse reactions within 28 days occurred in 56 (26 per cent) of 219 participants in the 1.5 g group.

Such adverse events were reported in 63 (29 per cent) of 217 participants in the 3 g group, and 27 (24 per cent) of 114 in the control group.

Only one serious adverse reaction -- a case of pneumonia -- was reported in the control group, however, this was unrelated to the Covid-19 vaccination, the researchers said.

In phase 1, 100 per cent of participants in both the 1.5 g and 3 g groups generated antibodies against SARS-CoV-2.

Stronger immune responses -- determined by the amount of antibodies produced that can neutralise the virus -- were detected among the 3 g group compared with the 1.5 g group.

In phase 2, 97 per cent of participants in the 1.5 g group produced antibodies against SARS-CoV-2, compared with 100 per cent in the 3 g group.

Participants in the 3 g group again produced a stronger immune response than those in the 1.5 g group.

The researchers noted that immune responses among children and adolescents were higher than those measured in adults aged 18-59 years and elderly aged 60 years and older.

No significant differences in immune response were detected in an analysis by age group.

More than 93 per cent of those in the 1.5 g and 3 g groups aged 3-5 years, 6-11 years, and 12-17 years produced antibodies against SARS-CoV-2 at day 28 after the second dose.

The researchers recommend two 3 g doses of the vaccine for children and adolescents aged 3-17 years based on their results.

They acknowledged some limitations in their study.

T cell responses -- which play an important role in SARS-CoV-2 infections -- were not assessed in the study, though these have been investigated in related studies.

The study involved a small number of participants and all were of Han ethnicity, highlighting a need for larger studies in other regions and involving multi-ethnic populations.

The researchers noted that long-term safety and immune response data were not available, though participants will be followed for at least one year.

They said the results should be interpreted with caution as it was not possible to draw strong statistical conclusions owing to the small number of participants in the study.

"Herd immunity against Covid-19 is the prerequisite to end this pandemic, either through vaccinations or natural infection, said Professor Bin Cao, of the China-Japan Friendship Hospital, who was not involved in the study.

Cao noted that most estimates placed the threshold at 65-70 per cent of the population gaining immunity, mainly by vaccination.

"However, widely circulating virus variants and persistent hesitancy on vaccine make this threshold difficult to reach. Thus, the calculation has to be revised upward and children must be covered in the immunisation campaign," he added.