As children and adolescents are at lower risk of severe COVID-19 disease, countries should prioritise adults and sharing vaccine doses with the COVAX program to bring supplies to poorer countries, the World Health Organisation says.

Some rare cases of heart inflammation called myocarditis have been reported in younger men who received vaccines based on mRNA technoloy - Pfizer-BioNtech and Moderna - but these were generally mild and responded to treatment, it said.

Although that risk had not been fully determined, it was less than the risk of myocarditis linked to SARS-CoV-2 infection, it said.

The WHO's interim guidance was issued as more regulatory agencies authorise certain vaccines for use in children, including the United States, China, European Union, India and Israel, and most recently Canada last week.

"As children and adolescents tend to have milder disease compared to adults, unless they are in a group at higher risk of severe COVID-19, it is less urgent to vaccinate them than older people, those with chronic health conditions and health workers," the WHO said.

Children can experience "long COVID-19" with prolonged symptoms but this was still under investigation, it said.

Several risk factors for severe COVID-19 in children have been reported including obesity and pre-existing conditions including type 2 diabetes, asthma and heart disease, it added.

Maintaining education for all school-aged children should be an important priority during the pandemic although transmission mitigation measures might be needed in schools, the WHO said.

Given vaccine supply constraints, immunisation programs should focus on protecting groups at high risk of hospitalisation and death, the WHO said.

"As many parts of the world face extreme vaccine shortages, countries with high coverage in at-risk populations should prioritise global sharing of COVID-19 vaccines before vaccinating children, adolescents," it said.

Australian Associated Press