Teachers and staff at schools need to be prioritized for COVID-19 vaccines as schools must remain open for the well-being of students, the World Health Organization (WHO) and UNICEF said on Monday.

Measures to ensure that schools can stay open throughout the pandemic "include offering teachers and other school staff the COVID-19 vaccine as part of target population groups in national vaccination plans," the U.N. agencies wrote in a statement.

The recommendation, already made by a group of WHO experts in November 2020 before the vaccination roll-out, should be done "while ensuring vaccination of vulnerable populations," the statement said.

As schools reopen after the summer holidays, the agencies said it was "vital that classroom-based learning continue uninterrupted," despite the spread of the more contagious delta variant of the coronavirus.

"This is of paramount importance for children's education, mental health and social skills, for schools to help equip our children to be happy and productive members of society," the director of the WHO European region, Hans Kluge, said in the statement.

"The pandemic has caused the most catastrophic disruption to education in history," he added.

The agencies urged countries to vaccinate children over the age of 12 who have underlying medical conditions that put them at greater risk of severe COVID-19 disease.

It also recalled the importance of measures to improve the school environment during the pandemic, including better ventilation, smaller class sizes, social distancing and regular COVID-19 testing for children and staff.

According to the WHO's recommendations published in early July, testing of children in risk groups who display COVID-19 symptoms should be a priority.