The World Health Organization has updated its coronavirus vaccination guidelines. It recommends people at high risk of developing serious symptoms receive routine inoculations.

The WHO's advisory group released a revised roadmap for the use of COVID-19 vaccines on Tuesday. The group says the revisions were made after taking into consideration the immunity that many people have developed through vaccination or infection.

The revised guidelines include three classification levels -- high, medium and low. A group's level determines where it falls on the vaccination priority list.

The "high-priority" group includes frontline health workers, the elderly, people with underlying conditions, such as diabetes and heart disease, people with compromised immune systems and pregnant women.

The experts recommend that people in this group receive an additional routine booster either six or 12 months after the last vaccination.

The "medium priority" group is comprised of healthy adults under the age of 60, as well as children and young people with underlying conditions. Individuals in this group are advised to receive just the first set of booster shots. The experts say additional boosters are safe, but they note that the public health benefits from people receiving them are "comparatively low."

The "low priority" group consists of healthy children and adolescents. The experts say they will leave decisions about vaccinating this group up to each country.

They say the public health benefits obtained by vaccinating this group are "comparatively much lower" than those gained by vaccinating children against measles and other diseases.

The advisory group says countries should not compromise on providing routine vaccinations for the "high-priority" group.