A COVID-19 booster vaccination is an extra dose of vaccine that is administered after a longer period, after the basic series of COVID-19 vaccinations has been completed. This booster vaccination is intended as a boost, to restore protection and immunity when these have diminished over time.

Advisory report of the Health Council of the Netherlands

A booster for the entire population is not yet necessary.
The vaccines are very effective and offer strong protection. Protection against serious illness from COVID-19 has not waned, even among older people and against the Delta variant. Protection also remains strong among the people who were vaccinated first. The vaccines offer 95% protection against hospital admission and 97% against ICU admission. 

Ongoing monitoring by RIVMNational Institute for Public Health and the Environment

RIVM constantly keeps a close eye on admission data from hospitals, the situation in nursing homes, and research studies being done in other countries. If the protection provided by COVID-19 vaccination diminishes, a booster campaign will be implemented quickly.