Vaccine effectiveness against SARS-CoV-2 infection and mild COVID-19 disease provided by the standard number of primary vaccine doses declines over time, while protection against severe disease and death remains high, including against the Omicron variant.
A booster dose can significantly restore immune responses, particularly in older adults. Vaccine effectiveness studies for booster doses are published by an increasing number of countries, but remain limited in follow-up time. All studies demonstrate an improvement in protection against severe disease and death.
WHO recommends that countries offering a booster dose should focus first on the most vulnerable groups and healthcare workers to minimize the risk of increased severe cases and deaths, and maximize the resilience of health care services.
Update (March 2023):
A first booster dose is administered usually 4–6 months after completion of the primary series. Booster doses can be a vaccine with the ancestral strain, or a vaccine incorporating a variant strain. As vaccine effectiveness wanes over a period, a second booster dose also should be offered 4–6 months after the last dose or, if this time period is missed, as soon as possible thereafter, especially for priority groups at highest risk.