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 vaccine dose can significantly restore immune responses, particularly in older adults. Vaccine effectiveness studies for booster dose 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.