A first booster dose is usually given 4-6 months after completing the primary series. The booster can be a vaccine with the ancestral strain or one containing a variant strain. Since the effectiveness of the vaccine decreases over time, a second booster dose should be offered 4-6 months after the last dose. If this time period is missed, it should be administered as soon as possible, especially to priority groups at higher risk. To maintain the highest level of protection, it is essential to receive all vaccine doses offered.

According to information released by the Centers for Disease Control and Prevention (CDC), vaccine effectiveness may vary depending on specific populations, such as older people or those with immunocompromising conditions. Additionally, factors such as the type of vaccine, the total number of doses received, and the time since the last dose can influence its effectiveness. The CDC regularly evaluates vaccine effectiveness to identify changes that could be attributed to emerging SARS-CoV-2 variants or a decline in vaccine protection over time.

As mentioned by the WHO Collaborating Center for Vaccine Safety, the duration of protection varies depending on the vaccine and the immunocompetence of the recipient. Immunocompromised individuals may require an additional dose in the primary series to elicit a stronger immune response. Immunity against mild infection may wane in the months after vaccination, especially in older adults. This has led to the implementation of booster doses in many countries, initially targeting older age groups.

The effectiveness of the standard number of primary doses of the vaccine against SARS-CoV-2 infection and mild COVID-19 disease decreases over time. However, protection against severe disease and death, including against the Omicron variant, remains high. Administration of a booster dose significantly increases immune responses, especially in older adults. The WHO recommends that countries prioritize the most vulnerable groups and health workers when offering booster doses. This approach aims to minimize the risk of an increase in severe cases and deaths and strengthen the resilience of health services.