In early 2021, the primary vaccination campaign against COVID-19 was rolled out to the adult population in Belgium, followed by the first booster campaign towards the end of the year. At the beginning of 2022, booster doses were recommended and offered to people meeting the definition of immunocompromised conditions (from January 24, 2022) and to people who had received the Jcovden vaccine (Johnson&Johnson®) as a primary COVID-19 vaccination schedule (from February 16, 2022) . From May 2022, Flanders offered a second booster vaccination to nursing home (NH) residents and people aged over 80 years. In early July 2022, a second booster vaccination was offered to people working in the healthcare sector and those living in nursing homes in all regions of Belgium.

From the 12th of September 2022 onwards, the Interministerial Conference (IMC) organised an autumn vaccination campaign with a systematic invitation for a new booster dose, consecutively for immunocompromised people, people aged over 65 years, healthcare workers and people aged between 50 and 64 years. People aged 18-49 years were allowed to receive a booster dose as well but were not systematically invited. People who had already had a booster dose earlier in the year were also eligible to receive a subsequent (third) booster during this new campaign, with a minimum interval of 3 months between the 2 doses. As such, this autumn campaign included mainly administrations of second, but also third booster doses.

Here we assessed the COVID-19 vaccine effectiveness against symptomatic infection and hospital admission among those who received the second and third boosters in Belgium administered during and following the autumn vaccination campaign of 2022, compared to those who did not receive a vaccine during this time.