Background

To tackle waning protection against severe COVID-19 infection, booster doses of the COVID-19 vaccines were recommended among individuals who received their primary course. A first COVID-19 vaccine booster campaign was performed in winter of 2021 (age ≥ 18 years) and a second campaign for (age ≥ 50 years) in autumn of 2022. During these campaigns persons were actively invited to get a booster dose against COVID-19. We aimed to explore (first and second) booster uptake among fully vaccinated adults in Belgium and investigate demographic (age, region of residence and sex) and COVID-19 related (having had a documented COVID-19 infection) factors associated with the first and second booster uptake.

Methods

We used individually-linked data from the Belgian COVID-19 vaccine registry and the COVID-19 test database to determine booster uptake among the 18+ Belgian population having completed the primary vaccination course (PC). We reported coverage by demographics (age, region of residence and sex) and COVID-19 infection history before or after completion PC (based on PCR tests). We used a quasibinomial logistic regression model to determine influential factors on the booster uptake.

Results

Among the Belgian adult population (N = 9270186), the coverages reached on the 31st January 2023 were 72.65% and 40.85% for the first and second booster respectively. The highest coverage (number boosted over number PC completed) was observed in the 75-84 age group (96.73%) and 85+ age group (82.30%) for the first and second booster respectively and the lowest for both boosters in the 18-24 age group (69.86% and 17.85%). Individuals who had previous infections were significantly less likely to receive the first booster dose compared to individuals with no infection, especially when the infection occurred after completion of PC and with multiple infections (COVID-naive reference, minimum 1 infection before PC: OR=0.73, 95%CI 0.67-0.79, 1 infection after PC: OR=0.29, 95%CI 0.27–0.31; multiple infections before and after; OR=0.15, 95%CI 0.12–0.18; multiple infections after, none before: OR=0.09, 95%CI 0.03–0.29). Older age and living in Flanders were positively associated with uptake in both boosters.

Conclusions

We report a high booster uptake among the 18+ Belgian population (72.65% and 40.85%) compared to the European average (first booster: 57.54% ± 21.71%, second booster: 14.61% ± 11.91%). Individuals previously infected were less likely to receive the first booster dose while older individuals were more likely to receive any booster dose. These results offer important insights for future booster campaigns for COVID-19 vaccines.