COVID-19 Vaccination Uptake in Belgium : Sociodemographic and Socioeconomic Disparities

• Study design and data sources

In this retrospective cohort study, the investigators aimed to unravel SE and SD disparities in the uptake of the first COVID-19 vaccine dose using data from the LINK-VACC project. This project was launched by Sciensano, the institute legally responsible for the surveillance of infectious diseases in Belgium (Royal Decree of 21/03/2018), to perform the post-authorization surveillance of COVID-19 vaccines in Belgium.

To that aim, selected variables from multiple existing national health and social sector registries were linked at an individual level using the unique Belgian social security number. The databases were linked within a pseudonymised environment hosted by Healthdata.be of Sciensano. For the present study, four databases were used :

• Vaccinnet+, the Belgian vaccine registry, which contains information on all COVID-19 vaccine doses administrated in Belgium. It includes demographical data of the vaccinated person (sex, age, postal code of residence), as well as data on the vaccinator (type (person or organization) and postal code) and on the administrated vaccine (brand of the vaccine, lot number, date of administration, date of registration).
• The COVID-19 Healthdata database containing data from COVID-19 laboratory tests performed in Belgium (data of sampling and test result).
• The STATBEL database provided by the Belgian Statistical Institute which contains variables related to socioeconomic and sociodemographic characteristics.
• The Common Base Registry for HealthCare Actor (CoBRHA) allowing the identification (by profession and specialty) of people who have been licensed to practice a healthcare profession in Belgium.
• Cohort definition

The population of interest includes all individuals of 18 years and older tested for COVID-19 at least once in Belgium until the 31st of August 2021 (which is the date of linkage between STATBEL, Vaccinnet+ and the COVID-19 Healthdata database) for whom SD and SE information from STATBEL database was available.

The cohort is built from Vaccinnet+ and the COVID-19 Healthdata database which includes all doses of vaccines administered and PCR and rapid antigenic tests performed in Belgium, respectively. As of 31 August 2021, 10 475 908 individuals had been tested and/or vaccinated at least once in Belgium according to Vaccinnet+ and the COVID-19 Healthdata database. On January 1, 2021, the official Belgian population included 11 521 238 inhabitants, demonstrating a high coverage of the total Belgian population by these two data sources (90.92%).

The relevant SE and SD variables were then included after linking with STATBEL database based on the individual national identification number. The data source available from STATBEL contains SE and SD information for 10 309 300/10 475 908 individuals registered at least once either in Vaccinnet+ from 28 December 2020 to 31 August 2021 or in the COVID-19 Healthdata database before 31 August 2021, corresponding to a linkage rate of 98.41%. Information on the healthcare license available in CoBRHA were then added. After these linkages, people who were deregistered, migrated and deceased based on their status in the national registry were excluded. People whose age, region or gender were unknown were also excluded.

In order to be able to compare vaccination rate according to each SE and SD indicators, the investigators included only individuals tested before 31 August 2021 since it is the date of linkage between Vaccinnet+, COVID-19 Healthdata database and STATBEL. Finally, individuals under 18 years of age were excluded as they were considered as a group in their own right in the vaccination strategy by being vaccinated the latest in a separate phase of the vaccination campaign.

The final cohort for this study is therefore composed of 5 341 584 individuals of 18 years and over tested in Belgium until the 31st of August 2021.

• Outcome The outcome variable, provided by Vaccinnet+, is the uptake of the first dose of COVID-19 vaccine until the 31st of August 2021. The investigators considered that, at this date, all individuals over 18 years of age had received an invitation to be vaccinated with the first dose and had the opportunity to receive this dose of vaccine in Belgium.

Inclusion criteria

• Tested at least once in Belgium before 31 August 2021

Exclusion criteria

• people with a national registry status equivalent to deceased
• people with a national registry status equivalent to migrated
• people with a national registry status equivalent to deregistered
• people with an unknown age
• people with an unknown gender
• people with an unknown region