As things stand, the social security of persons moving within the European Union (EU) is governed by specific EU legislation protecting the social security entitlements of EU citizens. Consequently, EU citizens are at all times subject to the legislation of a single member state and therefore they pay social security or national insurance contributions in that country only.
These principles also apply to British citizens working or residing in Belgium, and will continue to apply to them until the UK leaves the EU, i.e. until the end of the extension of the article 50 process.
Exactly how social security will be arranged from that time on is something that will depend on any agreement which might yet be reached between the EU and the UK.
As of 1 February 2020, the United Kingdom is no longer part of the European Union. We are now in the transition period. This limited transition period has been agreed as part of the withdrawal agreement and will last at least until 31 December 2020. Until that time, there will be no changes for citizens, consumers, businesses, investors, students and researchers in the EU and the UK.