If an employee is seconded within the EU, s/he remains subject to the social security regime of the sending Member State and does not have to be affiliated to the social security of the host Member State, as long as a number of conditions are fulfilled. One of these is that the employee cannot be seconded to replace another seconded employee. 

On 6 September 2018, the ECJ ruled in the Alpenrind case (C-527/16) that the replacement of a seconded employee by an employee seconded by another employer is also not permitted. In such a case, the secondment conditions will not have been fulfilled and the employee must be made subject to the social security of the host Member State. 

The Court further confirmed its established case law on A1 certificates. Except in cases of fraud (see Update from April 2018), an A1 certificate binds the institutions of the host Member State as long as the certificate has not been withdrawn or declared invalid. 

Sophie Maes, partner at Claeys & Engels