On 6 September 2018, the European Court of Justice 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April 2018), an A1 certificate binds the institutions of the host Member State as long as the certificate has not been withdrawn or declared invalid.