Do I need an “A1” certificate?

An A1 certificate is required for Users who are employed by a home institute in the EU or in an EFTA country and travel to another EU/EFTA country for business.

The Home Institution Declaration by which the User’s home institution declares that they have adequate social security coverage in Switzerland, France, or other countries visited on duty travel, is an internal means for CERN to ensure that Users remain constantly covered, but the HID cannot be seen as material proof of such adequate coverage or as an exemption from the A1 requirement. The HID therefore does not replace the A1 certificate.
Although users are not requested to show their A1 Form to the Users Office upon registration or contract extension, they must carry an A1 Form when traveling to CERN.

Please check the following website: https://europa.eu/youreurope/citizens/work/social-security-forms/