There is no longer any requirement to have a certification of vaccination against Covid-19 for travel to Greece, but if you develop Covid-like symptoms and/ or have recently tested positive for Covid please, out of consideration for other participants, do not attend the Symposium in person. You may change your registration to attend online.

This is the official Covid advice for travel to Greece:

(February 22, 2023) *IMPORTANT NOTE*
The requirement for completion of Passenger Locator Form (PLF) by visitors from ALL countries is no more mandatory.
Furthermore, all travellers arriving in Greece, regardless of their country of origin, are no longer required to display a valid certificate of vaccination or recovery from COVID-19, or evidence of a negative test result from SARS-CoV-2 infection (PCR or Rapid Antigen test).

Protocol for arrivals in Greece: travel.gov.gr
The newest advice from Greece’s National Health Organization is here: advice
Useful links for information: reopen.europa.eu/en