Greece is one of four European countries that is no longer categorised as measles-free, according to the World Health Organisation (WHO).
Strains of the virus are now circulating in Greece, Albania, the Czech Republic and the United Kingdom, four countries where the virus had been declared "eliminated", until recently.
Measles is one of the most contagious viruses on the planet and can spread in the air even after an infected person has left the room.
According to data from WHO, there were 89,994 cases of measles in 48 European countries in the first six months of 2019, more than double the number in the same period in 2018. The deadly virus has also killed at least 37 people in Europe during the first half of this year.
Kate O'Brien, director of the WHO's Immunisation Department, stated that "we are backsliding, we are on the wrong track."
"This is the alarm bell that is ringing around the world: being able to achieve high national coverage is not enough, it has to be achieved in every community, and every family for every child," she continued.
The outbreak of the measles in Europe has been credited to the growth of the anti-vaccination movement in the last few years and misinformation about vaccines that is shared on social media.
Dr Gunter Pfaff, chair of the Regional Verification Commission for Measles and Rubella Elimination (RVC), noted that the "re-establishment of measles transmission is concerning. If high immunisation coverage is not achieved and sustained in every community, both children and adults will suffer unnecessarily and some will tragically die."