Vangelis Stathopoulos, who is in Greece's Larissa prison, is one of more than half a million people incarcerated in Europe amid the COVID-19 pandemic. And, like so many others, the prison where he is being held is an ideal breeding ground for viruses: it's overcrowded, with cramped living arrangements, and often poor hygiene conditions.

"When I got COVID last December, around half of the prisoners in here were sick at the same time," Stathopoulos says. "We were put into a ward with 60 people, in a space of around 110 square meters (1,200 square feet). It was a roll of the dice whether you were going to be severely or just mildly ill."