It's hard to tell... the medieval sources are not very reliable... These kind of plagues would affect cities more than the country regions. Modern sources believe around 40% of Constantinople and maybe 25% of rural areas population died in the Justinian plague. The Black Death probably killed around 50% cities population and 30% of rural areas... (It was 'a bit' more in south Europe and 'a bit' less in the north areas.) 
Again that's just averages.
