I am from Serbia and I assure you this is almost entirely due to anti-vaxxer activity there. Poverty has almost nothing to do with it. Most people that decided consciously to avoid against vaccination are the ones that would be considered middle class. The drop happened not during the 90ies i.e. the low point of economy but rather now when economy is doing completely OK.
Also the biggest drop in vaccination rate in Serbia is in Belgrade and Niš (my hometown and number 3 City in country) The biggest problem is that even a sizable chunk of medical professionals succumbed to anti-vaxxer agenda. Than again Serbia is the world capital of conspiracy theorists.
