Background/Objectives: Measles is a highly contagious, vaccine-preventable infectious disease. According to the ECDC report “Measles on the rise in the EU/EEA: considerations for public health response” [1], urgent action is required to address the increasing spread of measles and insufficient vaccination coverage across the EU. This study aims to develop a mathematical model for assessing the risk of measles outbreak/s and virus transmission in Bulgaria from 2001 to 2024. To achieve this, a Risk Index (RI) for measles outbreak was calculated based on age-structured immunization coverage and demographic trends.

Methods: Data on immunization coverage and demographic dynamics in Bulgaria were collected from 1971onward, encompassing a 50-year period. Census records dating back to 1970 were used to obtain age-structured demographic data. Established mathematical interpolation and extrapolation techniques were applied to estimate annual immunization coverage and demographic parameters. The Risk Index was calculated as a ratio of susceptible individuals to the total population.

Results: The Risk Index for measles outbreaks in Bulgaria was computed over a 50-year period. The key finding of this study is the definition and calculation of the Risk index, which reveals a concerning increase after 2015, with particularly alarming values projected for 2023 and beyond.

Conclusions: The high incidence of measles and its frequent complications pose a significant burden on healthcare systems, increasing hospitalizations and complicating prevention efforts, particularly among vulnerable populations. The proposed model provides valuable insight for health authorities to predict, prevent and localize outbreaks ensuring adequate healthcare preparedness, including sufficient medical resources and technical capacity. Additionally, the model can be adapted to assess the risk of outbreaks for other vaccine-preventable infectious diseases, such as influenza, pertussis, and, varicella, making it a useful tool for epidemic forecasting and prevention.