Objectives: The main objective of this research is to find the correlation between the effectiveness of vaccination and vaccination coverage and to determine the reduction scale of infectious diseases' incidence in Croatia. This is being done through the method of data processing and through calculating the percentage of vaccinated people in correlation with the number of diseased in the observed period from 2000. to 2019. Materials and methods: The methods used in this paper are primary and secondary data collecting and Annals of Health and Statistics were used as a primary source. Through the observed twenty-year period, using the method of analysis, the percentages of vaccinated people with obligatory vaccines are observed in relation with the number of those who developed the disease against which the vaccine is used. Using data comparison the effect of vaccination on the infectious diseases' morbidity is observed in the twenty-year period. Results: Owing to the consistently applied Vaccination Programme in Croatia during the twenty years preceding the Covid-19 pandemic, there is a significant success in eradicating the poliomyelitis (the last case was in 1989.), dyptheria (the last case was in 1974.), disseminated tuberculosis in children and tetanus in newborns. Mostly eradicated are also the following diseases: tetanus for all age ranges by 98%, rubella by 98%, parotitis by 99%, furthermore, significant reduction is seen in measles and pertussis, by 99%. The decline in the number of cases has been achieved due to very good vaccination rates which vary between 90 and 95%. There is a significant decline in the infectious diseases' rates in Croatia during the observed period. Apart from the eradication of poliomyelitis and dyptheria mentioned above, we are witnessing the dissapearence of rubeola in the last three years (pre-pandemic era), as well as the significant reduction of cases of tetanus and parotitis. Parotitis can be seen sporadically every year, but the numbers are not concerning as they show a tendency of decrease. Measles are the best example of strong correlation between disease incidence and vaccination rate, since there was a rapid increase of cases in areas of poor vaccination. Measles outbreaks point out a stronger need for implementing public health measures. Conclusion: High vaccination rates and well organised preventive healthcare are both proven to have led to a decrease in infectious diseases' rates in Croatia from 2000. to 2019. Some diseases have been eradicated, while most of the others have been significantly reduced. Strong media influence, as well as the influence of antivax movements in the last five years have made mandatory vaccination harder to carry out. It also led to the decrease of vaccination rates. Despite all the difficulties, it is necessary to continue with public health strategies and to try to eradicate infectious diseases in the population. The best tool we have for that are vaccines.