I am looking for a data set containing vaccination policies by country, and if possible by year. Ideally, the vaccination policy would be stored in some machine-readable format.
The WHO has some Excel tables that indicate vaccination policy for individual countries and individual vaccinations.
6. Immunization schedule. Data are available for:
6.1 Reported immunization schedules by vaccine in html and in excel
6.2 Year of introduction of selected vaccines database in excel
6.3 Immunization schedules by disease covered by antigens within age range in html
6.4 Immunization provided at school in excel
6.5 Slides on introduction status of selected vaccines
The the 2nd sheet in the Excel file for point 6.1 above has the data you are looking for.