The MD Department of Agriculture (MDA) has jurisdiction over animal health concerns in MD. So the first question is what state are you in, and did the horse cross state lines upon sale? Per the MDA web site (https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/animal-and-animal-product-import-information/...), horses over 9 months of age that enter MD must have a negative EIA (Coggins) Test within the immediate 12 months prior to their entry into the state. They also have to be accompanied by evidence of the negative test.

Since it is not clear from your question if the horse was shipped to another state, the state where the horse was shipped into would govern the requirements for horses entering there, vaccines included. Here is a reference from the American Veterinary Medical Association with various state requirements for entry (importation) for horses and other species--https://www.avma.org/Advocacy/StateAndLocal/Documents/CVI%20Chart.pdf.  

Depending on the area of the country, the age of the horses, uses/travels of horses and the prevalence of disease issues in the region, "routine" vaccination programs will differ widely. Consult a veterinarian in your area for that information. 

Typically, when buyers purchase horses it is a good idea to ask questions beforehand, such as when Coggins Testing was done and what vaccines were given to horses and when they were given, as well as many other questions. After purchasing, buyers don't have much leverage to get additional information about horses unless purchase contracts detail other concerns addressable after the sale.      

I hope this helps.