By 1800, Jenners work had been published in all the major European languages and had reached Benjamin Waterhouse in the United States – an indication of rapid spread and deep interest. : 262–67  Despite some concern about the safety of vaccination the mortality using carefully selected vaccine was close to zero, and it was soon in use all over Europe and the United States. In 1804 the Balmis Expedition, an official Spanish mission commanded by Francisco Javier de Balmis, sailed to spread the vaccine throughout the Spanish Empire, first to the Canary Islands and on to Spanish Central America. While his deputy, José Salvany, took vaccine to the west and east coasts of Spanish South America, Balmis sailed to Manila in the Philippines and on to Canton and Macao on the Chinese coast. He returned to Spain in 1806. The vaccine was not carried in the form of flasks, but in the form of 22 orphaned boys, who were carriers of the live cowpox virus. After arrival, "other Spanish governors and doctors used enslaved girls to move the virus between islands, using lymph fluid harvested from them to inoculate their local populations".The first state to introduce compulsory vaccinations was the Principality of Lucca and Piombino on September 25, 1806. On August 26, 1807, Bavaria introduced a similar measure. Baden followed in 1809, Prussia in 1815, Württemberg in 1818, Sweden in 1816, England in 1867 and the German Empire in 1874 through the Reichs Vaccination Act.
From 1770 to 1772 Jenner received advanced training in London at St Georges Hospital and as the private pupil of John Hunter, then returned to set up practice in Berkeley.Perhaps there was already an informal public understanding of some connection between disease resistance and working with cattle. The "beautiful milkmaid" seems to have been a frequent image in the art and literature of this period. But it is known for certain that in the years following 1770, at least six people in England and Germany (Sevel, Jensen, Jesty 1774, Rendall, Plett 1791) tested successfully the possibility of using the cowpox vaccine as an immunization for smallpox in humans. Jenner sent a paper reporting his observations to the Royal Society in April 1797. It was not submitted formally and there is no mention of it in the Societys records. Jenner had sent the paper informally to Sir Joseph Banks, the Societys president, who asked Everard Home for his views. Reviews of his rejected report, published for the first time in 1999, were skeptical and called for further vaccinations. Additional vaccinations were performed and in 1798 Jenner published his work entitled An Inquiry into the Causes and Effects of the Variolae Vaccinae, a disease discovered in some of the western counties of England, particularly Gloucestershire and Known by the Name of Cow Pox. It was an analysis of 23 cases including several individuals who had resisted natural exposure after previous cowpox. It is not known how many Jenner vaccinated or challenged by inoculation with smallpox virus; e.g.