The use of Jenner's new vaccine spread rapidly through Europe and had a significant positive impact on the severity and frequency of smallpox epidemics.

The efforts were well publicised in Spain and by 1804, dozens of papers, treatises, newspaper articles, and editorials had been published on the smallpox vaccine.

The boys were necessary because the vaccine consisted of infecting patients with cowpox, which is a virus closely related to smallpox but produces a much milder disease that confers immunity to both.

[7] The medical staff and caretakers for the boys consisted of Balmis, a deputy surgeon, two assistants, two first-aid practitioners, three nurses, and Isabel Zendal Gómez, the rectoress of Casa de Expósitos, an A Coruña orphanage.

[8] The mission took the vaccine to the Canary Islands, Puerto Rico, Venezuela, Colombia, Ecuador, Peru, Mexico, the Philippines and China.

In the Philippines, they received help from the Catholic church, which was initially reluctant until Governor-General Rafael Aguilar made an example by vaccinating his five children.

[11] On his way back to Spain in 1806, Balmis offered the vaccine to the British authorities in Saint Helena, despite the Anglo-Spanish War (1796–1808).