The world was riddled with smallpox, which killed one-third of all infected. Though Edward Jenner had discovered in 1797 that pus from a cow’s cowpox blisters could be used as a vaccine, the majority of the world had no access to the inoculation. Cowpox was such a local disease, mostly found in England and occasionally France or Italy, that it was unclear how anyone could scale vaccination to more people.Eventually, Jenner came to the realization that he could remove the cow from the vaccination equation. He discovered that by taking the pus from a vaccinated person’s cowpox blisters and putting it into the arms of others, he could create a "warm chain": arm-to-arm vaccination.
King Charles IV of Spain was watching in horror as family members and millions of people in his colonies were felled by smallpox. He conceived of a grand mission, the Royal Philanthropic Expedition of the Vaccine, which would take the vaccine to the Americas, save his people, and make the Spanish Empire the first with a robust plan against the pox.Smallpox was so incredibly infectious that any adult alive probably had already lived through the disease, and anyone with existing immunity would fail to develop the blisters needed to harvest more pus and propagate the vaccine.
Children were the only subpopulation that could keep the vaccine alive, so Balmis recruited 22 orphan boys, aged 3 to 10 years old. King Charles announced that the crown would take care of all these boys as compensation for their bravery, taking on all expenses related to the boys’ wellbeing and ensuring their schooling and financing until they were old enough to support themselves.
At times it looked like the cowpox vaccine would run dry and the expedition would be cut short. But despite close calls due to stormy weather and travel delays, Balmis persevered with the vaccine intact. At every stop he made sure always to instruct new physicians on how to schedule vaccinations to best keep the cowpox alive. He also helped local authorities set up vaccination institutions to oversee and track the administered doses. Versions of some of those vaccine boards still exist today. While records are incomplete, experts today believe the team managed to vaccinate 100,000 to 150,000 people in North and South America.