Edward Jenner and the End of Smallpox

A note: this piece is history and illustrative math, not medical advice. For decisions about vaccines for yourself or your family, talk to a doctor or your national public-health agency.

About this series: History-Makers profiles people whose specific, well-documented contributions changed human life at enormous scale. Jenner is here because his method led, through others' work, to the only human disease ever eradicated. But this is also the story of the people who came before him, and the piece names them.

Before Jenner: fighting smallpox with smallpox

Smallpox killed roughly three in ten of the people it infected and scarred many survivors. Long before vaccines, people noticed that survivors never caught it again, and some cultures learned to give it deliberately in a mild form. This practice, variolation or inoculation, put material from a smallpox sore into a scratch or, in China, blew powdered scabs into the nose. It killed perhaps 1–2 percent of those who received it, far fewer than natural smallpox.

Two people carried the idea into the English-speaking world around the same time:

Lady Mary Wortley Montagu, an English aristocrat scarred by smallpox in 1715, watched variolation in the Ottoman Empire and described it in a letter in 1717. She had her young son inoculated in Constantinople in 1718, and her daughter in London in 1721, the first such operation in England. The royal family followed after a trial on condemned prisoners at Newgate, all of whom survived.

Onesimus, an enslaved African man in Boston, probably Akan from what is now Ghana, told the minister Cotton Mather that he had been inoculated in Africa and described how it was done. When smallpox struck Boston in 1721, Mather pushed for inoculation, citing Onesimus, and was mocked for trusting an enslaved man. The physician Zabdiel Boylston went ahead:

Boston, 1721–22	People	Died	Death rate
Inoculated by Boylston	280	6	about 2%
Caught smallpox naturally	5,889	844	about 14%

Variolation worked, but it had a serious flaw: inoculated people had real smallpox and could spread it to others.

Jenner's experiment

Edward Jenner was a country doctor in Gloucestershire, already a Fellow of the Royal Society for his studies of how cuckoo chicks push their foster siblings out of the nest. Country people believed that milkmaids who caught cowpox, a mild disease from cows, were protected from smallpox. A Dorset farmer, Benjamin Jesty, had acted on that belief in 1774, giving his wife and children cowpox during an epidemic.

On 14 May 1796, Jenner took matter from a cowpox sore on the hand of a milkmaid, Sarah Nelmes, and put it into scratches on the arm of James Phipps, the eight-year-old son of his gardener. On 1 July he deliberately exposed the boy to smallpox material. Phipps didn't get sick.

The Royal Society didn't publish Jenner's first report. He gathered more cases and in 1798 published his Inquiry himself, describing 23 more subjects. The Latin name for cowpox, variolae vaccinae, "pustules of the cow," gave us the words vaccine and vaccination. Unlike variolation, the new method didn't spread smallpox.

Jenner didn't discover that cowpox protected people. He tested it, wrote it down, and kept pushing until the world adopted it.

Why vaccination can end a disease: the arithmetic

Every infectious disease has a basic reproduction number, R₀: the average number of people one case infects in a population with no immunity. If each case infects fewer than one new person, an outbreak shrinks and dies out. Immunity does the shrinking: if a fraction of people can't be infected, each case finds fewer targets. The share that must be immune to push spread below one is the herd immunity threshold, 1 − 1/R₀.

Try smallpox at 95% effectiveness: you need roughly 84% coverage. Now try measles: over 98%, which is why measles comes back the moment coverage slips a few points. And with a vaccine that's only 80% effective, some diseases can't be stopped by vaccination alone however many people take it. Smallpox's moderate R₀ was one reason eradication was possible at all.

This is the simplest textbook model. It assumes everyone mixes evenly. Real populations have clusters, so a community with low coverage can sustain outbreaks even when the national average looks safe, and real campaigns rarely rely on blanket coverage alone.

From one boy to a whole planet

Vaccination spread fast. Britain's Parliament granted Jenner £10,000 in 1802 and £20,000 more a few years later. In 1803 Spain sent the Balmis Expedition to carry the vaccine to its American and Asian territories. With no way to keep it alive in a bottle, it sailed with 22 orphan boys, infecting them two at a time, arm to arm, so that the live cowpox would survive the voyage. Isabel Zendal, the director of their orphanage, travelled to care for them.

Eradication took another 170 years. The key in the final campaign led by the World Health Organization was surveillance and containment: finding every case and vaccinating everyone around it, the "ring" strategy, rather than vaccinating whole populations. The last natural case was a hospital cook, Ali Maow Maalin, in Merca, Somalia, in October 1977. He survived. In 1978 the virus escaped from a laboratory in Birmingham, England, and killed the medical photographer Janet Parker, the last person to die of smallpox. The world was declared free of the disease in 1980.

The honest complications

The experiment. Jenner deliberately exposed a child, his employee's son, to smallpox. That followed the variolation practice of the day, but no ethics committee today would allow it.

The credit. The idea came from rural folk knowledge, Benjamin Jesty acted on it 22 years earlier, and inoculation reached the West through Ottoman, African and Asian practice, carried by a woman and an enslaved man whose contributions were long minimised. Jenner's genuine contribution was systematic testing, publication and relentless promotion.

The science was messier than the story. Jenner's conclusions were partly right and partly wrong, and the medical establishment argued over them for years. He believed the infection started in a horse disease called "the grease" before passing to cows. In a strange twist, modern genetic studies show that the vaccine virus later used worldwide, vaccinia, is not cowpox at all but most closely related to horsepox.

Resistance was immediate. Cartoons in 1802 showed vaccinated people sprouting cow parts. Arguments about compulsory vaccination began in the 19th century and continue today, with real questions about liberty and public health on both sides.

Honorable mentions

Onesimus and Lady Mary Wortley Montagu, who brought inoculation to Boston and London; Benjamin Jesty, the first known to use cowpox deliberately; Zabdiel Boylston, who gathered the first numbers; and D. A. Henderson and the thousands of health workers of WHO's Intensified Smallpox Eradication Programme, who finished the job. (Louis Pasteur, who turned vaccination into a general method, has his own piece in this series.)