Inoculation was already a standard practice but involved serious risks, one of which was the fear that those inoculated would then transfer the disease to those around them due to their becoming carriers of the disease.
In 1721, Lady Mary Wortley Montagu had imported variolation to Britain after having observed it in Constantinople.
While Johnnie Notions had great success with his self-devised inoculation (and was reputed not to have lost a single patient), his method's practice was limited to the Shetland Isles.
Voltaire wrote that at this time 60% of the population caught smallpox and 20% of the population died of it.
Voltaire also states that the Circassians used the inoculation from times immemorial, and the custom may have been borrowed by the Turks from the Circassians.

By 1768, English physician John Fewster had realised that prior infection with cowpox rendered a person immune to smallpox.
In the years following 1770, at least five investigators in England and Germany (Sevel, Jensen, Jesty 1774, Rendell, Plett 1791) successfully tested in humans a cowpox vaccine against smallpox.
For example, Dorset farmer Benjamin Jesty successfully vaccinated and presumably induced immunity with cowpox in his wife and two children during a smallpox epidemic in 1774, but it was not until Jenner's work that the procedure became widely understood.
Jenner may have been aware of Jesty's procedures and success.
 A similar observation was later made in France by Jacques Antoine Rabaut-Pommier in 1780.

Noting the common observation that milkmaids were generally immune to smallpox, Jenner postulated that the pus in the blisters that milkmaids received from cowpox (a disease similar to smallpox, but much less virulent) protected them from smallpox.

On 14 May 1796, Jenner tested his hypothesis by inoculating James Phipps, an eight-year-old boy who was the son of Jenner's gardener.
He scraped pus from cowpox blisters on the hands of Sarah Nelmes, a milkmaid who had caught cowpox from a cow called Blossom, whose hide now hangs on the wall of the St. George's Medical School library (now in Tooting).
Phipps was the 17th case described in Jenner's first paper on vaccination.

Jenner inoculated Phipps in both arms that day, subsequently producing in Phipps a fever and some uneasiness, but no full-blown infection.
Later, he injected Phipps with variolous material, the routine method of immunization at that time.
No disease followed.
The boy was later challenged with variolous material and again showed no sign of infection.

Donald Hopkins has written, "Jenner's unique contribution was not that he inoculated a few persons with cowpox, but that he then proved [by subsequent challenges] that they were immune to smallpox.
Moreover, he demonstrated that the protective cowpox pus could be effectively inoculated from person to person, not just directly from cattle."
Jenner successfully tested his hypothesis on 23 additional subjects.

Jenner continued his research and reported it to the Royal Society, which did not publish the initial paper.
After revisions and further investigations, he published his findings on the 23 cases, including his 11 months old son Robert.
Some of his conclusions were correct, some erroneous; modern microbiological and microscopic methods would make his studies easier to reproduce.
The medical establishment deliberated at length over his findings before accepting them.
Eventually, vaccination was accepted, and in 1840, the British government banned variolation – the use of smallpox to induce immunity – and provided vaccination using cowpox free of charge (see Vaccination Act).

The success of his discovery soon spread around Europe and was used en masse in the Spanish Balmis Expedition (1803–1806), a three-year-long mission to the Americas, the Philippines, Macao, China, led by Dr. Francisco Javier de Balmis with the aim of giving thousands the smallpox vaccine.
The expedition was successful, and Jenner wrote: "I don’t imagine the annals of history furnish an example of philanthropy so noble, so extensive as this".
Napoleon, who
Inoculation was already a standard practice but involved serious risks, one of which was the fear that those inoculated would then transfer the disease to those around them due to their becoming carriers of the disease. In 1721, Lady Mary Wortley Montagu had imported variolation to Britain after having observed it in Constantinople. While Johnnie Notions had great success with his self-devised inoculation (and was reputed not to have lost a single patient), his method's practice was limited to the Shetland Isles. Voltaire wrote that at this time 60% of the population caught smallpox and 20% of the population died of it. Voltaire also states that the Circassians used the inoculation from times immemorial, and the custom may have been borrowed by the Turks from the Circassians.
By 1768, English physician John Fewster had realised that prior infection with cowpox rendered a person immune to smallpox. In the years following 1770, at least five investigators in England and Germany (Sevel, Jensen, Jesty 1774, Rendell, Plett 1791) successfully tested in humans a cowpox vaccine against smallpox. For example, Dorset farmer Benjamin Jesty successfully vaccinated and presumably induced immunity with cowpox in his wife and two children during a smallpox epidemic in 1774, but it was not until Jenner's work that the procedure became widely understood. Jenner may have been aware of Jesty's procedures and success.  A similar observation was later made in France by Jacques Antoine Rabaut-Pommier in 1780.
Noting the common observation that milkmaids were generally immune to smallpox, Jenner postulated that the pus in the blisters that milkmaids received from cowpox (a disease similar to smallpox, but much less virulent) protected them from smallpox.
On 14 May 1796, Jenner tested his hypothesis by inoculating James Phipps, an eight-year-old boy who was the son of Jenner's gardener. He scraped pus from cowpox blisters on the hands of Sarah Nelmes, a milkmaid who had caught cowpox from a cow called Blossom, whose hide now hangs on the wall of the St. George's Medical School library (now in Tooting). Phipps was the 17th case described in Jenner's first paper on vaccination.
Jenner inoculated Phipps in both arms that day, subsequently producing in Phipps a fever and some uneasiness, but no full-blown infection. Later, he injected Phipps with variolous material, the routine method of immunization at that time. No disease followed. The boy was later challenged with variolous material and again showed no sign of infection.
Donald Hopkins has written, "Jenner's unique contribution was not that he inoculated a few persons with cowpox, but that he then proved [by subsequent challenges] that they were immune to smallpox. Moreover, he demonstrated that the protective cowpox pus could be effectively inoculated from person to person, not just directly from cattle." Jenner successfully tested his hypothesis on 23 additional subjects.
Jenner continued his research and reported it to the Royal Society, which did not publish the initial paper. After revisions and further investigations, he published his findings on the 23 cases, including his 11 months old son Robert. Some of his conclusions were correct, some erroneous; modern microbiological and microscopic methods would make his studies easier to reproduce. The medical establishment deliberated at length over his findings before accepting them. Eventually, vaccination was accepted, and in 1840, the British government banned variolation – the use of smallpox to induce immunity – and provided vaccination using cowpox free of charge (see Vaccination Act).
The success of his discovery soon spread around Europe and was used en masse in the Spanish Balmis Expedition (1803–1806), a three-year-long mission to the Americas, the Philippines, Macao, China, led by Dr. Francisco Javier de Balmis with the aim of giving thousands the smallpox vaccine. The expedition was successful, and Jenner wrote: "I don’t imagine the annals of history furnish an example of philanthropy so noble, so extensive as this". Napoleon, who 
Edward Jenner