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Long before the word vaccine existed, communities across Asia and Africa practised variolation, the deliberate infection of a healthy person with material taken from the pustules of a smallpox victim. The procedure, documented in China by the sixteenth century and in parts of the Ottoman Empire and West Africa even earlier, carried real danger: roughly two percent of those treated died of the disease itself. Yet survivors gained lasting protection, and the practice spread along trade routes. When the English aristocrat Lady Mary Wortley Montagu observed variolation in Constantinople in 1717, she championed it at home, where it was tested on prisoners and found broadly effective, although never entirely safe.
The decisive advance came in 1796, when the English physician Edward Jenner tested an old dairymaids' belief that catching cowpox, a mild animal disease, prevented smallpox. Jenner inoculated an eight-year-old boy, James Phipps, with pus from a cowpox sore and later exposed him to smallpox material; the boy remained healthy. Jenner called the procedure vaccination, from the Latin vacca, meaning cow. His findings, published in 1798, were initially mocked by some colleagues, but within a decade vaccination had spread across Europe and the Americas, and by 1840 Britain had banned variolation outright.
For most of the nineteenth century, however, no one could explain why vaccination worked. The germ theory of disease, developed by Louis Pasteur and Robert Koch, finally provided a framework. Pasteur discovered that weakened, or attenuated, microbes could train the body without causing illness, and he applied this insight to chicken cholera, anthrax and, most famously, rabies in 1885. The twentieth century then produced a cascade of vaccines against diphtheria, tetanus, whooping cough, polio and measles, turning several killers of childhood into rarities.
Vaccines do not work by magic; they exploit the immune system's own memory. When a vaccine introduces an antigen - a harmless fragment or weakened form of a pathogen - the immune system manufactures antibodies and, crucially, produces memory cells that patrol the body for years. If the real pathogen later appears, these cells trigger a rapid, targeted response that usually neutralises the invader before symptoms develop. Some vaccines, such as those for tetanus, require periodic boosters because this memory fades, while others confer protection lasting decades.
The newest major shift arrived in the 1990s, when researchers began testing vaccines made not from the pathogen itself but from genetic instructions that teach the body's own cells to produce a single viral protein. Messenger RNA, or mRNA, vaccines took decades to mature because the molecule is fragile and provokes unwanted inflammation. The breakthrough came when scientists stabilised the mRNA and wrapped it in tiny fat bubbles called lipid nanoparticles. When COVID-19 emerged in 2020, this groundwork allowed developers to design candidate vaccines within days of reading the virus's genetic code, compressing a process that once took ten to fifteen years into less than one.
The impact of vaccination on public health is difficult to overstate. The World Health Organization estimates that immunisation currently prevents between two and three million deaths every year. Smallpox, which killed an estimated 300 million people in the twentieth century alone, was declared eradicated in 1980 after a global campaign - the only human disease ever eliminated in this way. Polio remains endemic in just two countries, and measles deaths fell by more than seventy percent between 2000 and 2018.
Despite this record, vaccination programmes face persistent obstacles. In low-income countries, the so-called cold chain, the unbroken refrigeration that many vaccines require from factory to patient, is costly and fragile. In wealthy nations, the challenge is often public confidence: a 1998 paper wrongly linking the measles vaccine to autism, though fully retracted and discredited, fuelled hesitancy that health authorities still battle. Coverage fell in more than a hundred countries during the COVID-19 pandemic, and outbreaks of measles have since returned to several nations that had eliminated the disease.
Scientists are now pursuing goals that would have seemed fanciful a generation ago. A universal influenza vaccine, designed to protect against all strains rather than just those predicted for a given season, has entered late-stage trials. Researchers are also testing vaccines against malaria, tuberculosis and even certain cancers. Whether these ambitions are realised will depend not only on laboratory science but on manufacturing capacity, funding and, above all, public trust - the fragile ingredient on which every immunisation programme ultimately depends.