In 1721, as smallpox swept through London, Lady Mary Wortley Montagu had a doctor deliberately infect her young daughter with material from a smallpox sore. She had seen the practice, called variolation, in the Ottoman Empire, where it was an old folk custom. A mild case usually followed, and survivors were protected for life. The same year in Boston, the minister Cotton Mather, who had learned of the practice from an enslaved African man named Onesimus, persuaded a doctor to try it during an epidemic. Variolation carried real risk, since a small share of people died and others could spread the disease, but it was far safer than catching smallpox naturally.

From cowpox to eradication

In 1796 the English doctor Edward Jenner tested a safer idea. Milkmaids who caught cowpox, a mild disease from cattle, seemed to be protected against smallpox. Jenner took material from a cowpox sore on the hand of a milkmaid, Sarah Nelmes, and inserted it into the arm of a boy named James Phipps. Weeks later he exposed the boy to smallpox, and the boy did not fall ill. Such an experiment on a child would be unthinkable today. The word vaccine comes from vacca, Latin for cow.

Even so, smallpox killed an estimated 300 million people in the twentieth century alone. In 1967 the WHO launched an intensified eradication campaign that relied on surveillance and ring vaccination: finding each case and vaccinating everyone around it. The last natural case occurred in Somalia in 1977, in a hospital cook named Ali Maow Maalin, who survived and later worked on polio vaccination campaigns. In 1980 the world declared smallpox eradicated, the first human disease ever wiped out.

How vaccines work

A vaccine trains the immune system to recognize a germ without causing the disease. Some vaccines use weakened or killed germs, others use pieces of them, and newer mRNA vaccines give cells instructions to make a harmless piece of a virus's protein. Either way, the body builds memory cells, so if the real germ arrives later, the defense is fast. Vaccines are tested in large trials before approval and monitored afterward by safety systems that look for rare side effects. Serious reactions do occur, but they are rare, and for the vaccines in routine use the risks of the diseases themselves are far greater.

Herd immunity

Vaccines protect not only the person vaccinated but the people around them. Each infectious disease has a basic reproduction number, written R0 and pronounced "R-nought": the average number of people one case infects in a population with no immunity. If R0 is 3, each case leads to 3 more, then 9, then 27. But if enough people are immune, the germ keeps hitting dead ends, and chains of transmission die out. This is herd immunity, also called community immunity. A simple estimate of the share that must be immune is 1 minus 1 divided by R0. Measles is one of the most contagious diseases known, with an R0 often estimated at 12 to 18, so about 95 percent of a community needs to be immune to stop its spread. Herd immunity shields those who cannot be vaccinated, such as young babies, people receiving cancer treatment and people with certain immune disorders.

What vaccines have achieved

• Wild polio cases have fallen by more than 99 percent since 1988, when the Global Polio Eradication Initiative began. Wild poliovirus now circulates only in Pakistan and Afghanistan.
• The WHO estimates that measles vaccination averted almost 60 million deaths between 2000 and 2023.
• A 2024 study led by the WHO estimated that global immunization efforts had saved about 154 million lives over the previous 50 years, most of them infants.

Because vaccines prevent disease so well, people can forget what the diseases looked like. When vaccination rates fall, the diseases come back, as recent measles outbreaks in Europe and North America have shown.

Fair access

Vaccines help only those who receive them. Programs such as Gavi, the Vaccine Alliance, founded in 2000, help lower-income countries buy and deliver vaccines. During the COVID-19 pandemic, wealthy countries received doses far sooner than poorer ones. That gap cost lives and weakened global protection, since new variants can arise wherever a virus spreads freely.

Worked example: setting a target

Suppose a new respiratory virus has an R0 of about 4. Using the formula, 1 minus 1/4 gives 0.75, so roughly 75 percent of the population must be immune to halt its spread. But no vaccine is perfect. If the vaccine prevents infection in 90 percent of the people who receive it, coverage would need to reach about 0.75 divided by 0.9, or around 83 percent. Then come the human questions. Who gets the first doses when supply is short? Usually those at greatest risk of severe illness and those most likely to spread the virus, such as health workers. How do you reach everyone else? Through convenient clinics, clear information and trusted local voices. The arithmetic sets the goal; organizing is what reaches it.