Teaching Your Body to Win in Advance
A vaccine is essentially a training exercise for your immune system. It contains an agent that resembles a disease-causing microorganism, often a weakened or killed form of the microbe, one of its toxins, or one of its surface proteins.
The body treats this harmless stand-in as a real threat. It attacks, destroys it, and, crucially, remembers it. If the real pathogen ever shows up, the immune system recognizes the invader and destroys it before the disease can take hold.
Most vaccines are preventive, protecting against a future infection, but some are therapeutic, designed to fight a disease that has already begun, such as cancer. Either way, the verdict of the medical world is clear: vaccination is the most effective method of preventing infectious diseases, and its safety and effectiveness have been widely studied and verified.
An Idea Much Older Than You Think
Long before modern medicine, people had noticed that surviving a disease often meant never catching it again. In China, hints of deliberate smallpox inoculation, a practice known as variolation, appear as early as the 10th century, with recorded use by the 16th century.
The idea traveled west in 1721, when Lady Mary Wortley Montagu brought the practice from Turkey to Britain.
The true breakthrough came from an English country doctor. Edward Jenner noticed that a mild disease called cowpox seemed to protect against deadly smallpox. In 1798 he published his findings under a title that gave the field its name: Variolae vaccinae, Latin for "smallpox of the cow". In 1881, Louis Pasteur proposed extending the words vaccine and vaccination to all protective inoculations, in honor of Jenner.
The Killer That No Longer Exists
Vaccination's greatest trophy is an empty one: smallpox, one of the most contagious and deadly human diseases, is simply gone, eradicated worldwide. It was also the first disease for which a vaccine was ever produced.
Other killers have been pushed to the brink. Polio is now restricted to only parts of three countries: Afghanistan, Nigeria, and Pakistan. The numbers for measles tell the story vividly: in 1958, the United States recorded 763,094 cases and 552 deaths, while after the vaccine's introduction, cases fell to fewer than 150 per year.
Today the World Health Organization reports that licensed vaccines exist for 25 different preventable infections, and the measles vaccine alone is estimated to prevent about a million deaths every year.
Many Recipes, One Goal
Not all vaccines are built the same way. Live attenuated vaccines use weakened viruses, as with measles, mumps, rubella, and yellow fever. Inactivated vaccines use killed microorganisms, as with polio, hepatitis A, and influenza. Subunit vaccines use only a fragment of the pathogen, like the hepatitis B shot, and newer mRNA vaccines, such as the Pfizer-BioNTech and Moderna shots, take a genetic approach.
Vaccines protect communities, not just individuals. When the vast majority of people are vaccinated, outbreaks struggle to spread, shielding even those who cannot be vaccinated. This effect is called herd immunity.
No vaccine is perfect: they fail to produce immunity in an estimated 2 to 10 percent of individuals, and adverse effects, when they occur at all, are generally mild, with severe reactions extremely rare. Against that stands one of the greatest life-saving records in the history of medicine.
