Photograph by Lynn Johnson, Nat Geo Image Collection
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A trial flu vaccine is injected into a volunteer. Vaccines play an integral role in keeping humans safe from deadly disease.

Photograph by Lynn Johnson, Nat Geo Image Collection

Why vaccines are critical to keeping diseases at bay

Vaccines have protected us from deadly pathogens for millennia. Explore how they shield the human body.

Scientists around the world are racing to develop a vaccine for the novel coronavirus that has killed tens of thousands of people since late December. Dozens of companies and institutions are leading the charge at a record pace, and some already have begun the first phase of clinical trials. Yet researchers continue to warn that it could take at least a year to 18 months before a vaccine is ready for public use—a long time to wait for what many see as the best hope to stem the spread of the SARS-CoV-2 virus, which causes COVID-19.

Most vaccines don’t cure diseases; they prevent you from getting infected in the first place. Vaccines contain the same germ (or part of a germ) that causes a disease, but in a killed or weakened state so that it doesn't actually make you sick. The immune system learns about the pathogen, stores information about it, and produces antibodies against it so that the next time it appears, the body can fight it off.

Vaccines have been around only for a couple hundred years, but the concept of inoculating ourselves against diseases has a long history.

The invention of vaccines

Smallpox was one of the early scourges of humankind—and the first and only one to be eradicated with the use of a vaccine. By 430 B.C., humans had figured out that people who survived smallpox developed an immunity to it. Sometime over the next 2,000 years—some say as early as 200 B.C.—people learned how to inoculate themselves against it.

Early accounts from China and India indicate that people fought the deadly disease using a technique called variolation, which involved grinding up smallpox scabs and deliberately infecting someone with it by blowing it up a nostril or scratching it into their skin. Variolation caused a milder form of the disease and was far from perfect: Not only was there still a 2 to 3 percent fatality rate, but the infected could pass on smallpox. Still, by the early 18th century, the technique had become popular in Europe and the Americas.

In 1796, an English doctor named Edward Jenner revolutionized the way we approach diseases like smallpox. He showed that inoculation using a weakened strain of cowpox—a mild zoonotic disease that at the time typically transferred from cattle to humans—could also protect against smallpox. During the next several decades, Jenner’s vaccination method gradually replaced variolation. Thanks to that discovery and developments in the ensuing years, smallpox began to fade. In 1980, nearly 200 years later, the World Health Organization (WHO) declared it eradicated.

Jenner’s breakthrough paved the way for vaccines that today prevent widespread epidemics of a variety of diseases, including influenza, measles, polio, rabies, tetanus, typhoid, yellow fever, and cervical cancer.

How solving this medical mystery saved lives What do milk, sheep, and vaccines have in common? Louis Pasteur. Learn how he helped prove to the world that germs cause disease and usher in an unprecedented era of medical breakthroughs.

How vaccines work

Your body’s immune system is designed to seek and destroy invading pathogens—but it’s not always easy, and pathogens can be clever. For example, the flu virus disguises itself as it enters your body and then begins to replicate before your immune system realizes that it’s there. Vaccines give your immune system a leg up in the fight by teaching it how to quickly recognize a pathogen.

There are several different types of vaccines, but they all essentially serve to introduce a germ or part of a germ into your body in a way that can’t make you sick—though it may cause minor symptoms such as fever as your body builds immunity. Some vaccines use the entire pathogen, but in a killed or weakened state; some use only the parts of the organism that alert the immune system; some use a toxin made by the germ, and some rely on the pathogen’s genetic material.

When you receive a vaccine, the germ sends up an alert to your immune system to start producing antibodies to fight it. Once your immune system has beaten the pathogen, it knows how to quickly destroy it. When you’re exposed to the real thing, your body recognizes the bug and can fight off the infection before it begins.

Sometimes that immunity from a vaccine can last for years or even the rest of your life, while other vaccines require boosters at regular intervals. All adults and children need the influenza vaccine every year to prevent infection against the viral strains likely to be common that season.

Misinformation and waning trust in science and government has spurred an anti-vaccine movement among those who question their safety. Yet vaccines remain as crucial as ever to keeping dangerous diseases such as measles and polio at bay. The WHO estimates that vaccines save two million to three million lives each year.

Many are now pinning their hopes on a vaccine to do the same for the novel coronavirus. But it’s too soon to say when that might be—or what type of vaccine will be most effective against the coronavirus that continues to spread around the world.