In the doctor’s office, a little toddler smiles joyfully as the doctor releases the plunger and withdraws the syringe from his arm. This was the first of a series of shots that he was scheduled to receive as part of his vaccination regimen. The doctor applies a bandage to the toddler’s arm, adds the vaccination to his records, and asks the family if they had any questions before leaving for the day.  

Little did the toddler know that within the next few weeks, his body would develop long-lasting immunity to chickenpox, a disease that once took millions of lives each year in the U.S. In fact, vaccination has become one of the most effective methods for preventing the spread of diseases. Today, people of all age groups receive vaccines, and vaccines prevent 2 to 3 million deaths each year. So how did vaccination start?

The Beginning of Vaccination

The history of vaccination traces back to crude attempts of exposing individuals to viruses as protection against diseases. There is some evidence that the Chinese have tried to develop immunity against smallpox in 1000 AD by inhaling powered sores from smallpox lesions or by smearing smallpox pus onto open cuts. Early practices were termed “variolation” from the Latin term varus, meaning “mark on the skin,” and spread to Turkey, England, and eventually America around the 18th century. Though a small percentage of people still died from smallpox after variolation, the practice became quite popular in Europe and in the U.S. colonies as the main method of protection. Still, practitioners sought to find more effective methods.

Towards Modern Vaccines: Milestones in History

discovery of vaccines
In the late Eighteenth century, Edward Jenner used cowpox lesions from the arm of a dairymaid to inoculate others against smallpox)

A major landmark in the discovery of safer, modern vaccines occurred in the late 1700s. Edward Jenner, an English physician, noticed that a dairy farmer who previously contracted cowpox was immune to smallpox. As an experiment, Jenner inoculated a little boy with cowpox lesions and later with smallpox virus and noted that the boy did not catch smallpox. This began the practice of vaccination, a more developed form of inoculation, that we know of today.

Nearly a century later, Louis Pasteur stumbled upon a cholera vaccine by serendipity. In his studies of chicken cholera, he injected fresh cultures of bacteria into chickens, many of which died soon after exposure. One day, he accidentally left the bacteria cultures exposed to air before injection, and the chickens showed only mild symptoms of cholera. In fact, the chickens were now immune to fresh inoculations of chicken cholera. Pasteur concluded that the bacteria were weakened after air exposure and could be used for vaccination; this marked the birth of “live attenuated” vaccines.

We’ve come a long way since the days of Jenner and Pasteur! Today, there are many vaccines on the market, including those used to prevent measles, meningitis, and tetanus. The government has set up safety regulations: before a vaccine is licensed for public use, it undergoes extensive testing by the Food and Drug Administration (FDA) to determine its effectiveness, side effects, and dosage requirement. Each vaccine is also monitored after it is available to ensure public safety. In addition to live-attenuated vaccines, many other types of vaccines have been FDA-approved, including inactivated vaccines, subunit vaccines, and toxoid vaccines.

So How Do Vaccines Work?

discovery of vaccines
A healthy human T lymphocyte (also called a T cell). Credit: NIAID

So now that we’ve talked about how vaccines originated, how do they work in the body? We now know that the body’s immune system has a couple of tools for fighting infection, including T and B cells in the blood. B cells produce antibodies that bind to foreign substances  in the blood to mark them for destruction by other immune cells. T cells destroy cells that have taken up bacteria or viruses. Because these cells attack only one type of foreign substance, the body must generate new ones each time the body encounters a new virus or bacteria. However, the body takes several days to generate these cells to fight the infection. This makes the process of fighting diseases very slow.

Vaccines work by activating the immune system to produce memory cells that wait in the person’s body for the virus or bacteria that was injected. Since the tools for fighting the infection have already been generated, your body can mount a faster and stronger immune response during a second exposure to the same pathogen. The symptoms of a disease are often milder upon secondary exposure. But this “immunological memory” can wear down over time, so some vaccines require booster doses in adulthood to re-expose the immune system to the infectious agent.

The next time you receive a vaccine, remember that your immune system has gotten smarter. They can remember things too!

Though many vaccines have been developed, there is still room for more discovery. For example, there are currently no vaccines for some viruses, such as HIV, that rapidly evolve to escape our immune system. Other viruses, such as Hepatitis C, occur in many variants, and no vaccine can target all genetic forms of Hep C.

Today, scientists are still tackling these challenges. Meanwhile, tap into tips for staying safe from common viruses like the seasonal flu at: Being sick can be tough to deal with!


  • Andrea Dao

    Andrea Dao is a first-year PhD student at University of Chicago in the Molecular Engineering Program. She is interested in developing vaccines and understanding how they work in the body. Find her on LinkedIn.

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