STEMscopes Asks: How do Vaccines Work?

Posted by Marissa Alonzo on July 02, 2015
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Vaccines are a key development in medicine because they enable us to prevent known communicable (highly contagious) diseases before people get sick, rather than just fighting the disease as it appears. While there are many different types of vaccines, they all work in the same basic way. Vaccines introduce illness-causing microbes into the body in a controlled manner in order to trigger an immune system response. The body produces antibodies that would be used to fight an actual infection, which then prepares the immune system to recognize and fight the pathogen more quickly if it appears again. This reaction boosts the immune response in the individual so that if it ever has a real encounter, the person is far less likely to get sick.

In addition to protecting individuals from disease, vaccines also create “herd immunity.” This is the idea that when enough people in a community are vaccinated, those who are not are also protected from the disease because there are so many vaccinated people that it makes it harder for the disease to pass among those who are not. This is essential to preventing disease among those who are too sick, young, or old to be vaccinated safely and effectively.

How Vaccines are Made

The exact process for making vaccines depends on the type of vaccine, but the general process is common across almost all vaccines.

First, the disease-producing pathogen—a virus or bacterium—must be grown. This is often done by infecting cells grown in a tissue culture with the pathogen to mass-produce it in a lab setting.

Next, the pathogen needs to be altered to prevent the vaccine from triggering the actual disease, rather than just a “practice run” immune response. There is a variety of ways of doing this. The most common alteration is attenuating (weakening) the pathogen by growing it repeatedly and selecting for a less dangerous strain. Secondly, inactivating (killing) the pathogen or separating the part that causes the immune response and using only that part in the vaccine. Lastly, inactivating the toxin made by the pathogen for use in the vaccine as a toxoid is an alternative alteration. 

Finally, the treated pathogen is combined with other ingredients, such as stabilizers, preservatives, and suspending fluids, to prepare the vaccine for transportation, storage, and administration to the patient.


Vaccines as we know them today were first developed in England in 1796, when Edward Jenner, a doctor, discovered that people could be immunized against smallpox using matter from cowpox lesions on cows. Several attempts to prevent smallpox by deliberate introduction (inoculation) can be identified in China as early as 1000 CE and began to be widely practiced in Asia as early as the 1600s and in the 1700s in Europe. These early attempts came in the form of variolation, which involved transferring material from someone infected with a disease to a healthy person—for example, by pulverizing smallpox scabs and blowing them into the nostril of a child or by scratching particles from a smallpox sore into a healthy person’s skin, as a Chinese statesman attempted in 1000 CE. Even though it was not understood at the time, variolation worked in essentially the same way as vaccines—it introduced a pathogen in a controlled manner, prompting an immune response, which in turn prepared the body to work more quickly and effectively and fight off the pathogen before it could cause an illness.

A growing understanding of how diseases work happened in the 1800s and 1900s that became critical to the acceleration of vaccine development. The advent of germ theory and the identification of the underlying bacteria and viruses that caused various diseases were essential to developing methods of preventing them. Additionally, advances in our ability to culture—or grow—pathogens improved our ability to protect against them, because creating a successful vaccine ultimately requires isolating the cause of the illness, figuring out how to grow it in a lab, and then developing a way to safely introduce it into the human body to trigger an immunity-building response.

Interestingly, the military utility of vaccines—which prevent an army from being weakened or incapacitated by communicable disease—provided a catalyst for progress in vaccine research and development. The military has played an important role in researching, testing, and popularizing vaccines.

The progress of vaccines since their introduction has been enormous. In 1980, nearly two centuries after Jenner introduced the world’s first vaccine, the World Health Assembly declared smallpox eradicated. By that point, vaccines had been developed for a wide range of diseases, and what used to be common childhood diseases (e.g., measles, mumps, rubella, diphtheria, whooping cough, and polio) had become increasingly rare in the world’s wealthier countries. By 1994, polio was declared eliminated from the Americas, and by 2002 it had vanished from Europe as well.

 Researchers are currently developing experimental vaccines using DNA and recombinant vectors. DNA vaccines use genes that code for antigens, rather than whole or partial pathogens, that cause the body’s own cells to produce the antigen molecules, allowing an immune system response that establishes immunity without ever introducing the disease-causing microbe. This method of vaccination is currently being explored to fight influenza and herpes. Another progressive vaccine is the recombinant vector vaccine—which is similar to DNA vaccines, except they use harmless, attenuated viruses or bacteria to introduce the antigen-producing DNA. The microbe produces antigens as if it were the disease-causing pathogen but lacks the capacity to actually cause the illness. Scientists are exploring this strategy for use in preventing HIV, rabies, and the measles.

Today, once-feared diseases are but a distant memory in many countries, and researchers are working to push the boundaries of vaccination ever further. Vaccines for some diseases—such as malaria and HIV—remain exploratory, but advances in research into DNA- and recombinant vector-based vaccines may prove promising in improving existing vaccines and tackling diseases that have not been successful with vaccinations.