The Meaning of Vaccine Is the Same as It Was in 1796, Regardless of Online Conspiracy Theories

A reader sent in an email to our inbox at 2AM last night:

“It has come to my attention that the general definition of vaccine has changed, at least wrt Merriam Websters Dictionary. That publication has removed the use of the term “immunity.” Is that where the medical definition is pulled from or is there a medical journal or dictionary that is used by the health care industry?

If not, I would think there needs to be a concerted effort, in these confusing times, to make very clear that the current usage of the word “vaccine” was different, prior to 2020-21.
Perhaps an alert or notation on your landing page warning that changes in the english language may make historical references difficult or unclear.”

The origin of his question was unclear until a web search yielded an article from the Miami Herald and several mentions of this on social media. It appears that CDC did some editing on their website, and now — magically — the word “vaccine” has a completely different meaning. The Miami Herald explains it this way:

“Before the change, the definition for “vaccination” read, “the act of introducing a vaccine into the body to produce immunity to a specific disease.” Now, the word “immunity” has been switched to “protection.”

The term “vaccine” also got a makeover. The CDC’s definition changed from “a product that stimulates a person’s immune system to produce immunity to a specific disease” to the current “a preparation that is used to stimulate the body’s immune response against diseases.””

“Why did CDC change its definition for ‘vaccine’? Agency explains move as skeptics lurk” The Miami Herald. September 27. 2021.

And, yes, the Merriam-Webster definition of vaccine has changed from what it had in 2013 to what is there now, in 2021. But the change was from “immunity” to “immune response.” Further reading of the definition of immunity defines it as what happens when an organism develops an immune response. It seems that the folks at Merriam-Webster did some editing:

  • 2013 definition of VACCINE: “a preparation of killed microorganisms, living attenuated organisms, or living fully virulent organisms that is administered to produce or artificially increase immunity to a particular disease
  • 2021 definition of VACCINE: “a preparation or immunotherapy that is used to stimulate the body’s immune response against noninfectious substances, agents, or diseases
  • 2021 definition of IMMUNITY: “the quality or state of being immune; especially : a condition of being able to resist a particular disease especially through preventing development of a pathogenic microorganism or by counteracting the effects of its products

The origin of the word vaccine is quite interesting, if you want to read all about it.

Below is the response sent to the reader:


In science, when we are analyzing variables, we have an operational definition of a term. Going back to the first vaccine, people outside of science believed that vaccines protected against infection, like some magic force field that did not allow viruses and bacteria to land on someone once the vaccine was given. That is not the case.

When a person’s immune system responds to a vaccine, the immune system is given advanced knowledge of what a pathogen looks like. That way, when the pathogen lands on or in someone, the immune system can clear it as soon as possible.

For most diseases, this means no signs and symptoms of the disease, even when there is an infection. This was known of the first vaccine, the one against smallpox, and the latest vaccines against the novel coronavirus causing the disease known as COVID-19. So, no the definition of a vaccine, how it works or what it does has not changed.

What CDC did, and vaccine skeptics and others like to make an enormous deal of for no reason, is refine the operational definition of a vaccine. Nothing about their new set of words put in a particular order to define vaccination is wrong. Vaccines are not 100% safe, and they are not 100% effective, but they are safer by orders of magnitude and more effective by orders of magnitude than the alternatives to dealing with infectious pathogens.

There is a famous image of two boys, one vaccinated and the other not, from over a century ago, where the authors of the paper describing the fact that one is covered in pustules while the other has one or two pocks mention very clearly that the smallpox vaccine reduces to near zero the probability of a clinical smallpox infection. At the same time, the lack of vaccine raises to 30% the probability of death when infected.


Take a guess as to which boy is vaccinated?

It is easy to see why people working outside of science — and even some scientists — thought that vaccines prevented infection. If a person is vaccinated, there is a good chance that they clear infections quickly, before any symptoms appear. If they don’t have symptoms, they are less likely to spread the disease. For example, they’re not coughing or sneezing to spread airborne diseases. Or they don’t get a lesion from which skin-to-skin contact can spread a sexually transmitted infection. If the immunized do not have symptoms, they will also not go seek care. If they do not seek care, they do not get counted as cases. And, if they do not get counted as cases, the disease is seen as dwindling and disappearing.

Does this mean that there are polio cases in the United States 30 years after elimination? No, but it does mean that if any of the recent arrivals from Afghanistan, or tourists from Pakistan, came here and were unfortunate enough to be incubating polio, the wide use of the polio vaccine makes it very probable that any infections are subclinical and not likely to spread further.

At the end of the day, the definition of vaccine or vaccination has not changed. It is still the material used (vaccine) or the act of using that material (vaccination) to give advanced knowledge of an infectious pathogen to the immune system so that, if the real pathogen appears, the immune system can clear it as fast as possible, hopefully preventing disease. If, suddenly, everyone in the world understood the word “vaccine” to be a type of tree or a species of cat, then we’d have a change in definition.

Then the entire concept of History of Vaccines would change.

Author: René F. Najera, DrPH

I am the editor of the History of Vaccines site, an online project by the College of Physicians of Philadelphia. All opinions expressed on these blog posts are not necessarily those of the College or any of my employers. Check out my professional profile on LinkedIn: https://www.linkedin.com/in/renenajera Feel free to follow me on Twitter: @EpiRen