A Quick Guide of COVID-19 Vaccine Terms

As you're staying up-to-date on the latest COVID-19 news, you may come across words you haven't heard before. We've provided definitions of common vaccine-related terms.

Feb. 11, 2021 4   min read

Vaccine terms

As the phased distribution of COVID-19 vaccines continues, it’s more important now than ever to stay informed and up-to-date with the latest research and news. In doing so, you may come across new terms and ideas that you haven’t heard before. That’s why Rochester Regional Health has created a list of the most common COVID-19 vaccine related-terms.

Antibody / Antibodies

A protein found in the blood that is produced in response to foreign substances (e.g., bacteria or viruses) invading the body. Antibodies protect the body from disease by binding to these organisms and destroying them. The COVID-19 vaccine will help protect those who get the vaccine by creating an antibody response without the vaccinated individual having to experience infection.

Clinical Trial

A way of testing whether a new therapy, test, device or vaccine is safe and effective in people. Often, clinical trials are used to test if a new treatment is more effective and has less harmful side effects than a current standard treatment.

Clinical Trial Phases

Clinical trials advance through four key phases as scientists determine whether the therapy, test, vaccine or device they’re investigating works, find the appropriate dosage and look for any harmful side effects.

  • A phase I trial tests the therapy on a small group of healthy people, with an emphasis on the therapy’s safety.
  • A phase II trial involves more people, with an emphasis on whether the therapy works.
  • A phase III trial recruits an even a larger population and gathers more information about safety and effectiveness, as well as examining different populations, dosages and whether the drug/device can be used in combination with other drugs. If the U.S. Food and Drug Administration agrees that the results are positive at this stage, it will approve the experimental drug or device.
  • A phase IV trial is continual monitoring of the drug or device in large and diverse populations after FDA approval.

Emergency Use Authorization (EUA)

Allows the U.S. Food and Drug Administration to evaluate new or repurposed medical products during a public health emergency. Vaccines or medicines are still subject to rigorous evaluation, but the process takes less time than the typical FDA approval process. When the health emergency is over, the authorization is no longer in effect.

Herd Immunity

This is achieved when enough people in a population have developed immunity to a disease that it becomes difficult for it to spread. According to the World Health Organization, a substantial proportion of the population must be vaccinated to achieve herd immunity, but more data is necessary to determine the exact percentage.

Learn More About the Vaccines and Herd Immunity

Immune System

The complex network of cells and proteins that protect the body from infection.


Protection against disease. Immunity is often indicated by the presence of antibodies in the blood.


The action of making an individual immune to infection from a disease or virus.

Messenger RNA (mRNA)

Genetic molecules that direct cells to make specific proteins. The Moderna and Pfizer/BioNTech vaccines contain mRNA molecules that are instructed to make a harmless spike protein unique to COVID-19. Once these proteins are produced, our body recognizes they shouldn’t be there and fights them off. After this process, our body remembers how to fight off the protein, protecting us from COVID-19 infection.


A harmless, inert substance often used in clinical trials as a means of testing the effectiveness of a new medical treatment or intervention. In the COVID-19 vaccine trials, for example, volunteers received either the experimental vaccine or a placebo. Scientists then compared how many people in each group develop symptoms of the virus.

Serious Adverse Event (SAE)

A term used in clinical trials to document reactions to a medical product. An adverse event is any undesirable experience associated with the use of a medical product in a patient.

Serious events are reported to the FDA when the patient outcome is:

  • Death
  • Life-threatening
  • Hospitalization
  • Disability or permanent damage
  • Birth defects
  • Required intervention to prevent permanent impairment or damage
  • Other serious events

Recovered/Resolved: When a Serious Adverse Event (SAE) is marked “recovered” or “resolved,” the patient’s health has returned to the status held before the trial.

Spike Protein

A molecule on the surface of a virus that, in the case of SARS-CoV-2, appears to stimulate a strong antibody response.


The process of administering a vaccine to the body, typically done through an injection.


A substance that prompts the body's immune system to recognize a particular pathogen, such as a virus, and create antibodies to destroy it, resulting in better preparedness to fight off the virus if a person becomes infected.

Vaccine Efficacy

A measure of how well a vaccine works. If a vaccine is 90% effective, it means that those who received the vaccine were 90% less likely to become sick than those who did not.

How effective are the COVID-19 vaccines?