A guide to understanding vaccine vocabulary

Since the COVID-19 pandemic you would have undoubtedly been exposed to information related to vaccines. While there is a lot of information about vaccines available, there may be some commonly used vaccine terms you may be less familiar with. So here’s a quick guide to understanding some of the terminology you may have seen or heard.


An adjuvant is a substance that is added to a vaccine to make it more effective for longer. It works by enhancing the intensity and durability of the immune response. The first adjuvant was added to a vaccine over 100 years ago.

Adverse event2

The term ‘adverse event’ is used to describe any unfavorable effect associated with the use of a medicinal product. You may have heard ‘adverse events’ being described as ‘side effects’. 


An antigen is the part of a pathogen (see below for definition of ‘pathogen’) that stimulates the body’s immune system to produce antibodies.


An antibody is a protein that’s created in response to an antigen, as part of the body’s immune response. The purpose of an antibody is to help destroy the antigen, which it does by attaching to the antigen and neutralising it.


A booster, or a booster shot, in the context of vaccines, is an additional dose of the vaccine following the completion of a primary vaccination series, to help protect against a specific disease, like COVID-19.


Efficacy describes a medicine’s ability to produce a desired effect under ideal circumstances, such as a controlled clinical trial. It can sometimes be confused with the term ‘effectiveness’, which describes how a medicine performs in the real world.


A disease is described as endemic when it is considered to be consistently present but limited to a region. Malaria is an example of a disease that is endemic in certain countries.


An epidemic happens when a disease suddenly breaks out in a particular region, community or population, and affects a larger number of people than expected. An epidemic may happen if a virus, bacteria, or other cause of disease has grown stronger, is introduced somewhere new, or finds new ways to infect people.

Herd immunity

When a significant proportion of a population becomes immune to a disease (through vaccination or infection and recovery), the immunity threshold is reached and therefore herd immunity is achieved. This helps control the spread of the disease within the population, and even those who are not immune are protected because the spread of the disease is limited.12 For example, over 80% of the global population has been immunised against smallpox, so it is a virus that we have achieved herd immunity against.13

Investigational vaccine14

An investigational vaccine refers to a vaccine that is approved for clinical trials but is still being tested, and as such is not yet licensed for use in the general public.


Messenger ribonucleic acid, mRNA, carries genetic codes from the DNA to the cytoplasm where it is translated into a protein.15 In mRNA vaccines, the synthesised gene from the antigen is delivered into the body to produce the antigen protein and trigger an immune response.16


Nanoparticles are submicroscopic units measuring less than 100 nanometres (nm), and are used in a variety of areas, including medicine.17 Nanoparticles have been used in several COVID-19 vaccines to protect the antigen within the vaccine, so it can be delivered effectively into the body.18


A pathogen is an organism that causes disease to its host, generally through the production of toxins. Pathogens can trigger immune reactions that may increase transmission to others. One example is influenza, where the pathogen causes symptoms like sneezing and coughing, and the aerosol created by these symptoms helps transmit the disease.


A pandemic occurs when the growth of a disease is exponential, with cases increasing more and more each day, across several countries and populations.10 Only the World Health Organization (WHO) can declare a pandemic.20


Recombinant vaccines combine DNA from the virus with bacterial or yeast cells to create the protein that will be used as the active ingredient in the vaccine.


Viruses have to replicate in order to spread, but they don’t always produce an identical copy of themselves. As a result, the genetic sequence of the virus may begin to change. These new genetic versions of the virus are called variants.

Virus23, 24

A virus is an organism that causes diseases such as chickenpox, measles, and hepatitis. It infects cells and uses parts of the cell to multiply.

Flu and COVID-19 vaccination guidance in the 2023-2024 season in Spain25

In Spain, the seasonal flu vaccine is recommended for children between 6-59 months of age, persons from 5-59 years of age who present a higher risk of complications, as listed in the guidance document, and people with direct occupational exposure to animals. COVID-19 vaccinations are recommended for persons over 60, and those under 60 with the defined risk conditions listed in the guidance document, pregnant women, and persons living with those who have a high degree of immunosuppression.

Full guidance document

More information

We hope this guide to vaccine terminology has helped demystify the language around vaccines. If you have any further questions about vaccines and/or what certain terms mean, please speak to your doctor or healthcare professional.

Dirk Poelaert of Novavax
Dirk Poelaert, MD
Senior Director, Medical Affairs
  1. Pulendran B et al. Nat Rev Drug Discov. 2021;20(6):454–475.
  2. ICH topic E2A Clinical safety data management: Definitions and standards for expedited reporting. Available at