Confused about COVID-19 vaccines? Here’s your guide to vaccine vocabulary

When people talk about COVID-19 vaccines, they can sound like they’re speaking a foreign language. Don’t worry! Here’s your guide to vaccine vocabulary.


The experts have been saying it for months: The fastest way to end this pandemic is a COVID-19 vaccine.

Now the first ones are almost here, and everyone is talking about them. At times, it can sound like people are speaking a foreign language. But don’t worry — you’ll be able to follow along just fine with the help of this handy glossary.

What’s the difference between vaccination and immunization? Regular immunity and herd immunity? A clinical trial and a challenge trial? Read on for all the vaccine vocabulary you need to know.



A type of virus whose outer surface is studded with spike-shaped proteins that resemble a crown. SARS-CoV-2, the specific coronavirus responsible for this pandemic, probably jumped from an animal to a human in late 2019.


The respiratory disease that can result from a SARS-CoV-2 infection. The Centers for Disease Control and Prevention estimates that about 40% of infections do not result in any symptoms.


A substance that prompts the body’s immune system to recognize a particular pathogen (such as a virus) and create antibodies to destroy it. That way, you’re better prepared to fight off the real thing if you later become infected.

Vaccine candidate

An experimental vaccine that is still being tested.


The process of administering a vaccine to the body. This is typically done through an injection, though some vaccines can be swallowed or given as nasal sprays.


More or less a synonym for vaccination, though it can also describe other methods of inducing immunity to a disease (such as early methods to fight the spread of smallpox).



A process that makes someone immune to a disease. This is the goal of vaccination.


The body’s ability to fight off an infection, often with the help of antibodies. In active immunity, antibodies are generated through exposure to a pathogen or a vaccine. In passive immunity, a person doesn’t make their own antibodies but receives them from another source. Active immunity is more enduring than passive immunity.

Herd immunity

When enough people in a population have developed immunity to a disease that it becomes difficult for it to spread. In the case of COVID-19, scientists estimate that about 70% of the population will need to be vaccinated to achieve herd immunity.

Vaccine efficacy

A measure of how well a vaccine works. If a vaccine is 90% effective, it means that people who got it were 90% less likely to become sick than people who didn’t get it.

Clinical trial

A way of testing a vaccine’s efficacy by comparing the health outcomes of people who take a vaccine and people who take a placebo, or sham treatment. Researchers also use clinical trials to see whether a vaccine (or other type of medication) causes problematic side effects.

Human challenge trial

A type of clinical trial for a vaccine in which study volunteers are deliberately exposed to a virus or other infectious organism. These trials can produce faster results, but they are controversial in the case of COVID-19 vaccines because there is no guarantee that volunteers who become sick can be cured.


An ingredient sometimes used in a vaccine that helps it work better by boosting the body’s immune response.


Adverse drug reaction

A fancy term for side effects that are unwanted, uncomfortable and in some cases dangerous. The risks posed by these side effects are weighed against the benefits of the drug, or in this case the vaccine.

Emergency Use Authorization

This is a faster way for 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 review, but it’s less comprehensive than with the typical FDA approval process. When the health emergency is over, the authorization is no longer in effect.

Booster injection

An additional dose of a vaccine that primes the immune after the effects of an earlier dose wear off. It is not yet clear whether booster shots of a COVID-19 vaccine will be necessary.


A measure of a vaccine’s strength. Potency can diminish over time.

Vaccine hesitancy

When someone delays getting a vaccine or outright refuses it. This can be due to skepticism about the need for it; a lack of trust in the vaccine itself or in the medical professionals who administer it; or if it just seems too inconvenient to get.