The immune system is the body’s natural defense mechanism. Most of the time, it works efficiently to combat invading pathogens. However, certain pathogens can overwhelm the immune system, causing serious infection and illness.

Vaccines are products that help the immune system combat invading disease-causing pathogens. They are a safe and effective method for preventing certain diseases.

This article outlines what vaccines are and how they work. It will also discuss the various stages that vaccines must go through to receive medical approval.

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Vaccines, which often take 10–15 years to develop, work to prevent certain diseases.

Vaccines are products that help prevent disease. They do this by preparing the body for exposure to disease-causing pathogens.

Today, vaccines help control and prevent a range of serious diseases. These include but are not limited to:

Vaccines train the immune system to detect and combat disease-causing pathogens. To understand how vaccines work, people will need to know how the immune system works.

How does the immune system work?

Pathogenic viruses and bacteria have molecules on their surface called antigens. These antigens stimulate the immune system to produce proteins called antibodies. The antibodies bind to the antigens, thereby killing or disabling the pathogen.

Different pathogens have different antigens on their surface. A pathogen can only be killed or disabled by an antibody that is capable of binding to its antigens. These antibodies will only exist if the immune system has encountered the antigen in the past.

How do vaccines work?

Vaccines safely expose the body to pathogens so that the immune system can make antibodies capable of binding to their antigens. This allows the immune system to fight off a specific disease-causing pathogen should it encounter that pathogen in the future.

Vaccines expose the body to pathogenic antigens. To do this, they contain one or more of the following:

  • an inactive or dead type of pathogen
  • a weakened pathogen
  • a sugar or protein derived from a pathogen
  • a toxoid containing toxin from a pathogen

Vaccine injections contain several other components, including:

  • adjuvants, which ensure that the immune system responds to the antigen as an invading foreign body
  • stabilizers, which ensure that the vaccine remains effective
  • formaldehyde, which helps prevent bacterial contamination during the manufacturing process

It is worth noting that in most cases, the manufacturer will remove the formaldehyde before packaging.

There are four main types of vaccine. These are:

  • Live attenuated vaccines: These contain a weakened form of the disease. Live attenuated vaccines are effective in providing people with lifelong immunity. People with compromised immune systems cannot receive this type of vaccine, however.
  • Inactivated vaccines: This type of vaccine contains a dead or inactive form of the pathogen. Despite this, the vaccine still stimulates the body to recognize its antigens. A person will require several doses or booster shots of an inactivated vaccine to guarantee its effectiveness.
  • Conjugate, or subunit, vaccines: This type of vaccine contains an isolated protein or sugar from the pathogen. The protein or sugar teaches the immune system to recognize and react to the invading pathogen.
  • Toxoid vaccines: Some diseases harm the body by releasing toxins. Toxoid vaccines use deactivated toxins to help prepare the immune system to fight off live pathogens.

DNA and recombinant vector vaccines are two additional types of vaccine that are still in the experimental stages of development.

Below is a list of current Food and Drug Administration (FDA)-approved vaccines:

It takes about 10–15 years to develop a vaccine for a specific disease.

Vaccine development and testing stages vary from country to country. Below is the process for vaccine approval in the United States.


The developmental stage consists of the following three substages:

  • Exploratory stage: In this stage, researchers conduct laboratory studies to identify antigens that may help treat or prevent diseases. This stage lasts around 2–4 years.
  • Preclinical stage: This stage involves laboratory studies and animal testing to verify vaccine effectiveness and safety before clinical trials. Vaccines that fail to prompt an immune system response in testing do not make it past this stage of development. This stage can take up to 2 years.
  • Application: During this stage, a research sponsor submits a request to the FDA for an Investigational New Drug. The response time is about 30 days.


Approved applications go on to the following three phases of testing:

  • Phase I trials: These involve testing in small groups of adults to assess vaccine safety and immune response.
  • Phase II trials: These involve randomized, controlled testing in a larger group of individuals, including a placebo group. This step assesses the safety of the vaccine and determines an appropriate dosage and delivery method.
  • Phase III trials: This phase assesses safety and efficacy in a larger population consisting of thousands of people. Testing during this phase is double-blind and involves a placebo group for comparison.


If the phase III trial is successful, developers will send a Biologics License Application to the FDA. The FDA will then inspect the manufacturing location and either approve or reject the vaccine for labeling.

Once the FDA license a vaccine, inspections and monitoring will continue. Monitoring activities include phase IV trials (such as optional studies for testing safety and effectiveness) and the use of a Vaccine Adverse Event Reporting System (which allows people to report adverse reactions that may be associated with the vaccine).

Vaccines are products that prepare the immune system to detect and eradicate certain disease-causing pathogens. Ultimately, vaccines help reduce the devastating impact of preventable diseases.

Each vaccine must go through a process of development, testing, and monitoring to ensure that it is safe and effective. The FDA will only approve a particular vaccine once it has passed this rigorous process.