How vaccines are tested before they reach people

Vaccine development begins long before any human studies take place. Early research happens in the lab, where scientists study how the vaccine interacts with the body and whether it can trigger an immune response.

This stage is sometimes called preclinical research. It involves testing in laboratory settings and, in some cases, animal studies. The goal is to determine whether the vaccine is safe enough to study in people.

By the time a vaccine reaches clinical trials, researchers have usually spent years studying and understanding how the vaccine works and what risks might need attention.

The phases of vaccine clinical trials

Clinical trials happen in phases, with each phase aiming to gather more information to create a complete picture, supporting safe and effective use of the vaccine.

Unless there are exceptional circumstances, such as a pandemic or public emergency, each phase must be completed before the next one begins. Researchers can only move forward when the results prove an acceptable level of safety and effectiveness.

Phase 1 focuses on initial safety. A small group of healthy volunteers (usually around 20–100 people) receives the vaccine so researchers can understand how the body responds and what dose may be appropriate.

Phase 2 expands the study. The vaccine is given to a larger group of healthy volunteers, around 100 to 300 people. Researchers continue to monitor safety and look more closely at how the immune system reacts.

Phase 3 looks at large scale use. The vaccine is given to a large group of volunteers, often thousands to tens of thousands of people across multiple locations. Researchers compare those who receive the vaccine with those who do not to confirm how well it works at preventing the disease, and to identify side effects that may be more common when more people are involved.

Phase 4 takes place after approval. Even after a vaccine is approved and made available to the wider public, monitoring continues as the vaccine is used more widely in order to track safety and effectiveness over time.

How vaccine safety and side effects are tracked during trials

Clinical trials are specifically designed to identify possible side effects and understand the risk profile associated with a vaccine.

Researchers follow participants closely throughout the study, collecting detailed reports on what happens after vaccination. This includes short-term reactions and longer-term outcomes during the study period.

Independent groups also review the data. These are known as Data Safety Monitoring Boards. They’re separate from the teams running the trial and review safety results as they come in. If there is a concern, they can recommend pausing or stopping the study.

How long vaccine testing usually takes

Vaccine development and testing typically takes several years. This process includes early laboratory and animal research, followed by multiple phases of clinical trials in people to evaluate safety and effectiveness, and reviews by regulatory agencies like the U.S. Food and Drug Administration (FDA) before a vaccine is approved and recommended.

Much of the work happens before a vaccine is ever studied in humans, as researchers test ideas, refine how the vaccine works, and gather early safety information. Once clinical trials begin, they may run for months or years depending on what researchers need to observe. Early studies often take around one to two years, while larger, later-stage studies may take several years to complete.

A big part of that timeline comes down to enrollment. Large trials may need thousands of volunteers from different locations and backgrounds, and researchers need time to follow participants after vaccination to understand how the vaccine performs over weeks or months.

What happens when vaccine development is accelerated

Vaccine development can sometimes move faster in response to urgent public health needs. An example of this accelerated process was in response to COVID-19, when the risk to public safety from the virus demanded an expedited approval. Even with an accelerated timeline, the process still ensures the vaccines undergo rigorous testing to maintain safety and effectiveness.

In a traditional timeline, each stage happens one after another. When development is accelerated, some steps can happen at the same time.

The key point is that safety requirements do not change. Each phase still needs to meet the same standards before moving forward.

How vaccines are approved and what happens next

Once trials are complete, the data is submitted to regulatory agencies such as the FDA. Submissions include detailed information from every stage of testing, how the vaccine is made, and how it performed in clinical studies.

Teams of scientists review the evidence, and independent advisory groups made up of outside experts may also evaluate the data and provide recommendations. In the United States, this often includes a panel called the Vaccines and Related Biological Products Advisory Committee, or VRBPAC. This group reviews the data in public meetings and provides recommendations before a decision is made.

Based on this review, the vaccine may be approved or authorized for use. In some cases, vaccines are first made available through an emergency authorization while data continues to be collected. Full approval requires a more complete set of evidence over time.

Similar review processes are used by regulatory agencies in other countries, adding additional layers of evaluation.

Even large clinical trials have limits. They can identify common side effects, but very rare effects may only become visible when a vaccine is used more widely by much larger populations.

To address this, several monitoring systems are in place after approval. These include reporting systems that collect information on possible side effects, such as the Vaccine Adverse Event Reporting System (VAERS) in the U.S. Additionally, large data networks, like the Vaccine Safety Datalink (VSD) are in place to look for patterns across populations. Health professionals and individuals both contribute to this process.

In the U.S., the Advisory Committee on Immunization Practices (ACIP) is a group of medical and public health professionals that reviews evidence and develops recommendations on the use of vaccines, including who should receive them and when for the U.S Centers for Disease Control and Prevention (CDC) to consider.

This ongoing monitoring helps ensure that new information is identified and assessed over time.

Why ongoing monitoring matters

Rather than being decided at one moment, vaccine safety is built up over time through multiple layers of evidence:

• Early research provides the starting point.
• Clinical trials add structured data from controlled studies.
• Regulatory review brings independent evaluation.
• Post-approval monitoring continues to build on that knowledge using real-world data.

An example of this system in action can be found in the post-approval monitoring of the COVID-19 vaccine, which identified a risk of myocarditis in certain groups. Upon further investigation, the guidance was updated in some cases to reflect the new information.

Each layer of review adds more information. Together, they form a system designed to identify benefits and risks.