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Vaccine Safety and Effectiveness

What is Vaccine Development?

Vaccine development begins in the laboratory before any tests in animals or humans are done70. The U.S. Food and Drug Administration (FDA) must license and approve a vaccine before it can be used in the United States. FDA regulations for the development of vaccines help to ensure their safety, purity, potency, and effectiveness.70

Before a vaccine is approved by FDA for use by the public, results of studies on safety and effectiveness of the vaccine are evaluated by highly trained FDA scientists and doctors. FDA also inspects the vaccine manufacturing sites to make sure they comply with Current Good Manufacturing Practice regulations.71

The United States’ long-standing vaccine safety system ensures that vaccines are as safe as possible. Each vaccine development typically goes through three phases of clinical trials. The FDA sets guidelines for the phases of clinical trials to ensure the safety of the volunteers as well as ensuring the highest level of scientific and ethical standards are upheld. In addition to evaluating the results of the clinical trials, FDA scientists and medical professionals carefully evaluate a wide range of information including results of studies on the vaccine’s physical, chemical, and biological properties, as well as how it is manufactured, to ensure that it can be made consistently safe, pure, and potent.70


What Happens During Clinical Trials?

The trials and all other data must show that the vaccine’s benefits outweigh the potential risks (side effects) for people who will be recommended to receive the vaccine. Only if a vaccine’s benefits are found to outweigh its potential risks does the FDA grant a license for the vaccine, allowing it to be used by the public.70

Click on the trial phases below to learn more.

  • During Phase 1 of clinical trials, the main focus is on safety. The trial includes 20-100 healthy volunteers. The questions answered during this phase focus on if the vaccine seems to work, if there are any serious side effects, and how the size of the dose is related to those side effects. It is important to note that vaccines that are being developed for children are first tested on adults.70

  • If no serious side effects are found in Phase 1- the trail moves to Phase 2. Phase 2 includes several hundred volunteers, where the main focus is on short-term side effects and immune system responses.70

  • The last phase of clinical trials (Phase 3) involves double-blind placebo-controlled studies. This trail includes hundreds to thousands of volunteers. The main focus is on comparison between the volunteers who received the vaccine and those who did not. The vaccine is studied for effectiveness & safety.70

Is that the end?

After vaccines are approved and licensed, they are continually monitored closely to ensure no adverse events arise. Even though the vaccine was extensively monitored in the laboratory and in clinical trials, the risk of side effects may still exist. Some clinical trials, even though are comprised of thousands of volunteers, may not be big enough. Side effects may only happen in 1 in 100,000 or 1 in 500,00071. The clinical trials, even though diverse, may not account for all the chronic medical conditions or pregnancy related risks.70

FDA regularly inspects vaccine manufacturing facilities to make sure they are following strict regulations. Vaccines are manufactured in batches called lots, and vaccine manufacturers must test all lots of a vaccine to make sure they are safe, pure, and potent. Vaccine lots cannot be distributed until released by FDA.70

The Advisory Committee on Immunization Practices (ACIP)73, a group of medical and public health experts, carefully reviews safety and effectiveness data on vaccines as a part of its work to make recommendations for the use of vaccines. The ACIP modifies recommendations, if needed, based on safety monitoring. If a link is found between a possible side effect and a vaccine, public health officials take appropriate action by first weighing the benefits of the vaccine against its risks to determine if recommendations for using the vaccine should change.70

For more information regarding Ensuring Vaccine Safety, visit the CDC70. To learn about vaccine safety specifically within the United States, visit CDC- Understanding Vaccines and Vaccines Safety.70

How is all the data monitored?

Centers for Disease Control and Prevention (CDC) and FDA manage several programs using several different methods, to provide the U.S. with rapid, consistent, and thorough assessment of vaccines.74

Below outlines a few data monitoring systems. Click on each monitoring system to learn more.


  • Is a national system used by scientists at FDA and the CDC to collect reports of adverse events (possible side effects) that happen after vaccination. Health care professionals, vaccine manufacturers, vaccine recipients, and parents or family members of people who have received a vaccine are encouraged to submit reports to VAERS if they experience any adverse events after getting any vaccine. It is important to understand that a VAERS report does not prove cause and effect between a vaccine and a reported event. There are reports in VAERS of common conditions that may occur by chance alone that are found shortly after vaccination. Investigation may find no medical link between vaccination and these conditions.75

  • Is a collaborative project between CDC’s Immunization Safety Office, integrated health care organizations, and networks across the U.S. The VSD uses electronic health data from participating sites to monitor and assess the safety of vaccines. The studies help determine if possible side effects identified using VAERS are actually related to vaccination. VSD is a network of eight managed care organizations across the United States. The combined population of these organizations is more than 24 million people.76

  • PRISM is the largest vaccine safety surveillance system in the United States. The system uses a large database of health insurance claims data to monitor for potential vaccine safety signals. Because PRISM has access to information for over 190 million people, FDA is able to identify and analyze rare health outcomes that would otherwise be difficult to assess.74

  • Is a collaboration between CDC and seven medical research centers. The CISA Project provides U.S. clinicians who have vaccine safety questions about a specific patient residing in the U.S. with consultation services. Vaccine safety experts conduct individual case reviews and clinical research studies about vaccine safety.74

What even is vaccine effectiveness?

Vaccine effectiveness is a measure of how well vaccines work in the real world. Effectiveness is measured by observing how well the vaccines work to protect communities as a whole. As stated earlier, clinical trials include a wide range of people, but they cannot be a perfect representation of the whole population. The efficacy seen in clinical trials applies to specific outcomes in a clinical trial.77