Before the U.S. Food and Drug Administration (FDA) determines whether to approve a vaccine or authorize a vaccine for emergency use, clinical trials are conducted to determine how well it works. This is known as effectiveness.
After FDA approves a vaccine or authorizes a vaccine for emergency use, it continues to be studied to determine how well it works under real-world conditions. CDC and other federal partners will be assessing COVID-19 vaccine effectiveness under real-world conditions.
CDC will further assess vaccine effectiveness now that a COVID-19 vaccine has been authorized for emergency use by FDA. Many of these assessments will build on existing CDC programs, such as the Emerging Infections Program, Coronavirus Disease 2019-Associated Hospitalization Surveillance Network (COVID-NET), and systems used to estimate the effectiveness of influenza vaccines.
Do clinical trial results show whether vaccines are effective?
Yes. Clinical trial provide data and information about how well a vaccine prevents an infectious disease and about how safe it is. The FDA evaluates these data, as well as manufacturing information, to assess the safety and effectiveness of vaccines. FDA then decides whether to approve a vaccine or authorize it for emergency use in the United States.
However, more assessments take place after a vaccine is either approved or authorized for emergency use by FDA and then recommended for public use. The goal of these assessments is to understand more about the protection a vaccine provides under real-world conditions, outside of clinical trials.
CDC will further assess the effectiveness of COVID-19 vaccines after they are approved or authorized for emergency use by FDA and recommended for public use. These real-world assessments will compare groups of people who do and don’t get vaccinated and people who do and don’t get COVID-19 to assess how well COVID-19 vaccines are working to protect people.
Why do experts continue to assess vaccine effectiveness in real-world conditions?
The major reason for conducting additional vaccine effectiveness assessments is to make sure a vaccine protects people from getting a disease under real-world conditions, outside of the strict setting of clinical trials. There are many factors that can affect a vaccine’s effectiveness in real-world situations. These factors can include things such as how a vaccine is transported and stored or even how patients are vaccinated. Vaccine effectiveness can also be affected by differences in the underlying medical conditions of people vaccinated in the real-world compared to those in the clinical trials. Vaccine effectiveness assessments can also provide important information about how well a vaccine is working in groups of people not included or not well represented in clinical trials.
How will experts evaluate the effectiveness of COVID-19 vaccines in real-world conditions?
Experts are working on many different types of real-world vaccine effectiveness assessments, and each uses a different method.