Important update: Healthcare facilities
CDC has updated select ways to operate healthcare systems effectively in response to COVID-19 vaccination. Learn more
To maximize protection from the Delta variant and prevent possibly spreading it to others, get vaccinated as soon as you can and wear a mask indoors in public if you are in an area of substantial or high transmission.
Given new evidence on the B.1.617.2 (Delta) variant, CDC has updated the guidance for fully vaccinated people. CDC recommends universal indoor masking for all teachers, staff, students, and visitors to K-12 schools, regardless of vaccination status. Children should return to full-time in-person learning in the fall with layered prevention strategies in place.
The White House announced that vaccines will be required for international travelers coming into the United States, with an effective date of November 8, 2021. For purposes of entry into the United States, vaccines accepted will include FDA approved or authorized and WHO Emergency Use Listing vaccines. More information is available here.
Travel requirements to enter the United States are changing, starting November 8, 2021. More information is available here.

Why CDC Measures Vaccine Effectiveness

Why CDC Measures Vaccine Effectiveness

After the U.S. Food and Drug Administration (FDA) approves a vaccine or authorizes a vaccine for emergency use, experts continue to assess how well the vaccine is working in real-world conditions. These are known as vaccine effectiveness (“VE”) studies. The goal is to understand how a vaccine protects people outside of strict clinical trial settings.

CDC and other partners assess how COVID-19 vaccines work in real-world conditions. Assessing how vaccines work in the real-world is important to:

  • Learn if vaccines offer the same protection seen in clinical trials.
  • Adjust vaccine recommendations, as needed.
  • Learn why and how often breakthrough cases (people getting sick after vaccination) occur.
  • Learn how vaccines protect against COVID-19 variants.
  • Inform vaccine policy and vaccine distribution.
  • Inform future development of vaccine technologies.

Clinical Trial Results Show COVID-19 Vaccines Are Effective

Clinical trialsexternal icon provide data and information about how well a vaccine prevents a disease and about how safe it is. The Food and Drug Administration (FDA) evaluates these data, along with information from the manufacturer, to assess the safety and effectiveness of the vaccine. FDA then decides whether to approve a vaccine or authorize it for emergency use in the United States.

Based on evidence from clinical trials, all COVID-19 vaccines approved for use in the United States were effective at preventing laboratory-confirmed COVID-19 illness in people who received two doses and who had no evidence of being previously infected.

For more information on clinical trial data for each of the COVID-19 vaccines approved for use in the United States:

More assessments take place after a vaccine is either approved or authorized for emergency use by FDA. The goal of these assessments is to understand more about the protection a vaccine provides under real-world conditions, outside of strict clinical trial settings.

Real-World COVID-19 Vaccine Effectiveness Can Differ

CDC and partners assess the effectiveness of COVID-19 vaccines approved or authorized for emergency use by FDA and recommended for public use in the United States.

CDC is assessing how well COVID-19 vaccines work in real-world conditions. Some real-world assessments observe both people who get vaccinated and those who don’t to see how many people in each group become ill with COVID-19. Some assessments look at how COVID-19 vaccine effectiveness differs for people who are partially vaccinated compared to those who are fully vaccinated.

Many factors can affect how a vaccine works in real-world conditions. These factors include:

  • Host factors such as people not included in clinical trials who may respond differently to the vaccine
  • Virus factors such as variants
  • Programmatic factors such as following dosing schedules or storing and handling vaccines properly

Understanding how COVID-19 vaccines work in real-world conditions will allow CDC, FDA, and other partners to ensure vaccines offer real-world protection against COVID-19.