Important update: Healthcare facilities
CDC has updated select ways to operate healthcare systems effectively in response to COVID-19 vaccination. Learn more
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.

Monitoring COVID-19 Vaccine Effectiveness

Monitoring COVID-19 Vaccine Effectiveness

Why CDC Tracks How Well the Vaccines Are Working

Updated June 23, 2022

CDC is reviewing this page to align with updated guidance.

CDC continuously monitors vaccine effectiveness to understand how COVID-19 vaccines protect people in real-world conditions.

Vaccine effectiveness is a measure of how well vaccination protects people against infection, symptomatic illness, hospitalization, and death.

CDC monitors COVID-19 vaccine effectiveness to understand how well the vaccines:

  • Protect different age groups, such as children, adolescents, and adults, including adults ages 65 and older
  • Protect specific groups, such as people with underlying health conditions or healthcare workers
  • Protect against new variants
  • Lower the risk of infection, including infection without symptoms
  • Protect against milder COVID-19 illness
  • Prevent more serious outcomes, such as hospitalization or death
  • Prevent complications from COVID-19, such as post-COVID conditions and multisystem inflammatory syndrome (MIS)
  • Prevent spreading COVID-19 to others
  • Provide long- and short-term protection
  • Perform among people who have received one or more booster doses

Assessing How Vaccines Work in the Real World Helps Us:

Adjust vaccine recommendations, as needed, such as booster doses

Guide vaccine policy and vaccine distribution

Inform development of vaccine technologies

COVID-19 Vaccines and New Variants of the Virus

Viruses are constantly changing to create new types of the virus, called variants. So far, research shows that the COVID-19 vaccines used in the United States continue to protect against severe disease, hospitalization, and death from known circulating variants. They may not be as effective in preventing infection from these variants. CDC will continue to monitor vaccine effectiveness to see what impact, if any, variants have on how well COVID-19 vaccines work in real-world conditions.

How CDC Monitors COVID-19 Vaccine Effectiveness

CDC uses multiple strategies to monitor how well COVID-19 vaccines are working in real-world conditions.

Read MMWR COVID-19 vaccine effectiveness and safety reports