Monitoring COVID-19 Vaccine Effectiveness


How and Why CDC Tracks How Well the Vaccines Are Working


CDC continuously monitors vaccine effectiveness to understand how a vaccine protects 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 (e.g., people with underlying health conditions, healthcare workers)

  • Protect against new variants (e.g., Delta, Omicron)

  • Reduce the risk of infection, including infection without symptoms and other breakthrough cases

  • Protect against milder COVID-19 illness

  • Prevent more serious outcomes, such as hospitalization or death

  • Prevent spreading COVID-19 to others

  • Provide long- and short-term protection

  • Perform among people who have received vaccine boosters

COVID-19 vaccines and new variants of the virus

Viruses are constantly changing and new types of the virus, called variants, occur. New variants of the virus that causes COVID-19 are spreading in the United States and in other parts of the world. Research shows that the COVID-19 vaccines used in the United States protect against severe disease, hospitalization, and death from some known variants of concern; they may not be as effective in preventing asymptomatic infection. CDC will continue to monitor effectiveness to see if variants have any impact on how well COVID-19 vaccines work in real-world conditions.


How CDC monitors COVID-19 vaccine effectiveness

CDC uses several methods to study vaccine effectiveness and safety. Before vaccines are released to the public, the FDA and vaccine manufacturers study vaccine efficacy, which is measured in randomized controlled trials. After vaccines are released to the public, CDC measures how well vaccines are working in real-world conditions through various types of observational studies, also known as vaccine effectiveness studies. Along with studies performed by vaccine manufacturers, by other government agencies, and by academic investigators, CDC also works on many types of studies to determine COVID-19 vaccine effectiveness in real-world conditions. Many of these studies are published in the Morbidity and Mortality Weekly Report (MMWR) and other academic journals.


Why CDC monitors vaccine effectiveness

Vaccine effectiveness studies help CDC understand issues such as declining vaccine effectiveness over time after vaccination, and the need for—or the benefits provided by—vaccine boosters in different age groups and special populations (adults ages 65 and older, people with underlying health conditions, and healthcare workers).

Assessing how vaccines work in the real world helps us:

  • Adjust vaccine recommendations, as needed, such as for booster doses

  • Inform vaccine policy and vaccine distribution

  • Inform future development of vaccine technologies

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