COVID-19 Vaccine Effectiveness Research

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 vaccine efficacy.

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.

Such evaluations will help us understand if vaccines are performing as expected outside the more controlled setting of a clinical trial. As vaccine uptake increases nationally, we will also try to understand how well the vaccines:

Perform in specific subpopulations
Reduce the risk of infection (including infection without symptoms)
Protect against milder COVID-19 illness
Prevent more serious outcomes, including hospitalization
Prevent spread of illness (e.g., whether people who have been vaccinated can still spread COVID-19 to others)
Provide long-term protection (i.e., assess duration of protection)
Protect against changes in the virus (new variants)
Protect against COVID-19 when the vaccine is administered using a single dose or when the second dose is delayed, if these dosing regimens occur under real-world conditions

Several factors can affect real-world vaccine effectiveness, including:

Population host factors (e.g., people not included in clinical trials who may respond differently to the vaccine)
Virus factors (e.g., variants)
Programmatic factors (e.g., adherence to dosing schedules or storage/handling of vaccines)

CDC will use several methods to study all of these factors, as they can all contribute different information about how a vaccine is working. Descriptions of planned evaluations CDC is conducting with partners, and, where relevant, links to protocols that detail the evaluation designs may be found below.

Participating Sites
Name	Outcome	Population	Design	Participating Sites
Arizona Healthcare, Emergency Response, and Other Essential Workers Surveillance (AZ Heroes) Study	Asymptomatic and symptomatic re-infection with SARS-CoV-2	Healthcare providers/first responders	Prospective cohort	University of Arizona, Tucson, AZ Mel and Enid Zuckerman College of Public Health, Tucson, AZ
Estimating SARS-CoV-2 vaccine effectiveness among adults age 65 years and older using CMS data	Hospitalization and severe disease with SARS-CoV-2	≥65 years	Retrospective case-control	COVID-NET Sites
Evaluating SARS-CoV-2 Vaccine Effectiveness Among Health Care Personnel During Early Phase Vaccination	Symptomatic SARS-CoV-2 infections	Healthcare providers	Test-negative prospective case control	PREVENT Sites Baystate Medical Center, Springfield, MA Brigham and Women’s Hospital, Boston, MA Jackson Memorial Hospital, Miami, FL Olive View-UCLA, Los Angeles, CA Thomas Jefferson University, Philadelphia, PA Truman Medical Centers, Kansas City, MO University of Alabama Hospital, Birmingham, AL University of California, San Francisco and Fresno, CA University of Chicago, Chicago, IL University of Iowa, Iowa City, IA University of Massachusetts Memorial Medical Center, Worcester, MA University Medical Center, Louisiana State University, New Orleans, LA University of Mississippi Medical Center, Jackson, MS University of Texas Southwestern Medical Center, Dallas, TX University of Washington, Seattle, WA Valleywise Hospital, Phoenix, AZ SHEPheRD sites Duke University, Durham, NC Johns Hopkins University, Baltimore, MD Rush University, Chicago, IL University of Utah, Salt Lake City, UT Washington University, St. Louis, MO University of Wisconsin, Madison, WI Emerging Infection Program Sites CDC Artic Investigations Program sites Alaska Native Tribal Health Consortium (ANTHC)external icon, Bethel, AK Yukon-Kuskokwim Health Corporation (YKHC)external icon, Anchorage, AK
Research on the Epidemiology of SARS-CoV-2 in Essential Response Personnel (RECOVER)	Symptomatic and asymptomatic SARS-CoV-2 infections	Healthcare providers/first responders	Prospective cohort	Baylor Scott and White Health, Dallas, TX Kaiser Permanente Northwest, Portland, OR St. Luke’s Hospital, St. Louis, MO University of Arizona, Tucson, AZ University of Miami, Miami, FL University of Utah, Salt Lake City, UT
Research on the Epidemiology of SARS-CoV-2 in Essential Response Personnel (RECOVER) SARS-CoV-2 Household Transmission Study	Household transmission among healthcare workers with SARS-CoV-2	Households of healthcare providers/first responders	Prospective cohort	Baylor Scott and White Health, Dallas, TX Kaiser Permanente Northwest, Portland, OR St. Luke’s Hospital, St. Louis, MO University of Arizona, Tucson, AZ University of Miami, Miami, FL University of Utah, Salt Lake City, UT
US Influenza Vaccine Effectiveness Network Protocol for Influenza and Other Respiratory Viruses (including COVID-19)	Symptomatic infection with SARS-CoV-2	Outpatient adults and children ≥6 months of age	Test-negative prospective case control	Group Health Cooperative, Seattle, WA Marshfield Clinic Research Institute, Marshfield, WI University of Michigan, Ann Arbor, MI Henry Ford Health Systems, Detroit, MI University of Pittsburgh Medical Center, Pittsburgh, PA Scott & White Healthcare, Temple, TX
Investigating the Risk of Influenza-Associated Outcomes and Influenza Vaccine Effectiveness Using Integrated Medical and Public Health Records (VISION) + COVID-19 related addendum	Hospitalization and severe disease with SARS-CoV-2	Hospitalized adults and children	Prospective cohort	HealthPartners Institute, Minneapolis, MN Kaiser Permanente Northwest, Portland, OR Universities of California at Irvine, Los Angeles, and San Diego, San Francisco (COVID addendum only), and Davis (COVID addendum only) University of Colorado, Denver
Vaccine Safety Datalink (VSD)	Hospitalization and severe disease with SARS-CoV-2	Hospitalized adults	Prospective cohort	The health care organizations participating in VSD that will contribute data to this project will be confirmed soon.

Protocols for CDC COVID-19 Vaccine Effectiveness Evaluations

For more information on COVID-19 vaccine effectiveness: Ensuring COVID-19 Vaccines Work.

Page last reviewed: February 18, 2021

Content source: National Center for Immunization and Respiratory Diseases