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.
Name | Outcome | Population | Design | Participating Sites |
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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 |
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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
SHEPheRD sites
Emerging Infection Program Sites CDC Artic Investigations Program sites |
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 |
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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 |
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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 |
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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 |
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Vaccine Safety Datalink (VSD) | Hospitalization and severe disease with SARS-CoV-2 | Hospitalized adults | Prospective cohort |
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- Arizona Healthcare, Emergency Response and Other Essential Workers Surveillance Studypdf icon
- CMS and COVID-NET Case Control Among Adults >65 pdf icon
- Health Care Personnel During Early Phase Vaccination pdf icon
- RECOVER Healthcare Personnel Cohort pdf icon
- RECOVER Household Transmission Study pdf icon
- US Influenza Vaccine Effectiveness Network for Flu and Other Respiratory Viruses (including COVID-19)pdf icon
- VISION Cohort With COVID-19 Related Addendumpdf icon
- VSD Cohort for COVID-19 Hospitalization VE
For more information on COVID-19 vaccine effectiveness: Ensuring COVID-19 Vaccines Work.