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.

Vaccine Effectiveness Evaluations by Design

Select a study design below to get more information about the evaluation’s name, population, outcome, protocol and participating sites.

Page last reviewed: August 11, 2021