COVID-19 VACCINE

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 can be found here.