Vaccine Effectiveness

CDC’s COVID-19 vaccine effectiveness program

In collaboration with public health and academic partners, CDC conducts observational studies to evaluate the real-world effectiveness of FDA authorized and licensed COVID-19 vaccines.

Guiding principles for monitoring vaccine effectiveness

Researchers use information collected through public health surveillance systems, electronic health records, and prospective studies. CDC's COVID-19 vaccine effectiveness program evaluates current COVID-19 vaccines by:

1. Assessing real-world COVID-19 vaccine effectiveness in key populations and against key outcomes.
2. Providing timely data to evaluate effectiveness of new vaccine recommendations.
3. Detecting changes in COVID-19 vaccine effectiveness due to waning of vaccine-induced protection and emergence of new variants.
4. Monitoring populations at high risk for severe COVID-19.
5. Reviewing existing vaccine policy and informing new vaccine policy.
6. Communicating findings to policy makers, the scientific community, the public, and other stakeholders to increase vaccine confidence and uptake.

Types of vaccine effectiveness

Vaccine effectiveness is generally measured by comparing the frequency of health outcomes in vaccinated and unvaccinated people.

Absolute vaccine effectiveness is when a study compares vaccinated people to unvaccinated people.

Relative vaccine effectiveness is when a study compares people who have received one vaccine type or vaccine schedule to those who received a different vaccine type or vaccine schedule.

Incremental vaccine effectiveness refers to measures that compare people who received more doses with those who received fewer doses.

Factors that can influence vaccine effectiveness

Several factors influence vaccine effectiveness, including:

• Population-related factors, such as age, presence of underlying risk factors (e.g., diabetes, cancer, or pregnancy), and history of prior infection

• Pathogen-related factors, such as the virus variant(s) circulating

• Risk settings, such as residents of long-term care facilities or healthcare workers

• Vaccine factors, such as type of vaccine, vaccine dose, and time since vaccination

• Outcome factors, ranging from mild symptomatic infection to severe outcomes, like hospitalization and death

Collecting information on these factors and adjusting for important epidemiologic confounders allows CDC to minimize bias in measuring vaccine effectiveness.

Vaccine effectiveness can differ by the type of vaccine product, total number of vaccine doses given, and time since the most recent vaccine dose was received.

Vaccine effectiveness might also differ in high-risk populations, such as older individuals or those with immunocompromising conditions. To account for these differences, effectiveness estimates are often presented by:

• Age group

• Presence of immunocompromising conditions

• Populations at increased risk for severe COVID-19, such as long-term care facility residents

Vaccine effectiveness informs vaccine policy

Preventing severe COVID-19 is the priority of the U.S. COVID-19 vaccination program. The program monitors outcomes that are important to patients and that generate evidence relevant to COVID-19 vaccine policy including:

• Critical illness or death due to COVID-19

• Hospitalization for COVID-19

• Medically attended COVID-19 (e.g., urgent care and emergency department visits)

• Symptomatic SARS-CoV-2 infection

CDC's vaccine effectiveness monitoring platforms serve as a pillar of the nation's early warning detection and response system for routine respiratory and other vaccine preventable disease threats which can easily overwhelm our healthcare systems.

Vaccine effectiveness evaluations

Critical illness or death due to COVID-19, hospitalization for COVID-19, and medically attended COVID-19

Hospitalization, critical illness and death are widely recognized measures of severe COVID-19. Prior CDC studies have shown that COVID-19 vaccines may provide lasting protection against critical illness or death due to COVID-19.

CDC works with partners to evaluate vaccine effectiveness in preventing hospitalizations, emergency department visits, and urgent care visits for COVID-19 using the VISION network. This platform also collects information on the severity of COVID-19-associated hospitalizations and COVID-19-associated deaths.

VISION

Outcome:

• COVID-19-associated emergency department and urgent care visits

• COVID-19-associated hospitalization

• Critical illness defined as COVID-19-associated intensive care unit admission or death

Population:

Individuals of all ages who have an emergency department, urgent care visit, or hospitalization with a COVID-19-like illness within the facility network

Participating Sites:

• Columbia University (New York)

• HealthPartners (Minnesota and Wisconsin)

• Intermountain Healthcare (Utah)

• Kaiser Permanente Northern California (California)

• Kaiser Permanente Center for Health Research (Oregon and Washington)

• Regenstrief Institute (Indiana)

• University of Colorado (Colorado)

Symptomatic SARS-CoV-2 infection

The primary goal of the COVID-19 vaccination program is to prevent severe illness and death. However, monitoring effectiveness of vaccines against infection provided insights into the impact of vaccines against new variants and provided an early signal of waning immunity. Studies monitoring vaccine effectiveness against SARS-CoV-2 infection can be subject to bias due to changes in COVID-19 testing practices, including increased use of at-home testing, or different testing practices among vaccinated and unvaccinated individuals. CDC employed multiple study designs and various statistical methods to account for the influence of such factors when measuring vaccine effectiveness.