Vaccine Effectiveness Studies

CDC’s COVID-19 Vaccine Effectiveness Program

In collaboration with public health partners, CDC evaluates vaccine effectiveness through multiple observational studies employing a variety of methods and using information collected through surveillance platforms, electronic health records, and prospective studies.

Several factors influence vaccine effectiveness, including:

• host-related factors, such as age, presence of underlying medical conditions (e.g., diabetes, cancer), and history of prior infection

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

• vaccine-related factors, such as type of vaccine and time since vaccination

Collecting information on these factors and adjusting for important epidemiologic confounders allow CDC to minimize bias in measuring vaccine effectiveness. Vaccine effectiveness can differ by the type of vaccine product, total number of vaccine doses received, and how long it has been 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, by presence of immunocompromising conditions, or among other populations at increased risk for severe COVID-19, such as long-term care facility residents.

Guiding principles for monitoring vaccine effectiveness

Goals of CDC’s COVID-19 vaccine effectiveness program are to evaluate existing COVID-19 vaccines and inform decisions by the U.S. Advisory Committee on Immunization Practices regarding COVID-19 vaccine policy. CDC accomplishes these goals by:

1. Assessing COVID-19 vaccine effectiveness in key populations and against key outcomes (see below).
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. Including populations at high risk for severe COVID-19
5. Communicating findings to policy makers, the scientific community, the public, and other stakeholders

Outcomes of interest for vaccine policy

Preventing severe COVID-19 is the priority for the U.S. COVID-19 vaccination program.  CDC’s COVID-19 vaccine effectiveness program monitors several patient-important outcomes in the evaluations it conducts to generate evidence most relevant for COVID-19 vaccine policy:

• 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

Terms for vaccine effectiveness

Vaccine effectiveness is a measure of how well vaccination protects people against health outcomes such as infection, symptomatic illness, hospitalization, and death. Vaccine effectiveness is generally measured by comparing the frequency of health outcomes in vaccinated and unvaccinated people.

Absolute vaccine effectiveness is a term that can be applied when the study compares vaccinated people to unvaccinated people.

Relative vaccine effectiveness is the term used when the study measures vaccine effectiveness by comparing people who have received one vaccine type or regimen to those who received a different vaccine type or regimen.

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

Measuring relative and incremental vaccine effectiveness can help vaccine policy makers decide whether people who have already been vaccinated might benefit from receiving an extra dose or another vaccine product.

Vaccine effectiveness evaluations by outcome

Below are descriptions of some of the vaccine effectiveness evaluations CDC conducts with partners.

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

Hospitalization is a widely recognized measure of severe COVID-19. The limitations of this measure as a marker of severe disease include the challenge in distinguishing hospitalizations due to COVID-19 from hospitalizations in which SARS-CoV-2 infection is incidentally identified, which can lead to differences in vaccine effectiveness estimates generated across various platforms.

CDC works with partners to evaluate vaccine effectiveness in preventing hospitalizations and emergency department and urgent care visits for COVID-19 using the following platforms: VISION, IVY, and Overcoming COVID.  These platforms also collect information on severity of COVID-19-associated hospitalizations and COVID-19-associated deaths.

VISION

IVY

Overcoming COVID-19

Symptomatic SARS-CoV-2 infection

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

Vaccine effectiveness against SARS-CoV-2 infection is monitored using the ICATT platform.

ICATT

Search MMWR reports by topic, using vaccine effectiveness as a search term. For the latest CDC data on COVID-19 vaccine effectiveness: CDC COVID Data Tracker: Vaccine Effectiveness