Important update: Healthcare facilities
CDC has updated select ways to operate healthcare systems effectively in response to COVID-19 vaccination. Learn more
Given new evidence on the B.1.617.2 (Delta) variant, CDC has updated the guidance for fully vaccinated people. CDC recommends universal indoor masking for all teachers, staff, students, and visitors to K-12 schools, regardless of vaccination status. Children should return to full-time in-person learning in the fall with layered prevention strategies in place.
The White House announced that vaccines will be required for international travelers coming into the United States, with an effective date of November 8, 2021. For purposes of entry into the United States, vaccines accepted will include FDA approved or authorized and WHO Emergency Use Listing vaccines. More information is available here.

COVID-19 Vaccine Effectiveness

COVID-19 Vaccine Effectiveness
Updated Mar. 7, 2024

Vaccine effectiveness is a measure of how well vaccination works under real-world conditions to protect people against health outcomes such as infection, symptomatic illness, hospitalization, and death.

Results of vaccine effectiveness studies are critical to the CDC’s vaccine program and national vaccine policy decision-making.

The overall goal of CDC’s vaccine effectiveness program is to generate the comprehensive evidence needed to inform COVID-19 vaccine policy decisions and CDC guidance on other prevention measures. To accomplish this, CDC in collaboration with public health and academic partners, conducts observational studies to evaluate the real-world effectiveness of authorized and licensed COVID-19 vaccines in the United States.

These studies generate data on how well vaccines work according to:

  • Age group (e.g., young children, adolescents, adults, and adults ages 65 and older)
  • Risk group (e.g., people with underlying health conditions and pregnant women)
  • Risk setting (e.g., residents of long-term care facilities and healthcare workers)
  • Outcome (e.g., against severe outcomes, such as hospitalization or death; and milder outcomes, such as symptomatic infection)
  • Vaccine product (e.g., original monovalent, bivalent, or updated [2023-24] monovalent)
  • Vaccine dose (e.g., primary series, additional doses, time since last dose)

CDC is committed to routinely evaluating vaccine effectiveness to detect changes that could be due to:

  • Emerging SARS-CoV-2 variants
  • Waning of vaccine protection

This work helps CDC identify population subgroups who may benefit from additional doses in the future.

Updates summarizing the results of CDC led vaccine effectiveness evaluations are provided on COVID Data Tracker.

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. Provide 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