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.
Travel requirements to enter the United States are changing, starting November 8, 2021. More information is available here.

How CDC Monitors Vaccine Effectiveness

How CDC Monitors Vaccine Effectiveness
Updated Mar. 22, 2023

CDC uses a multilayered approach to understand how well COVID-19 vaccines work in the real world. This approach includes ongoing analyses of surveillance data to monitor impact of vaccines and studies of vaccine effectiveness.

Monitoring Vaccine Impact

CDC uses public health surveillance data to monitor rates of COVID-19 cases, hospitalizations, and deaths by vaccination status to help identify overall patterns or trends in the population level impact of vaccines. This information might also signal if there are any potential changes in how well vaccines work that can be further studied through more robust, controlled vaccine effectiveness studies.

Cases, Hospitalizations & Deaths by Vaccination Status

Monitoring Vaccine Effectiveness

Vaccine effectiveness is a measure of how well vaccination protects people against infection, symptomatic illness, medically attended illness, including emergency department and urgent care visits, and severe illness, including hospitalization and death.

CDC conducts observational studies specifically designed to estimate protection provided by vaccination under “real-world” conditions. These vaccine effectiveness studies provide estimates that help us better understand how well the vaccines work in different groups of people, against different health outcomes (such as infection, symptomatic illness, hospitalization, and death), and during periods when different virus variants are circulating (such as Omicron and Delta). Results from these studies generate the evidence needed to inform COVID-19 vaccine policy decisions.

COVID-19 Vaccine Effectiveness

Vaccine Effectiveness Studies

CDC works with partners to study COVID-19 vaccine effectiveness using several data collection platforms and study designs. Vaccine effectiveness studies vary based on the outcome (e.g., infection, hospitalization) and population (e.g., residents of long-term care facilities, children).

CDC-Led Vaccine Effectiveness Studies