Important update: Healthcare facilities
CDC has updated select ways to operate healthcare systems effectively in response to COVID-19 vaccination. Learn more
UPDATE
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.
UPDATE
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.
UPDATE
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 June 23, 2022
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To understand how well COVID-19 vaccines are working in real-world conditions over time, CDC uses a multi-layered approach, which includes ongoing analyses of surveillance data and studies of vaccine effectiveness.

Surveillance data allow us to monitor rates of COVID-19 cases, hospitalizations, and deaths by vaccination status.

Analyses of surveillance data can provide signals of potential changes in vaccine effectiveness.

These signals can then be further studied through more robust, controlled vaccine effectiveness studies conducted by CDC and partners.

The vaccine effectiveness studies help us better understand how well COVID-19 vaccines are working in different groups of people, against different health outcomes, and against different variants. Vaccine effectiveness is a measure of how well vaccination protects people against infection, symptomatic illness, hospitalization, and death.

Monitoring Data Based on Vaccination Status

Learn about how CDC monitors COVID-19 cases, hospitalizations, and deaths by vaccination status

Data Based on Vaccination Status
How to Study Vaccine Effectiveness

Learn about how CDC studies COVID-19 vaccine effectiveness

Studying Vaccine Effectiveness