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.

About COVID-19 Epidemiology

About COVID-19 Epidemiology

Investigating COVID-19: The Science Behind CDC’s Response

When a new infectious disease is discovered, scientists called epidemiologists work with other scientists to find who has it, why they have it, and what CDC can do about it. From the beginning of the COVID-19 outbreak, scientists at CDC and around the world have been working to:

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Identify the source of the outbreak

Epidemiologists went to the area in China where the disease first appeared and conducted surveys in the community and health facilities. They collected nose and throat specimens for lab analyses. These field investigations showed them who was infected, when they became sick, and where they had been just before they got sick—and ultimately led them to a possible source.

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Monitor and track the disease

CDC keeps track of the number of COVID-19 cases and collects information on the disease from surveillance systems that report different kinds of data, such as new cases, hospitalizations, deaths, demographic information (like age, race/ethnicity, or sex), symptoms, and treatments.

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Study the disease

CDC scientists use surveillance data, including information from antibody testing and other kinds of studies, to find out more about the disease, such as how long someone with COVID-19 is contagious, risk factors for severe illness, and which medical treatments are most effective.

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Develop guidance for actions to slow the spread of the disease and lessen its impact

Using study findings, case counts, and surveillance, CDC publishes resources to help people in different risk groups (like healthcare workers or older people) stay safe in different settings (like grocery stores, home, or school). This guidance is constantly being updated as new information become available.

Terms to Know

Field Investigation: Study conducted in a defined population, including a community or setting where many people live close together, such as a prison.