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

Updated Dec. 13, 2022

When a new virus that causes disease in humans is discovered, scientists called epidemiologists work with other scientists to find who has it, why they have it, what health problems it causes, 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 identify the source of the outbreak, monitor and track the disease, study the disease, and develop guidance for actions.

Identify the source of the outbreak

Epidemiologists conducted surveys in the community and health facilities where the disease first appeared. They collected specimens using nose and throat swabs for laboratory analyses. These field studies took place in a community where many people live close together. The results showed who was infected, when they became sick, and whether they had traveled just before they got sick.

Monitor and track the disease

Using public heath surveillance systems, CDC keeps track of the number of COVID-19 cases over time and collects information on patient characteristics and risk factors. The types of data collected include case reports, hospital records, and death certificates, which have information like age, race/ethnicity, sex, symptoms, and health outcomes. With these data, CDC uses genomic surveillance to track the spread of variants, and monitor changes to the genetic code of SARS-CoV-2 variants and the impact on public health.

Study the disease

CDC scientists also use information from different kinds of studies to find out more about the disease and how it can be prevented. This includes how long someone with COVID-19 is contagious, risk factors for severe illness, the safety and effectiveness of vaccines, and longer-term health outcomes.

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

Surveillance informs health guidance and improves public health response. CDC publishes resources to help people in different risk groups (like healthcare workers or older adults) stay safe in different community and institutional settings. This guidance is updated as new information become available.­­­­­