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.

Monitoring and tracking the disease

Monitoring and tracking the disease

Defining Cases

As the virus that causes COVID-19 began to spread from person to person in communities (community transmission), scientists needed to track the disease and try to slow its spread. To do so, they needed a common definition for a case of COVID-19. Having a case definition helps to make sure cases are counted the same way everywhere. In the United States, a confirmed case of COVID-19 is defined as a person who tests positive for the virus that causes COVID-19.

COVID-19 became a nationally notifiable disease, meaning that health departments are required to report cases of COVID-19. Systems like the National Notifiable Diseases Surveillance System (NNDSS) collect and send data on cases of COVID-19 to CDC. This helps the agency monitor trends in cases within states and across the country. 

Collecting Information About Cases

As cases of COVID-19 are being reported, epidemiologists are conducting public health surveillance, the systematic collection, analysis, and interpretation of health data. Surveillance allows epidemiologists to calculate:

  • Incidence (number of new cases reported over a specific period of time).
  • Prevalence (number of cases at one specific point in time).
  • Hospitalizations (number of cases resulting in hospitalization).
  • Deaths (number of cases resulting in death).
U.S. mortality: death certificates listing pneumonia, influenza, and covid-19

Date gathered from surveillance are also used to create epidemiological models to predict where, how long, and how far COVID-19 will spread.

But surveillance isn’t just about counting cases. All kinds of information can be collected to learn more about the disease.

Collecting data from medical records (chart abstractions) can tell us more about COVID-19 patients and the course of their disease. These data might include demographic information (age, race/ethnicity, sex), as well as symptoms, treatments, and health outcomes. Scientists can use chart abstractions to learn who is more likely to become severely ill, what medical care patients have received, and if patients have recovered.

Reporting Cases

Once scientists collect and analyze the data, experts in data visualization help to create pictures, charts, and graphics to make the information easier to understand and use. This information is not only helpful for scientists working to understand the information but also for the public.

Displays of epidemiological data often include an epi curve. An epi curve shows what has happened, including the number of cases, hospitalizations, or deaths, over time. Epi curves for COVID-19 are being updated constantly as new data become available.

Because there is a delay between when someone gets sick and when that person’s case is reported, it can be hard to determine when cases actually start to decline. So an epi curve for the most recent few weeks might look like an outbreak is ending even when it is still active. The full shape of the curve is only clear after the outbreak is over.

Tracing Contacts

Contact tracing flow chart

Scientists and public health workers are also working to stop the spread of COVID-19 through contact tracing. In this strategy, public health workers talk to people with COVID-19 to learn about all the people they were physically close to while they were potentially able to spread the disease. Those people are their contacts. With this information, scientists can follow the chain of infection to understand how the disease might have spread from person to person. Contact tracing is used to prevent and control many other infectious diseases, such as tuberculosis and HIV.

Using the information learned through contact tracing, epidemiologists develop tables, called line lists, summarizing the data about the contacts. The connection between each person is called an epidemiological (epi) link.

Contacts of people with COVID-19 are at higher risk for developing the disease and spreading it to others.

Public health workers reach out to these people at higher risk to tell them they have had contact with someone with COVID-19 and because of this exposure, they might get sick. They recommend  prevention measures contacts should follow like self-quarantine (staying away from others while monitoring themselves for signs of illness), handwashing, and the use of cloth face coverings.

Contact tracing has helped slow the spread of other epidemics, including Ebola and SARS, and is crucial in slowing the spread of COVID-19.

Impact of Disease

A key role of epidemiologists during the COVID-19 pandemic is to estimate the burden of disease: the impact of a disease or other health outcome on a population. As scientists collect data from the COVID-19 studies, they are analyzing these data to estimate key outcomes, such as the number of infections, illnesses, medical visits, hospitalizations, and deaths.

Terms to Know

Burden of Disease: Impact of a health outcome or disease on a population.

Case Definition: Set of criteria for determining if a person has a particular disease or other health condition.

Chain of Infection: Links of infection from person to person.

Chart Abstraction: Recording information from patients’ medical records, including demographic data (age, race/ethnicity, sex), as well as symptoms, treatments, and health outcomes.

Community Transmission: Spread of disease from person to person in a community

Contact: Someone was physically close to a person who was potentially infectious with a pathogen.

Contact Tracing: A strategy for slowing the spread of disease in which public health workers communicate with infectious people to identify their contacts. They then follow up with those contacts to provide guidance on how to quarantine themselves and what to do if they develop symptoms of disease.

Data Visualization: Display of information in pictures, charts, and graphics to make the data easier to understand and use.

Epidemiologic (Epi) Curve: Graphic representation of what has happened with a disease such as changes in the number of cases, hospitalizations, or deaths over time.

Epidemiological (Epi) Link: Characteristic that links two cases, such as close contact between two people or a common exposure.

Epidemiological Model: Mathematical representation that predicts where, how long, and how far a disease will spread.

Exposure: Contact with a something that causes illness. In the case of an infectious disease, this represents interaction with a human, animal, or environment from which the pathogen can be transmitted.

Incidence: Number of people in a population who develop a disease or other health outcome over a period of time.

Infectious Disease: Something that causes an infection that can lead to a disease.

Line List: Table containing a list of people with a specific disease or exposures.

Nationally Notifiable Disease: Disease that healthcare providers must report to state or local health departments.

National Notifiable Diseases Surveillance System (NNDSS): Multifaceted program used by around 3,000 public health departments that includes a surveillance system that monitors over 100 diseases.

Public Health Surveillance: Systematic collection, analysis, and interpretation of health data.

Prevalence: Number of people in a population who have a disease or other health outcome at one point in time.

Self-quarantine: Staying away from others while monitoring oneself for signs of illness.