Studying the disease

CDC and other agencies and institutions around the world are conducting thousands of epidemiological studies to learn more about COVID-19 and the virus that causes it. These studies help us understand:

• The time between when someone is exposed to the virus and when they have symptoms (incubation period). We now know that someone can be infected with the virus for 2–14 days before they feel sick and that some people never feel sick.
• How long a person who is infected can shed (release from the body) the virus. To avoid spreading infection, we recommend that people infected with the virus avoid being around others until they have gone 3 days without fever, their symptoms have cleared, and 10 days have passed since their symptoms started.
• The range of signs, symptoms, and severity of the disease (spectrum of disease). Knowing this information helps people be on the lookout for early symptoms and helps healthcare professionals diagnose and treat the disease.
• The risk factors associated with severe disease. We now know that people who are older or have serious chronic health conditions are at higher risk for becoming very sick from COVID-19.
• How often the disease causes illness and death in a population (morbidity and mortality rate). This information helps epidemiologists understand the impact of COVID-19 on public health.

Cohort Studies

A cohort study is one that keeps track of a group of people (cohort) over time. If the information has already been collected for other purposes, it is a retrospective cohort study.

For example, CDC scientists who studied demographic characteristics of people in Georgia who were hospitalized with COVID-19 found that the percentage of hospitalized patients who were black was higher than the percentage of hospitalized patients in the overall population. These results help public health professionals prioritize COVID-19 prevention strategies for people who might be at increased risk for severe disease by prioritizing funds, test and additional research in those communities and developing specific clinical guidelines based on differences in resources to slow the spread in these communities.

In a prospective cohort study, the data collection begins when a cohort is formed, and the data are collected from that group going forward. Current COVID-19 studies are looking at:

• severity of illness and risk factors for severe disease,
• knowledge, attitudes, and practices of a specific population (such as pregnant people or people with underlying medical conditions),
• use of certain medicines, and
• infection prevention and control practices.

Serology Surveys

People who have been infected with a virus might develop antibodies (which are proteins in blood that to fight the virus) even if they don’t know they are infected. Serologic tests can be used to detect the antibodies. By counting the number of people with antibodies to COVID-19, scientists can learn how much the disease has spread in a population. Antibody tests are useful because they include infections that might have been missed because people had no symptoms (were asymptomatic) or mild symptoms and therefore did not get tested or receive medical care.

Antibody tests help answer other important questions about how COVID-19 infections are progressing through populations over time and help estimate how much of the population has not yet been infected, helping public health officials plan for healthcare needs.

CDC’s COVID-19 seroprevalence surveys include large-scale geographic surveys, community-level surveys, and surveys focusing on specific populations.