CDC wants to learn more about the percentage of people in select counties and communities in the United States who have been infected with SARS-CoV-2, the virus that causes COVID-19. Learning about infections in these communities can help CDC to understand more about how COVID-19 might be spreading in other communities with similar populations.
Looking for Past Infections in the Community
To better understand the percentage of infections in a community, CDC is collaborating with state and county health departments to conduct community-level seroprevalence surveys. These surveys use laboratory tests to check a person’s blood for antibodies, which can show if that person had a past infection with the virus that causes COVID-19. Antibodies are specific proteins made in response to infections. Antibodies are detected in the blood of people who are tested after infection; they show an immune response to the infection. Antibodies most commonly become detectable 1-3 weeks after symptom onset or infection. Antibody test results are especially important for detecting previous infections in people who had few or no symptoms. It is not known yet if having antibodies to the virus that causes COVID-19 can protect someone from getting infected again, or, if they do, how long this protection might last. CDC and its partners plan to study this issue.
Systematic Sampling
Community-level seroprevalence surveys test blood with serology (antibody) tests for antibodies that show previous SARS-CoV-2 infection in people sampled from select counties or communities. The selection of participants is completed in a systematic way using sampling methods. Sampling is a way to help keep the results meaningful and balanced; it means that you might not be selected, but your neighbor could be. Sampling provides a more representative population so that testing results might apply to other similar populations. People who have been sick and those who have not been sick may be selected to make sure we can get a broad picture of the disease.
Impact of the Results
The results can also help to identify areas where we can work with communities to help increase public awareness of actions that persons and communities can take to help slow the transmission of the virus, known as community mitigation strategies. CDC is working with state and county health departments to learn more about how COVID-19 is spreading in communities by performing serology tests in households in various communities. These community-level seroprevalence surveys are taking place in locations across the United States. Descriptions of these surveys are provided below.
In addition to these community-level surveys, CDC also is collaborating with public health and private partners on a variety of seroprevalence surveys of different sizes, locations, populations, and purposes. This includes large-scale geographic surveys to better understand how the virus is spreading through the U.S. population over time and smaller-scale surveys focusing on specific populations such as healthcare workers or pregnant women.