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 Serology Surveillance

About Serology Surveillance

Uncovering SARS-CoV-2 Infections through Serology (or Antibody) Testing

What COVID-19 Seroprevalence Surveys Can Tell Us. A link to the text version is provided below image

Read Text Equivalent (for Section 508 access)

Serology Surveillance Uncovers Missed Infections

Reported coronavirus disease, COVID-19, cases likely represent only a fraction of all SARS-CoV-2, the virus that causes COVID-19 infections. This may be because an unknown proportion of people:

  • have mild or no symptoms,
  • do not seek medical care, or
  • do not get tested when they sought medical care.

Early in the outbreak, viral testing was limited in many areas or was reserved for patients or healthcare workers with symptoms of COVID-19; since then testing availability has increased greatly.

Each of these issues could contribute to the underreporting of infections of SARS-CoV-2 from healthcare settings, such as hospitals or outpatient clinics.

CDC is using information from COVID-19 seroprevalence surveys to learn more about past SARS-CoV-2 infections, often not accounted for in reported cases. Seroprevalence surveys are investigations that involve the use of serology test (also known as antibody tests) to better understand how many SARS-CoV-2 infections have occurred at different points in time, in different locations, and in different populations in the United States.

Testing for Past Infections

Antibody tests look for antibodies—proteins made in response to infections—in a person’s blood, which indicates they may have been infected with SARS-CoV-2 since the beginning of the pandemic. These tests can be useful in identifying those who had mild or asymptomatic infection or who never received a diagnostic test despite having symptoms.

The percent of individuals in a population that have antibodies to an infectious agent is called seroprevalence. The results of seroprevalence surveys can help us understand how many people in a specific population may have been previously infected with SARS-CoV-2.

By determining the seroprevalence of a representative sample of a population, researchers can estimate the percentage of people in the larger population who have likely been infected with SARS-CoV-2. Using seroprevalence estimates and accounting for characteristics of a population, such as age distribution, researchers are able to estimate the number of infections that likely occurred in an area.

Types of Seroprevalence Surveys

CDC is collaborating with public health and private partners on a variety of seroprevalence surveys of different sizes, locations, and populations. The goal is to estimate the seroprevalence in the population, or proportion of the population with evidence of previous infection with SARS-CoV-2. The types of seroprevalence surveys CDC is conducting include:

What We Can Learn from Seroprevalence

Seroprevalence studies inform our understanding of the epidemiology of COVID-19. Initial seroprevalence estimates suggest that the number of infections was much greater than reported cases, potentially reflecting persons who had mild or no illness or who did not seek medical care or got tested, but who still may have contribute to ongoing virus transmission in the population.

Tracking population seroprevalence over time, in a variety of specific geographic sites, will inform models of virus transmission and policy decisions regarding the impact of social distancing and other preventive measures. CDC plans to conduct seroprevalence surveys across the United States on an ongoing basis.

Because people do not always know if they are infected with SARS-CoV-2, the public should continue to take steps to help prevent the spread of COVID-19, such as wearing cloth face coverings when outside the home, remaining six feet apart from other people, washing hands frequently, and staying home when sick.