Important update: Healthcare facilities
CDC has updated select ways to operate healthcare systems effectively in response to COVID-19 vaccination. Learn more
To maximize protection from the Delta variant and prevent possibly spreading it to others, get vaccinated as soon as you can and wear a mask indoors in public if you are in an area of substantial or high transmission.
UPDATE
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.
UPDATE
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.
UPDATE
Travel requirements to enter the United States are changing, starting November 8, 2021. More information is available here.

What COVID-19 Seroprevalence Surveys Can Tell Us

What COVID-19 Seroprevalence Surveys Can Tell Us
Updated July 8, 2020

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Infographic: A seroprevalence survey uses antibody tests to estimate the percentage of people in a population who have antibodies against SARS-CoV-2. This page includes a graphic explaining how seroprevlance surveys use antibody test to measure the percent of a population likely have a past infections with SARS-CoV-2, the virus that causes COVID-19.

What COVID-19 Seroprevalence Surveys Can Tell Us

Antibody against SARS-CoV-2

Serology, or antibody, testing checks a sample of a person’s blood to look for antibodies against SARS-CoV-2, the virus that causes COVID-19. Antibodies usually become detectable in the blood 1-3 weeks after someone is infected.

Person infected → 1-3 weeks → person has detectable level of antibodies*

*Some people may take longer than 3 weeks to develop antibodies, and some people may not develop antibodies. It is currently unknown how long antibodies are detectable after infection.

A positive result from this test may mean that a person was previously infected with the virus.

Positive (sample positive for SARS-CoV-2 antibodies)

Negative (Sample negative for SARS-CoV-2 antibodies)

Seroprevalence

The percentage of individuals in a population who have antibodies to an infectious agent is called seroprevalence.

Seroprevalence Survey

A seroprevalence survey uses antibody tests to estimate the percentage of people in a population who have antibodies against SARS-CoV-2.

This can tell us how many people in a specific population may have been previously infected with SARS-CoV-2.

A small sample of people participating in the survey represents a larger population, which could be a community, state, or special population (like healthcare workers or pregnant people).