The U.S. National Institutes of Health (NIH) is starting a study to determine how many Americans have already had COVID-19 without knowing it.

The researchers will analyze participants' blood samples for the presence of antibodies against SARS-CoV-2, the virus that causes COVID-19. These antibodies can show if a person has been infected with COVID-19 in the past.

The NIH plans to recruit as many as 10,000 volunteers for the study. Participants can live anywhere in the U.S., and must be over age 18 to participate. Individuals cannot participate in the study if they have a history of confirmed COVID-19 infection, or are currently experiencing symptoms of the disease.

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"This study will give us a clearer picture of the true magnitude of the COVID-19 pandemic in the United States by telling us how many people in different communities have been infected without knowing it, because they had a very mild, undocumented illness or did not access testing while they were sick," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID) said in a statement. "These crucial data will help us measure the impact of our public health efforts now and guide our COVID-19 response moving forward."

People who enroll in the study will first attend a virtual clinic visit and complete a health questionnaire. Next, they'll submit a blood sample by either having their blood drawn at the NIH Clinical Center in Bethesda, Maryland, or taking a blood sample at home with a special testing kit that will be sent to them, and which they will return by mail. The at-home test involves a small finger prick to obtain a blood sample.

"An antibody test is looking back into the immune system’s history with a rearview mirror," said Dr. Matthew Memoli, principal investigator of the study and director of NIAID's Laboratory of Infectious Diseases Clinical Studies Unit. "By analyzing an individual's blood, we can determine if that person has encountered SARS-CoV-2 previously."

People interested in participating in the study should email [email protected] for more information.

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