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Federal health officials are recruiting up to 10,000 volunteers nationwide as they investigate just how pervasive the novel coronavirus has been in the United States.

As of Monday, more than 550,000 people in the U.S. have tested positive for COVID-19 in the U.S. But with milder and asymptomatic cases often going undiagnosed, researchers are turning to antibody tests to figure out the extent of the virus’ spread, according to a news release from the National Institutes of Health.

Researchers are also hoping to gain insight into which populations and communities have been most impacted, officials said.

Using blood samples from up to 10,000 volunteers nationwide, the “serosurvey” will determine how many adults without a confirmed diagnoses have been infected, the release stated.

“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,” said Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, in the release.

The study is being conducted by researchers from the NIAID, as well as other centers within the NIH.

Participants will have blood samples tested for the presence of two types of antibodies, anti-SARS-CoV-2 S protein IgG and IgM, to determine if they were previously infected by and recovered from with the highly contagious respiratory illness.

“These crucial data will help us measure the impact of our public health efforts now and guide our COVID-19 response moving forward,” said Fauci, who is also a member of the president’s coronavirus task force.

The vast majority of testing in the U.S. conducted thus far has been used to identify active infections and can’t determine whether a person previously contracted COVID-19.

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

The federal agency isn’t the only one performing such an analysis. In California, scientists from Stanford Medicine launched antibody testing last week on a regional level, as did USC’s Price School of Public Policy in conjunction with Los Angeles County Department of Public Health.

Such testing could provide insight into how widespread the virus has circulated in the Golden State as some believe it may have cropped up in the U.S. earlier than initially thought.

Stanford researchers, for example, are investigating a possible viral surge in California back in February, according to the Los Angeles Times.

And data from the Centers for Disease Control and Prevention and local health departments indicate the virus may have been in the California as early as December — but at least “a lot longer than we first believed,” said Dr. Jeff Smith, the chief executive of Santa Clara County government, the Times reported.

“This wasn’t recognized because we were having a severe flu season,” Smith told the newspaper. “Symptoms are very much like the flu. If you got a mild case of COVID, you didn’t really notice. You didn’t even go to the doctor.”

For the NIH study, volunteers can participate from anywhere in the U.S., provided the individual is healthy and over the age of 18. Anyone who has previously tested positive for COVID-19 or shown symptoms of the virus currently is ineligible.

Those selected to participate in the study can have their blood drawn at the NIH campus in Bethesda, Maryland, or they can do it at home through an in-home blood collection provided by the Torrance-based medical device firm Neoteryx.

Those interested in joining the study are asked to contact the agency via email at clinicalstudiesunit@nih.gov. 

More information can also be found in a questions-and-answers guide here.