The United States began taking blood samples across the country to determine the true number of people infected with the new coronavirus, using a test that works retrospectively, according to reports.

The new tests are based on serological studies, which differ from nasal swabs used to determine if someone currently has the virus.

These types of tests analyze whether certain antibodies are present in the blood, showing that the person fought and then recovered from the disease, even if they never showed symptoms.

These tests are seen as the key to gradually making confinement more flexible by allowing those with proven immunity to resume their routine activities.

More than three-quarters of the US population is under some form of confinement, while coronavirus deaths in the country exceed 10,000.

“We are starting to test and will report on it very quickly,” said Joe Bresee, an epidemiologist at the Center for Disease Control and Prevention (CDC), according to the health website Stat News.

He added that the CDC will carry out three samplings: the first on blood samples from undiagnosed people from some of the country’s coronavirus hot spots, the second nationwide from different parts of the country, and the third a study on health workers.

The first study has already started over the weekend, but no precise timeline has been released for the other two.

It comes after the Food and Drug Administration (FDA) issued an emergency approval for the first serological test, performed by the Cellex company in North Carolina, last week.

Separately, Stanford University conducted its own serological survey on Saturday in the city of Santa Clara, Jay Bhattacharya, a professor of medicine at the university, told ..

“We collected fingerstick blood samples from around 2,500 volunteer participants selected to be representative of the county and around 500 of their children,” he said, adding that the results will be released soon.