More than 350,000 Americans have confirmed cases of coronavirus COVID-19. That’s just confirmed. Millions more may have the virus as well — testing is sporadic at best — and some who contract SARS-CoV-2 will never have a single symptom.

But testing to find out who HAS the virus will soon be far less important as testing for who has HAD the virus.

“Some of you could have had it already,” Dr. David Agus, a professor of medicine and engineering at the University of Southern California, said Tuesday on “The Howard Stern Show.” “There will be a test soon to see who has immunity, who had the virus. That test should come out over the next several days, and that will be key to understanding what’s happening in the country.”

The Centers for Disease Control and Prevention (CDC) has already begun preliminary studies to try to find out how many Americans have already been infected with the virus.

“The first, which has already begun, will be looking at blood samples from people never diagnosed as a case in some of the nation’s Covid-19 hot spots, to see how widely the virus circulated,” STAT reported on Saturday. “Later, a national survey, using samples from different parts of the country, will be conducted. A third will look at special populations — health care workers are a top priority — to see how widely the virus has spread within them. Bresee said the CDC hopes to start the national survey in the summer; he gave no timeline for the health workers study.”

The Food and Drug Administration (FDA) has also authorized the first blood test, known as a serology test, to look for antibodies in the blood. Cellex Inc., a medical device company based in North Carolina, says the test could help physicians determine how widespread the virus is and the duration of immunity for people after they recover.

“Based on the totality of scientific evidence available to FDA, it is reasonable to believe that your product may be effective in diagnosing COVID-19,” FDA chief scientist Denise Hinton wrote in a letter to James Li, CEO of Cellex. “The known and potential benefits of your product when used for diagnosing COVID-19 outweigh the known and potential risks of your product.”

Having antibodies in the blood means that that person likely — though not definitely — had the virus and, for at least a period of time, will have some immunity. That means testing for antibodies could help identify who can safely return to the workforce.

“Your body likely starts making antibodies against the coronavirus between seven and 21 days after you’re infected,” Vincent Racaniello, a professor of microbiology and immunology at the College of Physicians and Surgeons at Columbia University, told Mother Jones. “Those initial antibodies typically stick around in your system for about three months, while other antibodies take a little longer to develop and will likely be found in your blood years after you’ve had COVID-19.

“Their levels go down gradually,” Racaniello said. “But there will always be a low level in the blood that you can detect for years and years after infection.”

Assistant Health Secretary Admiral Brett Giroir said Monday that testing will be easy.

“When we get to the next phase, it will be a combination of the kind of tests we do now plus serologic tests which is a drop of blood, it’s very, very quick. They will tell you if you’ve been exposed to the virus. That’s very important as we think about reopening the country and the economy.”

“If you’ve had the virus, in all probability, you are safe and immune. For the serologic test we’re very optimistic,” the admiral said. “I’m personally very optimistic that by May, we’re going to have these in very large quantities.'”