Tests that reveal whether someone has been infected with the coronavirus are on their way. But they’re not perfect. Here’s what you need to know. When will life return to normal, or at least a new normal? A major answer to the question of when — and how — Americans can return to public places like work and school could depend on something called an antibody test, a blood test that determines whether someone has ever been infected with the coronavirus. People who are believed to be immune may be able to safely return to work. It would be especially important to know which health care workers are protected from getting infected and could continue to care for sick people.

The Centers for Disease Control and Prevention recently announced that it would begin using antibody tests to see what proportion of the population has already been infected. On Friday, the National Institutes of Health announced it would test 10,000 healthy volunteers around the country for the presence of antibodies.

“Within a period of a week or so, we are going to have a relatively large number of tests available,” Dr. Anthony S. Fauci, the leading infectious disease expert in the U.S., said Friday morning on CNN.

He said the White House coronavirus task force was discussing the idea of “certificates of immunity,” which could be issued to people who had previously been infected. “As we get to the point of considering opening the country,” Dr. Fauci said, “it is very important to understand how much that virus has penetrated society.” Immunity certificates, he said, had “some merit under certain circumstances.”

The idea of providing proof of immunity to allow workers to return to their jobs is being considered in many countries, including Britain and Italy. But as with any test, they are not perfect, and there have been problems with their accuracy.

(Adapted from: www.www.nytimes.com)

Choose the correct grammatical class for the underlined word in the sentence below:“a blood test that determines whether someone has ever been infected”.