The World Health Organization issued a warning Friday about coronavirus testing, saying there's no evidence serological tests can show whether a person has immunity or is no longer at risk of becoming reinfected.

"These antibody tests will be able to measure that level of serology presence, that level of antibodies, but that does not mean that somebody with antibodies" is immune, said Dr. Maria Van Kerkhove, head of WHO's emerging diseases and zoonosis unit.

So-called serological, or antibody, tests can indicate whether a person has had Covid-19 in the past and was either asymptomatic or recovered.

More than 560,000 of the 2.1 million coronavirus cases across the world are marked as recovered, according to data compiled by Johns Hopkins University. However, infectious disease experts say the case total is likely much higher as people go undetected and countries struggle with testing.

In the U.S., antibody tests have just begun to roll out. President Donald Trump has recommended states use the tests as they start relaxing some of the strict social distancing measures imposed to combat the pandemic, which has infected more than 671,000 people in the United States.

Kerkhove said WHO officials discovered many countries suggesting these tests would be able to "capture what they think will be a measure of immunity."

"What the use of these tests will do will measure the level of antibodies. It's a response that the body has a week or two later after they've been infected with this virus," she said at a news conference at WHO's Geneva headquarters. "Right now, we have no evidence that the use of a serological test can show that an individual is immune or protected from reinfection."

Dr. Mike Ryan, executive director of WHO's emergencies program, said scientists are also still determining the length of protection antibodies might give a person who has been infected with the coronavirus.

"Nobody is sure whether someone with antibodies is fully protected against having the disease or being exposed again," he said.

"Plus some of the tests have issues with sensitivity," he added. "They may give a false negative result."

Earlier this week, WHO officials said not all people who recover from the coronavirus have the antibodies to fight a second infection, raising concern that patients may not develop immunity after surviving Covid-19.

"With regards to recovery and then reinfection, I believe we do not have the answers to that. That is an unknown," Ryan said Monday.

A preliminary study of patients in Shanghai found that some patients had "no detectable antibody response" while others had a very high response, Kerkhove said Monday. Whether the patients who had a strong antibody response were immune to a second infection is "a separate question," she added.