Scientists still don't know whether coronavirus antibodies give a person immunity or reduce the risk of reinfection, even as some nations consider issuing passports or certificates that indicate whether someone has had the virus, World Health Organization officials said Monday.

Some countries are considering issuing so-called immunity passports or risk-free certificates to people who have antibodies against Covid-19, enabling them to travel or return to work assuming that they are protected against reinfection, Dr. Maria Van Kerkhove, head of WHO's emerging diseases and zoonosis unit, told reporters during a news conference at the agency's headquarters in Geneva.

But right now, scientists are unsure whether an antibody response means someone is immune from the virus, she said, adding researchers do know a person typically develops an antibody response about one to two weeks after becoming infected with Covid-19.

WHO officials are studying the so-called serological, or antibody, tests, which can indicate whether a person has had Covid-19 in the past and was either asymptomatic or recovered from the illness. U.S. officials and corporations across America are pouring money into antibody testing, hoping it will give people confidence to return to work and reopen parts of the economy.

The WHO has previously said many countries are suggesting these tests would be able to "capture what they think will be a measure of immunity."

"The serologic tests are the blood tests that test whether you've had the infection, to a greater or lesser extent of accuracy can say you've had the infection," said Dr. Mike Ryan, executive director of WHO's emergencies program. "It's a very different question to say, are you protected from another infection."

Scientists still don't know how much protection the antibodies from Covid-19 give someone from getting another infection, WHO officials said.

"Four months into this pandemic, we're not able to say an antibody response means someone is immune," Van Kerkhove said, adding that this topic is a "very active area of research" and that there are a number of ongoing studies. It doesn't mean a person doesn't have immunity, she added.

More than 876,000 of the 2.9 million coronavirus cases across the world have 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 nearly 1 million people in the United States.

The WHO also said Monday that children across the world will die as the coronavirus pandemic forces some countries to temporarily halt vaccinations for other deadly diseases such as polio.