World Health Organization officials said Monday 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," Dr. Mike Ryan, executive director of WHO's emergencies program, said at a press conference at the organization's Geneva headquarters on Monday.

A preliminary study of patients in Shanghai found that some patients had "no detectable antibody response" while others had a very high response, said Dr. Maria Van Kerkhove, WHO's lead scientist on Covid-19. Whether the patients who had a strong antibody response were immune to a second infection is "a separate question," she added.

More than 300,000 of the 1.87 million coronavirus cases across the world have recovered, WHO officials noted, adding that they need more data from recovered patients to understand their antibody response, whether that gives them immunity and for how long.

"That's something that we really need to better understand is what does that antibody response look like in terms of immunity," Van Kerkhove said.

Ryan said there are questions about whether the virus can reactivate after a patient recovers and tests negative for Covid-19.

"There are many reasons why we might see reactivation of infection either with the same infection or another infectious agent," he said. In general, "there are many situations in viral infection where someone doesn't clear the virus entirely from their system." Some patients can also clear the main infection but develop a secondary bacterial infection, he said.

The U.S. Centers for Disease Control and Prevention has said that it is developing a test to detect the presence of coronavirus antibodies to determine if a person could be immune to the disease. While such a test can determine who has been exposed to the virus, it's not clear if it can identify those immune to reinfection, according to the WHO.

WHO officials also warned Monday against lifting social distancing restrictions and reopening businesses, even as U.S. political leaders, from President Donald Trump to New York Gov. Andrew Cuomo, have said they hope to reopen businesses as soon as it is safe to do so.

"While Covid-19 accelerates very fast, it decelerates much more slowly. In other words, the way down is much slower than the way up," WHO Director-General Tedros Adhanom Ghebreyesus said at a press conference at the organization's Geneva headquarters on Monday. "That means control measures must be lifted slowly and with control. It cannot happen all at once."

Tedros outlined a checklist for countries before they should consider lifting social distancing measures:

Transmission of the virus should be controlled.

A surveillance system should be in place to detect, isolate and treat patients.

Outbreaks in hospitals and nursing homes should be minimized.

Preventive measures in essential locations such as schools and workplaces should be in place.

The risk of importing the disease from abroad should be under control.

"Control measures can only be lifted if the right public health measures are in place, including significant capacity for contact tracing," Tedros said.