People who have recovered from COVID-19 may or may not be immune to getting sick again – and it's too soon to know how long any immunity might last, World Health Organization experts say. The appraisal comes as WHO leader Tedros Adhanom Ghebreyesus says COVID-19 is "10 times deadlier than the 2009 flu pandemic."

"We know that in some countries, cases are doubling every three to four days," Tedros said at a news briefing in Geneva. "However, while COVID-19 accelerates very fast, it decelerates much more slowly. "

The worldwide number of COVID-19 cases is quickly approaching the 2 million mark, including more than 117,000 people who have died, according to a COVID-19 dashboard created by Johns Hopkins University's Whiting School of Engineering.

The 2009 H1N1 flu pandemic that Tedros referenced is estimated to have infected more than 60 million people in the U.S., according to the Centers for Disease Control and Prevention. The agency estimates the virus killed nearly 12,500 Americans in the span of one year. Nearly twice that number have already died from COVID-19 in the U.S., where some 570,000 people are confirmed to be infected.

More than 440,000 people worldwide are known to have recovered from COVID-19 — and their status is an important and lingering question. If people are immune after recovery, the thinking goes, they could resume normal life activities more quickly and provide both an economic boost and help in rendering essential services.

But for now, the answer to the question of whether people who have recovered can then be re-infected remains "an unknown," Dr. Mike Ryan, executive director of WHO's emergencies programs, said.

"One would expect that a person who generates a full-blown immune response with detectable antibodies should have protection for a period of time," Ryan said. "We just don't know what that period of time is. We would expect that to be a reasonable period of protection, but it is very difficult to say that with a new virus."

A preliminary study on antibodies in the blood plasma of 175 patients who recovered from the disease in China offers mixed information about the potential for immunity, according to Dr. Maria Van Kerkhove, an emerging disease expert who is the WHO's technical lead on COVID-19.

While noting that the study out of Shanghai has not been peer-reviewed, Van Kerkhove said the findings suggest different immunity levels for different patients.

The study "found some individual had strong antibody response," Van Herkhove said, adding, "Whether that antibody response actually means immunity is a separate question."

Researchers in that project, she added, "found some patients who had no detectable antibody response. And they found some individuals who had a very high response."

"Right now, we don't have a full picture of what immunity looks like," Van Herkhove said. "And until we do, we can't give a complete answer."

Citing the complex nature of many COVID-19 cases – including patients' underlying conditions and the body's sometimes dangerous immune response – Ryan said people can develop new infections from those circumstances, or possibly get sick with COVID-19 again because they didn't clear the coronavirus from their system.

The slow pace of controlling the respiratory disease means that restrictions such as business shutdowns and stay-at-home orders should only be lifted slowly, Tedros said. He added that such measures "can only be lifted if the right public health measures are in place, including significant capacity for contact tracing."

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