The World Health Organization caused widespread confusion and drew sharp criticism from health experts after one of its officials said recently that asymptomatic transmission of the novel coronavirus was very rare then walked back the remarks the next day.

Maria Van Kerkhove, the WHOs technical lead on COVID-19, had said nations that have conducted contract tracing studies are not finding secondary transmission onward [from asymptomatic cases]. Its very rare, The Washington Post reported.

She later clarified the comments during a live Q&A, saying that she had been referring to a small number of unpublished studies when she answered a reporters question and was not stating the policy of WHO, adding that much more research is needed.

So where do health experts stand on asymptomatic transmission? Heres what you need to know.

Health experts dont know for sure how many people have COVID-19 but dont show symptoms. But they have come up with estimates using models and available data.

In the Q&A, Van Kerkhove said WHO estimates that 6% to 41% of patients are asymptomatic, with a point estimate a single value given as an estimate of a parameter of a population of 16%.

But the U.S. Centers for Disease Control and Prevention has put its estimates somewhat higher. Based on data available before May 1, the CDC estimates 35% of cases may be asymptomatic, which is a 10% jump from the 25% estimate CDC Director Robert Redfield floated in March.

Dr. Anthony Fauci, the countrys top infectious diseases expert and the director of the National Institute of Allergy and Infectious Diseases, has given estimates closer to the CDCs numbers. He told ABCs Good Morning America that the number may be between 25% and 45%.

Other studies have found estimates within the CDCs range: A review paper published in early June in the Annals of Internal Medicine found that between 40% and 45% of patients dont have symptoms but noted that the data used to conduct the review had limitations.

Health experts say its hard to know for sure how many people are asymptomatic because many arent likely to get tested if they arent feeling sick. Individuals who contract the virus but dont show symptoms are more likely to be younger people with no underlying health conditions, health experts have said. And if their only symptoms are mild, such as feeling more tired than usual, they probably wont attribute them to COVID-19.

WHO health experts said its more important to focus on symptomatic cases first to slow the spread because most known transmissions have happened when people show symptoms, and because many nations still dont have the widespread testing capabilities needed to identify asymptomatic cases.

We have to focus our testing on those who we need to test most: health workers, people in long-term care facilities, people who are clinically unwell with the disease, said Mike Ryan, executive director of WHOs Health Emergencies Programme. I think there is much to be answered on this, there is much that is unknown. But we know enough to support the strategies that have already been put in place and continue with those strategies.

Its possible a person who isnt showing symptoms isnt passing the virus on to others, health experts say.

Besides the question of how many infected patients are asymptomatic, researchers are also trying to figure out what proportion of asymptomatic patients go on to make others sick.

Whatever proportion of the disease is transmitting from asymptomatic individuals, that is unknown, Ryan said. And that is occurring, Im absolutely convinced that that is occurring. The question is how much?

In guidance posted on its website earlier this month about the use of face masks, WHO states that evidence indicates that asymptomatically infected individuals are much less likely to transmit the virus than those who develop symptoms.

But other estimates indicate its much more common the CDC estimates that about 40% of transmissions occur before people show symptoms.

One study published in Nature Medicine in mid-April found that 44% of patients are infected by people who didnt have symptoms. And The New York Times previously reported that researchers in Hong Kong found that between 20% and 40% of infections in China may have happened before symptoms appeared.

The Annals of Internal Medicine paper found that based on patients in Iceland, Indiana and Italy, the rate of asymptomatic transmission could be as high as 45%. But the authors of that paper said that the data used in the paper are imperfect in many ways and that an ideal study of asymptomatic SARS-CoV-2 infection has yet to be done.

We simply dont know how much asymptomatic transmission happens, Eric Topol, a professor of molecular medicine at Scripps Research and one author of the study, told The Washington Post. Sometimes its important to just say that.

Health experts say its important to note that some definitions of asymptomatic include people who are pre-symptomatic or who dont show symptoms initially but later get sick.

The CDCs estimate, which says 35% of people may be asymptomatic, includes individuals who are pre-symptomatic, and the CDC has said people may be able to transmit the disease up to 48 hours before they begin showing symptoms. Some studies also include pre-symptomatic cases when estimating asymptomatic rates.

Van Kerkhove previously told ProPublica that the WHO has found few truly asymptomatic cases, or people who never show any symptoms throughout the course of the illness.

But some health experts say differentiating between the two only creates more confusion.

Its debating semantics because in a practical sense, there is no difference between people who simply havent developed symptoms yet and are infecting others, and those who are truly asymptomatic, Monica Gandhi, an infectious disease expert at the University of California at San Francisco, told The Washington Post. It looks the same in the early stages.

Ultimately, health experts say the most important thing for the public to know isnt the specific numbers. People need to understand that asymptomatic transmission does happen and that its important to take precautions as if you and those around you may be infected.

You can be vertical and feel 100% or virtually 100% and going about your daily business and unaware that youre infected and perfectly capable of transmitting the virus, Dr. William Schaffner, a Vanderbilt University professor and CDC adviser, told CNN. How do we inhibit transmission of the virus by these people who are doing their full range of normal activities? The answer is social distancing and wearing masks and good hand hygiene and stay away from crowds. Thats the formula.

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