The World Health Organization (WHO) sent the public health community abuzz this week when a technical consultant implied that asymptomatic transmission of Covid-19 is "very rare"—a statement that public health experts warned is misleading and could have detrimental effects on how closely people follow public health guidelines. But the incident begs the question: What do we know about asymptomatic spread?

How many Covid-19 patients have no symptoms? More than you might think.

WHO walks back comments

During a press conference Monday, Maria Van Kerkhove, an infectious disease epidemiologist at Institut Pasteur's Center for Global Health and a technical consultant for WHO, said, "From the data we have, it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual." She added, "It's very rare."

A CNBC article highlighting the statement quickly went viral, Vox reports. On Twitter, many individuals—including Sen. Rand Paul (R-Ky.)—shared the article as evidence that public health measures enacted to stem the spread of the new coronavirus could be eased for the general population. For instance, Vox reports that Paul in his tweet wrote: "Good News! People who catch coronovirus [sic] but have no symptoms rarely spread the disease. Translation: sending kids back to school does not require millions of test kits."

However, on Tuesday Van Kerkhove walked back her comments, saying they were misunderstood. "I wasn't stating a policy of WHO or anything like that," Van Kerkhove said. "We do know that some people who are asymptomatic, or some people who do not have symptoms, can transmit the virus on. … I think that it's a misunderstanding to state asymptomatic transmission globally is very rare."

Several public health experts criticized the misstep. For instance, Ashish Jha, director of the Harvard Global Health Institute, said, "The people running WHO are superb scientists, and I have no questions about their scientific credibility or expertise, but they need to communicate more effectively."

But many are now wondering, what do we know about asymptomatic carriers' ability to spread the virus? The answer, experts say, is very little.

How prevalent are asymptomatic Covid-19 patients?

The term asymptomatic refers to people who have been infected by the new coronavirus but never develop symptoms of Covid-19, the disease caused by the new coronavirus. Because these patients never actually become sick, it's hard to identify asymptomatic patients. Currently, Vox reports, the only way to identify these patients is through PCR tests, which can identify the viral load in a person's system.

Eric Topol, director of the Scripps Research Translational Institute, recently published a review of 16 studies in the Annals of Internal Medicine that estimated between 40-45% of all new coronavirus infections might be asymptomatic.

However, Oyuka Byambasuren, a postdoctoral research fellow at the Institute for Evidence-Based Healthcare at Bond University, in a separate study conducted a systematic review and meta-analysis that estimated an average of 15% of Covid-19 cases are asymptomatic.

WHO, meanwhile, cited evidence that 6% to 41% of coronavirus cases may be asymptomatic when it updated its guidance recommending the general public to wear fabric masks.

A CDC spokesperson in an email said health experts are certain that "some people without symptoms may be able to spread the virus. That is why measures like cloth face coverings in public are so important. We are looking at new data from domestic and international studies to better understand these routes of transmission."

What we know about asymptomatic spread

The harder question, experts say, is whether asymptomatic patients can spread the virus to others, as PCR tests cannot determine if a patient is infectious.

"That's the biggest unknown," Topol said.

But Topol and other public health experts say research into presymptomatic patients, which describes patients who do eventually go on to develop Covid-19 symptoms, can spread the disease during the five- to 14-day incubation period. 

Jha said, "We know that people with no symptoms but who are infected with the virus can and do transmit it to others."

For instance, the Harvard Global Health Institute in an email to STAT News said, "All of the best evidence suggests that people without symptoms can and do readily spread SARS-CoV-2, the virus that causes Covid-19. … In fact, some evidence suggests that people may be most infectious in the days before they become symptomatic—that is, in the presymptomatic phase when they feel well, have no symptoms, but may be shedding substantial amounts of virus."

For instance, one study published in the journal Nature found that people with Covid-19 are very infectious about one to two days before showing symptoms appear. The study also estimated that about 44% of infections were transmitted by presymptomatic individuals.

Experts say more research is needed to determine whether asymptomatic carriers can transmit the virus

Mike Ryan, director of WHO's health emergencies program, said, "I'm absolutely convinced that that is occurring, the question is how much."

For instance, research to date shows the new coronavirus is largely spread through viral particles when an infected person coughs or sneezes. While an asymptomatic person is unlikely to have these symptoms, Ryan said transmission could still possible when people are in close proximity or when they are projecting their voice around others, such as when at the gym or singing in a choir.

However, the "truth is we simply don't know how frequent" asymptomatic transmission is, Topol said. "At this point, we simply don't know how much asymptomatic transmission happens. Sometimes, it’s important to just say that. … And it doesn't change the facts we do know, which is that this virus is very transmissible and is very hard to combat" (Hinshaw, Wall Street Journal, 6/9; Ehley, Politico, 6/10; Joseph, STAT News, 6/9; Wan/Berger, Washington Post, 6/9; Parshley, Vox, 6/10).