The ‘Asymptomatic Carriers’ of COVID-19 Controversy, Explained

On June 9 2020, what was framed as a World Health Organization (WHO) finding about asymptomatic spread of COVID-19 was shared seemingly everywhere — but within 24 hours, the organization had stepped in to issue an important clarification.

A WHO Official’s June 8 2020 Statement on Asymptomatic Spread of COVID-19

During a June 9 2020 briefing in Geneva, Dr. Maria Van Kerkhove described the asymptomatic or presymptomatic spread of COVID-19 as “very rare”:

“From the data we have, it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual,” Dr. Maria Van Kerkhove, head of WHO’s emerging diseases and zoonosis unit, said at a news briefing from the United Nations agency’s Geneva headquarters. “It’s very rare.”

Focus on the “very rare” portion of Van Kerkhove’s remarks was predictably common. Less common were subsequent excerpts from articles like the one linked above, where the remarks seemed a bit less definitive:

More research and data are needed to “truly answer” the question of whether the coronavirus can spread widely through asymptomatic carriers, Van Kerkhove added.

In fact, that article — published by CNBC, a financial news outlet — included the following correction:

Correction: An earlier headline should have said most asymptomatic coronavirus patients aren’t spreading new infections. The word “most” was inadvertently omitted.

In the version of the article we accessed, “most” was not present in the headline.

Public Health Reactions to the Viral Claim

While financial news outlets were reporting and correcting the soundbite, public health officials reacted on forums like Twitter.

Andy Slavitt, former Acting Administrator of the Centers for Medicare and Medicaid Services, commented on the evening of June 8 2020 after speaking with “four reputable scientists” about the circulating quotes:

Slavitt noted that the purported basis of the claim appeared to be self-reported data from individuals who had contracted COVID-19:

Four studies cited by Slavitt contradicted the claim that asymptomatic spread was rare — but infected individuals were more likely to tell contact tracers they had been sickened by symptomatic people. Nearing the end of the thread, Slavitt said:

Slavitt also opined that “if your data is thin or not public or contradicted by lots of studies, making definitive statements is a mistake.”  We noted that Slavitt also referenced the CNBC article; CNBC’s primary function is financial news, not science and medicine. covered the story on June 9 2020, reporting:

To some, it came across as if the WHO was suggesting that people without symptoms weren’t driving spread. Some studies, however, have estimated that people without symptoms (whether truly asymptomatic or presymptomatic) could be responsible for up to half of the spread, which is why the virus has been so difficult to contain. Isolating people who are sick, for example, does not prevent the possibility they already passed the virus on to others. Some modeling studies have assumed quite widespread asymptomatic transmission.

“The WHO created confusion yesterday when it reported that asymptomatic patients rarely spread the disease,” an email from the Harvard Global Health Institute said [on June 9 2020]. “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.”

June 2020 Research Contradicts the Claim

On June 3 2020, research published in the Annals of Internal Medicine presented a very different picture of asymptomatic spread and SARS-CoV-2. Its abstract read in part:

[Study] authors sought to review and synthesize the available evidence on asymptomatic SARS-CoV-2 infection. Asymptomatic persons seem to account for approximately 40% to 45% of SARS-CoV-2 infections, and they can transmit the virus to others for an extended period, perhaps longer than 14 days. Asymptomatic infection may be associated with subclinical lung abnormalities, as detected by computed tomography. Because of the high risk for silent spread by asymptomatic persons, it is imperative that testing programs include those without symptoms. To supplement conventional diagnostic testing, which is constrained by capacity, cost, and its one-off nature, innovative tactics for public health surveillance, such as crowdsourcing digital wearable data and monitoring sewage sludge, might be helpful.

Immediately thereafter, a section labeled “Key Summary Points” reiterated a figure of 40 to 45 percent of infected people being asymptomatic, a possible longer period of transmissibility, and the presence of “subclinical” symptoms providing possible identification of asymptomatic SARS-CoV-2 spreaders:

The likelihood that approximately 40% to 45% of those infected with SARS-CoV-2 will remain asymptomatic suggests that the virus might have greater potential than previously estimated to spread silently and deeply through human populations.

Asymptomatic persons can transmit SARS-CoV-2 to others for an extended period, perhaps longer than 14 days.

The absence of COVID-19 symptoms in persons infected with SARS-CoV-2 might not necessarily imply an absence of harm. More research is needed to determine the significance of subclinical lung changes visible on computed tomography scans.

Walking Back the Comments

CNBC followed up their viral claim with a second article, reporting that “WHO scrambled to clarify its comments that transmission of the coronavirus by people who never developed symptoms is ‘very rare'”:

Dr. Maria Van Kerkhove, head of the WHO’s emerging diseases and zoonosis unit, said [on June 9 2020] that asymptomatic spread is a “really complex question” and much is still unknown. “We don’t actually have that answer yet,” she said.

“I was responding to a question at the press conference. I wasn’t stating a policy of WHO or anything like that. I was just trying to articulate what we know,” she said on a live Q&A streamed across multiple social media platforms. “And in that, I used the phrase ‘very rare,’ and I think that that’s misunderstanding to state that asymptomatic transmission globally is very rare. I was referring to a small subset of studies.”

That same, second article seemed to describe Van Kerkhove, a single person, as “WHO officials”:

On [June 8 2020], WHO officials said asymptomatic people aren’t driving the spread of the virus, casting doubt on concerns by some researchers that the disease could be difficult to contain due to asymptomatic infections.

“From the data we have, it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual,” Kerkhove said at a news briefing [June 8 2020] from the WHO’s Geneva headquarters. “It’s very rare.”

Asymptomatic Spread vs. Pre-Symptomatic Spread

It was almost immediately clear that Van Kerkhove’s statement was inadvertently or willfully misinterpreted, and perhaps clumsily worded.

Before the clarification on June 9 2020, scientists and public health experts tried to undo the potential damage caused by the claim:

Carl Bergstrom, a biologist at the University of Washington, wrote on Twitter that the WHO’s conclusions were based on “thin evidence,” at least when taking into account what has been published publicly.

Bergstrom also said the organization should have more clearly distinguished between people who are “truly” asymptomatic—those who never show symptoms—and those who may unwittingly spread the disease in the days before they become symptomatic. Topol’s study on asymptomatic transmission found that few people who test positive without symptoms go on to develop them, but studies suggest it takes an average of five days after exposure to the virus for symptoms to surface. People in this phase would be considered pre-symptomatic, not asymptomatic, but it’s difficult to tell the difference.

“Even if truly asymptomatic spread is very rare, pre-symptomatic transmission is likely to be important,” Bergstrom wrote on Twitter. “We still need to wear masks and distance to avoid spreading the virus during this period, probably concentrated in days 3-6 after infection.”

In a guide for journalists trying to clean up the mess caused by overeager, agenda-driven reporting of Van Kerkhove’s remarks, Poynter addressed the nuances of asymptomatic and pre-symptomatic spreaders of SARS-CoV-2:

A couple of weeks [prior], the New England Journal of Medicine said it was “clear” that people without obvious COVID-19 symptoms could transmit the virus. The Center for Disease Control and Prevention’s guidelines are based on the very presumption that asymptomatic carriers could be spreading the virus, and that we should socially distance to be safe.

The WHO’s statement sparked a quick response from Ashish Jha, director of the Harvard Global Health Institute, who said the WHO may be referring to “pre-symptomatic” and not “asymptomatic” people. He said there is every [indication] that infected people who aren’t showing symptoms still spread the virus.

Poynter embedded a tweet in which Jha described asymptomatic spread as the “Achilles heel” of the outbreak:

The outlet summarized the controversy and its drivers:

Without a doubt, this will light new fires among people who believe the entire COVID-19 pandemic has been overblown and that there was no need for stay-at-home orders that keep seemingly healthy people at home. Judging by internet traffic today, it will take a lot of effort to clear this one up.


During a conference on June 9 2020, Van Kerkhove’s statement about asymptomatic spread of COVID-19 being “very rare” was predictably seized upon by a financial news outlet (CNBC) and later others. Scientists and experts in public health voiced immediate concern that the claims would endanger the already uphill battle of encouraging the public to take precautions. Van Kerkhove and other WHO officials latter issued murky clarifications, none of which spread nearly as far as the initial misleading and dangerous claim that asymptomatic and pre-symptomatic carriers of SARS-CoV-2 did not exist.