WHO Says Asymptomatic Spread of COVID-19 Is “Rare,” but Doctors Have Concerns
On Monday, World Health Organization (WHO) epidemiologist Maria Van Kerkhove, Ph.D., said that asymptomatic transmission of COVID-19 is “very rare.”
However, public health officials, healthcare workers, and scientists say the message is confusing to the general public.
Doctors explain how asymptomatic and pre-symptomatic transmission differ, and why it makes a difference in how the WHO’s statement is interpreted.
On Monday, the World Health Organization (WHO) made a puzzling statement: Transmission of the novel coronavirus by asymptomatic people—those who never developed symptoms of COVID-19—is “very rare,” Maria Van Kerkhove, Ph.D., the WHO’s technical lead for coronavirus response and head of the emerging diseases and zoonoses unit, said during a briefing, seemingly walking back on prior messaging.
“From the data we have, it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual,” Van Kerkhove explained. “We have a number of reports from countries who are doing very detailed contact tracing. They’re following asymptomatic cases, they’re following contacts, and they’re not finding secondary transmission onward. It is very rare—and much of that is not published in the literature.”
Van Kerkhove also said that the WHO is “constantly looking at this data and “trying to get more information from countries to truly answer this question.” The statement drew swift criticism from the medical community and health journalists. Some called it “irresponsible” and believe that incorrect conclusions have been spread. Others say this is encouraging and that the “science is still evolving.”
Given that many countries went into lockdown for months over concerns about the spread of COVID-19 from infected people who don’t have symptoms, people are flooding Twitter with questions: Where is the specific evidence? What does “rare” actually mean? How does this change how we move forward? We asked doctors for their thoughts.
First: It’s important to understand what symptomatic, pre-symptomatic, and asymptomatic actually mean.
Symptomatic transmission is the standard: Someone is infected with the novel coronavirus and passes it on to another person after they’ve developed symptoms of COVID-19, like a fever, cough, sore throat, or shortness of breath.
Asymptomatic transmission means the novel coronavirus can be spread from an infected person who truly never has any symptoms throughout the entire period of their illness, the WHO explains.
But in the case of pre-symptomatic transmission, a person is infected with the virus and may have no symptoms early on during the illness, but still has the ability to transmit it before eventually developing symptoms, even if they’re just mild.
Van Kerkhove pointed out during the media briefing that there’s a difference between being asymptomatic and having mild symptoms. “When we actually go back and we say how many of them were truly asymptomatic, we find out that many have really mild disease,” she said. “They’re not ‘COVID symptoms,’ meaning they may not have developed fever yet, they may not have had a significant cough, or they may not have shortness of breath, but some may have mild disease. Having said that, we do know that there can be people who are truly asymptomatic.”
She later clarified her comments on social media, per CNBC. “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. “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.”
In fact, research has found that people who are infected with COVID-19 can spread the virus before developing symptoms.
A study published in the journal Nature Medicine in April found that infected people could infect others two to three days before they developed symptoms—but that simply means they’re pre-symptomatic, not asymptomatic. The Centers for Disease Control and Prevention (CDC) also clearly says in its planning scenarios that up to 40% of COVID-19 transmission happens before people have recognizable symptoms.
Robert Redfield, M.D., director of the CDC also told NPR in late March that up to 25% of people infected with COVID-19 are asymptomatic. “That’s important, because now you have individuals that may not have any symptoms that can contribute to transmission, and we have learned that in fact they do contribute to transmission,” he said at the time.
If you were confused by the contradictions, you’re not the only one.
“The WHO has now wonderfully confused the public health messaging,” says William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine. “Even our infectious disease sophisticates were all scratching their heads yesterday about this news, which came right on the heels of the WHO recommending much wider use of masks than they have previously. The reason you wear masks is because you can have essentially no symptoms or very few and still spread the virus. The WHO appears to be stepping on its message.”
However, what Van Kerkhove said wasn’t incorrect or even necessarily new information—it’s just confusing to the general public, says Amesh A. Adalja, M.D., senior scholar at the Johns Hopkins Center for Health Security.
Again, asymptomatic patients never develop symptoms and it can be difficult to figure out if a patient was truly asymptomatic or just pre-symptomatic with a milder case of COVID-19, Dr. Adalja says. “It’s very hard to distinguish when you’re interviewing patients and they’re trying to recall when they had symptoms,” he explains. “Many people don’t realize that the symptoms aren’t just fever, cough, and sore throat, and they may not volunteer other symptoms like muscle aches, pains, or GI symptoms.”
In this particular briefing, the WHO chose to focus on people who are truly asymptomatic, but didn’t address people who are pre-symptomatic or who have mild symptoms. “They were slicing the salami very thin here,” Dr. Schaffner says. “This is the kind of thing you debate at annual epidemiological meetings. This just confuses the public health message. They really were not thoughtful with these comments.”
Should this change anything you’re currently doing to prevent the spread of COVID-19?
Dr. Schaffner explains that the “usual recommendations” should still be practiced because pre-symptomatic spread of COVID-19—or spread from people who have very mild symptoms—is concerning. Those people can still be contagious and feel otherwise healthy until the further stages of their illness.
Until the WHO offers direct recommendations that differ from current public health precautions, you still should continue to do your best to practice social distancing, wash your hands frequently, apply hand sanitizer when needed, avoid touching your face with your hands, and wear a face mask when you’re in public and can’t maintain a six-foot space from others.
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