It has been widely believed for months that many people infected with the new coronavirus experience little or no symptoms, but may still be able to spread the virus to others. How many of these people without symptoms are “silent spreaders” was a mystery.
Despite the mystery, the public health message was clear: we should act as if we or someone else could be a silent spreader. We must take precautions such as wearing face masks, avoiding crowds and keeping physical distance.
But that conventional wisdom seemed to be called into question this week when Maria Van Kerkhove, an infectious disease epidemiologist at Institut Pasteur’s Center for Global Health and a technical advisor to the World Health Organization (WHO), noted at a news conference Monday that asymptomatic transmission of Covid -19 was ‘very rare’.
After a CNBC story the comment went viral, some people began to rejoice that this was great news, a reason not to worry too much. Some conservatives indicated that it proves that public health measures don’t have to be that strict.
Good news! People who catch coronovirus but have no symptoms rarely spread the disease. Translation: Sending kids back to school doesn’t need millions of test kits.
Asymptomatic spread of coronavirus is “very rare,” says the WHO https://t.co/reIyrFb0Au
– Senator Rand Paul (@RandPaul) June 8, 2020
Many public health experts, meanwhile, were annoyed and called Van Kerkhove’s comment (and the CNBC story about it) a misleading piece of scientific communication.
It seems to indicate that people without symptoms do not spread COVID19.
Does this mean that shoppers, students, protesters, etc. don’t need masks / distance?
– Carl T. Bergstrom (@CT_Bergstrom) June 8, 2020
Tuesday was Van Kerkhove for the press again, I went back a few comments and tried to explain what she meant. “I wasn’t talking about a WHO policy or something like that,” she said. “We know that some people who are asymptomatic, or some people who have no symptoms, can transmit the virus.”
Van Kerkhove said she had referred to a handful of preprints – studies not yet peer-reviewed – adding, “I think it’s a misunderstanding to explain asymptomatic transmission worldwide, it’s very rare.”
However, some public health experts were dissatisfied. Andy Slavitt, former Acting Administrator of the Centers for Medicare and Medicaid Services, tweeted, “This is such a mistake that I’m not sure how or if WHO statements can be treated now.”
What Slavitt and others fear is that the WHO statements will further politicize the debate about mask use, putting more people at risk of infection. “It’s a very serious mistake to interpret that you don’t need masks in public,” said Eric Topol, research method expert and director of the Scripps Research Translational Institute.
Here’s everything you need to know about the difference between asymptomatic and presymptomatic infection – and what it means for how to protect yourself.
What does asymptomatic mean and how does it differ from pre-symptomatic?
Asymptomatic means people who have been exposed and infected but never develop symptoms. Presymptomatic describes people who have been exposed to a disease and infected, but do not yet show any symptoms.
We know more about pre-symptomatic patients, in part because they get sick, so they are easier to identify. Covid-19 symptoms develop around on average five or six days after you are infected, although this may take up to 14 days. Some patients also only develop very soft symptoms.
Scientists are only just beginning to understand asymptomatic Covid-19 infections. Topol recently published a review of 16 different studies and the results, published in Annals of internal medicine, found that as many as 40-45 percent of all infections can be asymptomatic. Topol says to separate presymptomatic and asymptomatic cases, they have taken into account estimates from the five studies that conducted multiple PCR tests over time.
There were two notable conclusions, Topol explains, one of which was that you can become infected with, and even have lung injuries, from Covid-19 without knowing it. Two of the studies he reviewed, one in South Korea and one on the Diamond Princess cruise ship, looked at CT scans of lungs and found substantial abnormalities. “That tells you that something is going on, even among people who aren’t aware of it,” he says. “It suggests the possibility that you are asymptomatic and have chronic damage or injury.” The other was three separate studies shown the viral load of asymptomatic patients was very similar to that of patients who developed symptoms.
Another study, led by Oyuka Byambasuren, a postdoctoral researcher at Bond University’s Institute for Evidence-Based Healthcare, used a systematic review and meta-analysis, screening 998 primary studies and preprints to find nine studies from six countries that followed a first positive test to identify who later developed symptoms. Based on these data, Byambasuren estimated that an average of 15 percent of Covid-19 cases were truly asymptomatic.
A third study, by Swiss researchers, also found in a systematic review an asymptomatic mean of 15 percent, although they assessed primary studies other than Byambasuren. (Her team excluded small family clusters that included the Swiss study.) But, the Swiss authors wrote, from their data the share of “pre-symptomatic [patients] could not be summarized. ‘
According to the recent WHO update guidelines for masks, “The proportion of asymptomatic cases ranged from 6% to 41%, with a pooled estimate of 16 percent,” Van Kerkhove supported at a news conference on June 9.
These data were one reason why the WHO eventually decided to recommend that the general public wear cloth masks if they cannot keep social distance or fall into a higher risk category – because people who currently have no symptoms can spread the virus.
Can asymptomatic people spread the coronavirus?
Several peer-reviewed studies have shown that presymptomatic people often infect others before they feel sick – perhaps even most infectious before you have any idea you have Covid-19.
Since asymptomatic patients never feel sick, they can only be identified by PCR tests, which can determine the viral load. But these types of tests can only tell if viral particles are present – it doesn’t necessarily mean that these asymptomatic patients are contagious. “That is the greatest unknown,” says Topol, although he notes that evidence supports the idea of asymptomatic spread, especially the assessment of transfer on the USS Theodore Roosevelt.
Much more research will be needed to find out. But Ashish Jha, director of the Harvard Global Health Institute, says, “We know that people without symptoms who are infected with the virus can and cannot transmit it to others.” He points to several studies that suggest 40 percent to 60 percent of the transmission of Covid-19 is in people who have no symptoms, regardless of whether they develop it later or not. “That’s a shame about yesterday’s WHO statement.”
“The people who run the WHO are great scientists, and I don’t have questions about their scientific credibility or expertise, but they need to communicate more effectively,” he says. “They are doing better than our federal government, but that is a low bar.”
Meanwhile, it is difficult to even distinguish whether a person is pre-symptomatic or asymptomatic until the disease has progressed completely. Like Esther Choo, an associate professor of emergency medicine at Oregon Health & Science University, tweeted, “Plague presymptomatic COVID-19 (which develops symptoms) versus asymptomatic COVID-19 (which develops no symptoms) is operationally relevant for people with properly functioning crystal balls.”
Tara Smith, a professor of epidemiology at Kent State University, explains, “This is why masks and distances are still so important: both groups may be able to spread the virus to others while still feeling fine.” She adds, “Many asymptomatic individuals may never be identified because we still lack robust tests.” Jha says that this is why not having an effective testing and tracking strategy to identify those who spread the virus without symptoms remains “a major Achilles heel of this pandemic.” And without extensive testing and contact tracking, it will be difficult to know how many people are truly asymptomatic.
So in many ways, this whole conversation diverts from the main actions we know we need to take now. Since you may already be sick but don’t know it yet, it’s important not to expose others by taking social distance, wearing masks, and washing your hands. If you may be sick and infected others without it ever knowing it is just as important to take the same steps.
Smith recommends wearing masks when you are in close contact with others for an extended period of time, especially in indoor areas. For example, all 140 customers who agreed to test after being exposed by two hair stylists with Covid-19 in Missouri tested negative at the end of their incubation period. Both clients and hair stylists wore masks. But Smith adds, “Masks alone offer limited protection – combine them with as much physical distance as possible.”
Lois Parshley is a freelance investigative journalist and the Snedden Chair of Journalism 2019-2020 at the University of Alaska Fairbanks. Follow her Covid-19 report on Twitter @loisparshley.