You may actually be most contagious in the presymptomatic stage of the virus, before you start showing symptoms.

When COVID-19 first appeared in December 2019, it was a brand-new type of coronavirus (hence the initial name "novel coronavirus") that experts knew very little about. Now, six months later, we know more about the basics of the virus—that it's mainly spread through person-to-person contact via respiratory droplets, and older adults and those with underlying conditions are more at risk for severe disease—but we still don't know everything.

Case in point: A recent misstep by the World Health Organization regarding who can spread the virus. During a June 8 press briefing, Maria Van Kerkhove, PhD, the WHO's technical lead on the COVID-19 pandemic, said that asymptomatic spread of the coronavirus “appears to be rare.” That small statement caused an uproar, contradicting what many public health experts have been saying for months about asymptomatic spread.

Further clarifying, Van Kerkhove said that when health officials review cases that are initially reported as asymptomatic, "we find out that many have really mild disease." She revealed that there are some infected people who are “truly asymptomatic,” but countries that are carrying out in-depth contact tracing aren’t uncovering “secondary transmission onward” from those cases. “It’s very rare,” she added.

The WHO backtracked at a question-and-answer session the following day, STAT reported, and Van Kerkhove stressed that the actual rates of asymptomatic transmission aren’t yet known. "The majority of transmission that we know about is that people who have symptoms transmit the virus to other people through infectious droplets," she said. "But there are a subset of people who don’t develop symptoms, and to truly understand how many people don’t have symptoms, we don’t actually have that answer yet.”

Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases, told ABC’s Good Morning America on June 10 that the WHO’s initial comment “was not correct.” He said 25% to 45% of people who are infected with COVID-19 likely don't have symptoms, adding, “We know from epidemiological studies they can transmit to someone who is uninfected even when they're without symptoms. So to make a statement to say that's a rare event was not correct."

Part of the confusion here is that the term "asymptomatic" has been used widely by both the general public and public health experts to describe two different groups of COVID-19 patients—people who are infected and truly asymptomatic, and people who are infected but are what experts call "presymptomatic"—and there is a different between the two.

What's the difference between asymptomatic and presymptomatic?

“Asymptomatic is when a person does not have symptoms but is infected with a virus,” Jennifer Lighter, MD, an epidemiologist at NYU Langone Health, tells Health. “However, presymptomatic is the phase when an individual is infected and may be shedding virus but hasn’t yet developed symptoms.” That's especially important in terms of coronavirus because, as the CDC states, symptoms can show up in patients with COVID-19 two to 14 days after exposure. 

Essentially, the term asymptomatic is not associated with time, while presymptomatic is. “If a COVID-19 test comes back positive and the patient does not have symptoms, we don’t know if they will remain asymptomatic the whole time, or develop symptoms within a day or so,” Dr. Lighter says. 

Unfortunately, there's not much research that differentiates asymptomatic COVID-19 cases versus presymptomatic cases. The Centers for Disease Control and Prevention (CDC) estimates that 35% of all people with COVID-19 are asymptomatic, but says that those people are just as infectious as those with symptoms. The CDC also estimates that 40% of transmission happens before people feel sick. And one study, published in the journal Science, concluded that about 4 in 5 people with confirmed coronavirus in China were likely infected by people who didn't know they had it. 

How can you tell if someone is asymptomatic or presymptomatic?

The bad news: You can't. Both types of carrier look and feel “normal” until—in the case of presymptomatic carriers—symptoms develop. But with testing—and specifically more widespread testing currently—doctors are finding people with a positive test who don't have any symptoms at the time of testing, indicating they're either presymptomatic or asymptomatic. 

The polymerase chain reaction (PCR) test for COVID-19, which detects the genetic information of the virus (RNA), is very sensitive, epidemiologist Supriya Narasimhan, MD, chief of the infectious disease department at Santa Clara Valley Medical Center, tells Health. That means it has the ability to detect the virus even in asymptomatic or presymptomatic people. “It starts to pick up virus up to a week before symptoms develop, through the period of symptoms, and continues to detect particles of the RNA virus for up to and beyond six weeks after the patient has recovered,” Dr. Narasimhan explains.

Basically, in someone not showing symptoms but who has been exposed to COVID-19, a PCR test can show whether they have the virus. After a positive test, if that person goes on to develop symptoms, they were presymptomatic at the time of the test; if they never develop symptoms, they're asymptomatic.

Are presymptomatic or asymptomatic people more contagious?

People in the presymptomatic stage are highly contagious. “The peak of viral shedding occurs right before symptoms develop and immediately after, when the symptoms are still mild,” Dr. Narasimhan says. Sanjay Gupta, MD, CNN's Chief Medical Correspondent, also previously said that those who are presymptomatic are highly contagious. "People tend to be the most contagious before they develop symptoms, if they're going to develop symptoms," he said in a recent CNN article. But when it comes to asymptomatic patients, how much virus they shed and how contagious they are is still a matter of debate, Dr. Narasimhan says.

From a health care perspective, however, it’s important to distinguish between asymptomatic and presymptomatic patients wherever possible, particularly when it comes to planning elective surgeries for patients who test positive for COVID-19 without symptoms. “There is data to suggest that if we intubate someone for a surgery in their presymptomatic stage, we may end up doing them harm—they have worse surgical and respiratory outcomes,” Dr. Narasimhan says. That's because, if the patient develops COVID-19 after surgery, they'll have the illness to fight as they recover. “There is also a greater risk of transmission to health care workers because the viral load in the patient is the highest. It is also very important in investigating outbreaks in congregate living facilities, where affording every [resident] a private space may not be feasible,” Dr. Narasimhan says. 

Clearly, there’s still a lot to learn about how the new coronavirus spreads, including the risk posed by both asymptomatic and presymptomatic carriers. In the meantime, wearing face masks and keeping a physical distance from others can help reduce the risk of infection—whether you or someone else is symptomatic, asymptomatic, or presymptomatic.