(CNN)Last week, an official at the World Health Organization said asymptomatic transmission of the coronavirus appears to be rare. They've since clarified their remarks. CNN Chief Medial Correspondent Dr. Sanjay Gupta sets the record straight on who can transmit the virus.
Asymptomatic Infection: Clearing up Confusion. Dr. Sanjay Gupta's coronavirus podcast for June 16
You can listen on your favorite podcast app or read the transcript below.
CBC broadcast: The virus is very difficult to contain. Many experts studying the pandemic have said one reason for that is because of silent spreaders. That's why it came as a bit of shock to hear this yesterday from the World Health Organization.
Maria Van Kerkhove, infectious disease epidemiologist and technical lead for Covid-19, World Health Organization: It still appears to be rare that an asymptomatic individual actually transmits onward.
NBC broadcast: A bombshell assertion suggesting transmission from patients who do not show symptoms is actually not very common.
Dr. Sanjay Gupta: Recently there's been some confusion about how the coronavirus actually spreads.
Last week an official at the World Health Organization said asymptomatic transmission of the virus appears to be rare.
Now this seemed to contradict what many experts — myself included — have been saying for months. That is, even if you don't feel sick, you could still be spreading the virus.
Now, a day later, the World Health Organization did clarify their position.
But a lot of people are still left wondering. ... What is the truth here? Can you spread coronavirus to others, even if you don't have symptoms?
So today I'm going to clear up what we know — what the evidence is — about asymptomatic transmission.
I'm Dr. Sanjay Gupta, CNN's chief medical correspondent. And this is "Coronavirus: Fact vs. Fiction."
Right off the bat, I want to be clear.
Evidence suggests that yes, people can spread the coronavirus even if they don't appear to have any symptoms.
Here's Dr. Anthony Fauci last week on "Good Morning America."
Dr. Anthony Fauci, director, National Institute of Allergy and Infectious Diseases: The evidence that we have, given the percentage of people, which is about 25, 45% of the totality of infected people, likely are without symptoms. And we know from epidemiological studies that they can transmit to someone who is uninfected even when they're without symptoms.
Gupta: So what is all this confusion about then?
Some of it may boil down to how you define "asymptomatic." In fact, there are three terms that sometimes get grouped together under the asymptomatic umbrella. So let's go through them one by one quickly.
First, asymptomatic infection.
Patients with true asymptomatic infection have the virus, but don't have symptoms and never get symptoms.
Because of this, they may not seek out or qualify for testing for Covid-19. That makes it difficult then to know precisely how much asymptomatic transmission is actually happening.
Here's Dr. Mike Ryan, executive director of the WHO Health Emergencies Programme:
Dr. Mike Ryan, executive director, WHO Health Emergencies Programme: Whatever proportion of the disease is transmitting from asymptomatic individuals, that is unknown. And that is occurring. I'm absolutely convinced that that is occurring. The question is how much?
Gupta: So, we do believe asymptomatic spread is happening. We just don't know the extent.
But now let's look at the second term — pre-symptomatic infection.
That means you do have the infection, you don't feel sick now, but you do get symptoms later.
Now there has been a lot of study on pre-symptomatic transmission, and it does seem to play a big role in how coronavirus spreads.
A study published in the science journal Nature looked at 94 coronavirus patients. And they estimated 44% of those patients were exposed to the virus by someone who was pre-symptomatic.
You may be wondering — how can so many people have coronavirus with no symptoms yet?
Well remember, this virus does have a lengthy incubation period — that's the time period between when someone gets exposed to the virus to when they start showing symptoms. According to Harvard Medical School, the incubation period for Covid-19 is anywhere from three to 14 days, with symptoms typically appearing "within four or five days after exposure."
Now here is an important point.
Studies have also shown that people may be most contagious just before they start to show symptoms.
Let me repeat that: People may be most contagious right before they start to show symptoms.
During this period, people tend to have more virus in their nose and their mouth. And they can be more likely shedding that virus into the environment ... inadvertently infecting others.