Coronavirus: How asymptomatic carriers spread a disease like COVID-19

The COVID-19 outbreak has reached pandemic level, and with more than 179,000 cases confirmed globally as of March 17.

But the number of people confirmed to have the coronavirus does not necessarily reflect the number of people who have actually been infected — or who are carrying the SARS-CoV-2 virus without symptoms.

Understanding the difference between these groups — the obviously sick and the asymptomatic — explains why people like Tom Hanks feel the virus' effects, but people like Idris Elba feel ok.

Importantly, Elba is among the few people to test positive for COVID-19 despite not manifesting any symptoms, but there are many more out there who will not be tested for that very same reason. These asymptomatic carriers are critical to our understanding of coronavirus, because they may be the main drivers behind the new virus' spread, according to a study published Monday, March 16, in the journal Science.

So why do some people manifest symptoms, while others don’t? And what does this mean for the current pandemic?

Who is an asymptomatic carrier?

An asymptomatic carrier is a person who has contracted a virus, but isn’t manifesting any of the symptoms it causes.

Historically, asymptomatic carriers are sometimes referred to as a “Typhoid Mary”: Mary Mallon was a cook in the 1880s who infected all of her customers with salmonella. But she herself was healthy, and was in denial about having the disease.

The reason why is to do with how the body's immune system works. In some cases, the symptoms people experience are a result of the virus, while others are caused by the body's immune response to the virus as it tries to fight it, and not the virus itself.

In case of COVID-19, the SARS-CoV-2 virus infects cells involved in oxygen circulation, also known as alveolar cells, explains Catherine Beauchemin, a professor of virophysics at Ryerson University. The receptors on the surface of the cell make it an ideal host for the virus, which means it can affect your ability to take in oxygen. That's why one of the most-common symptoms of the current coronavirus is shortness of breath, and why doctors test oxygen levels in patients’ blood to confirm infection.

“If you failed to notice you've been invaded...you're going to have to bring some very heavier artillery if you're going to push all of them back out.”

But the cough and fever associated with COVID-19 are signs your body’s immune system is trying to fight off coronavirus.

“So once the infection is detected, your immune response shows up, like a whole bunch of soldiers show up and there's a combination of inflammatory cells and cell signaling,” Beauchemin tells Inverse. “And all of this is your immune response kicking in.”

But if you are an asymptomatic carrier, then the virus has infected your cells, but:

• The immune system is taking too long to react (and symptoms will show up later).
• The immune system kicked in early enough in the infection's course to not manifest symptoms.
• The infection is very mild and it's not enough to engage a strong immune response.

Imagine the virus is an invading hoard, Beauchemin says. If the invaders show up one at a time, and "you show up early in the invasion, when there's just one or two people there, you can push them out,” she says. As a result, you may remain asymptomatic.

“But, if you failed to notice you've been invaded and then when you show up, there's like 2000 of them, you're going to have to bring some very heavier artillery if you're going to push all of them back out.” And thus, you will likely be symptomatic.

Silent spreaders might make the most noise

But just because you are asymptomatic, that does not mean that the virus isn’t still replicating itself in your body. As a result, you may still "shed" the virus — that means give it to other people.

“The way you can shed the virus, you have to have enough virus to shed. If you're exhaling some of it and able to infect other people, then that's because you've got your own factory making more inside you,” Beauchemin says.

There’s been a lot of debate about whether people without symptoms spread the virus, especially from officials. But we know that coronavirus is spreading at the community level. This is when some people have been infected and are not sure how or where they became infected.

"Asymptomatic and mildly symptomatic transmission are a major factor in transmission for COVID-19."

The data so far suggests asymptomatic carriers make the difference in community spread. In some areas of China, according to simulations of the virus spread published in the journal Science, almost 9 out of 10 infections were caused by asymptomatic carriers. In another, preliminary study posted on March 6 to preprint server MedRxiv, researchers found that 48 percent of 91 infections in a Singaporean COVID-19 cluster were caused by asymptomatic carriers.

By comparing the Singaporean outbreak to another in Tianjin, China, the researchers estimate that the virus was passed from person to person between 2.55 days and 2.89 days before symptoms showed up.

This process of infection spread is known as stealth transmission, and, according to Jeffrey Shaman, the researcher who led the new Science study, this is a major driver of the current pandemic.

In a note in the New England Journal of Medicine, Bill Gates echoed the same sentiments. And other public-health experts agree with Shaman's assessment.

"Asymptomatic and mildly symptomatic transmission are a major factor in transmission for COVID-19," Dr. William Schaffner, a professor at Vanderbilt University School of Medicine, told CNN.

This is likely "because they are not isolated, or not quarantined, and they can spread the infection to other people in close proximity to them," Sam Basta, a professor of biochemistry at Queen’s University, tells Inverse.

"Whereas, a sick person will stay in bed isolated till the infection is cleared."

An emerging picture

It is important to note that right now, scientists don't know much about COVID-19 or the virus that causes it, SARS-CoV-2. To fully understand why some people show symptoms while others don't, and whether these asymptomatic people are the main driver behind the disease's spread, scientists need more data about the infection and its spread across the world.

In fact, there is very little understanding of how viruses spread and how they travel in the first place.

“We don't fully understand, even for influenza which we get every year, exactly what it is that transmits people to people,” Beauchemin says.

In the case of influenza, scientists can measure how much virus load people with the virus had in their nose; but knowing how much of a virus someone carries doesn't necessarily correlate with how much they might transmit to others.

“Does exhaling cause more of a threat than sneezing? It's not clear. We don't know,” Beauchemin says.

In the case of SARS-CoV-2, she stresses that there is too little information about the virus to understand how it spreads yet. Rather, the best work researchers can do right now is to try and track the spread, and stop it, she says.

If you are worried, Beauchemin personally advises you constantly monitor your temperature with a thermometer, every couple of hours, in order to detect any immune response to a possible infection.

You can also access the United States' Centers for Disease Control and Prevention guidelines on how to safeguard yourself against the virus, and what to do if you think you have it.