Coronavirus: What does viral load mean when it comes to COVID-19?

Studying a person's viral load could hold important clues.
/ Source: TODAY

“Viral load” is one of the more ominous-sounding medical terms emerging during the coronavirus outbreak.

It’s an important metric that virologists consider, but what exactly does it signify?

Viral load simply means how much virus is present in any sample taken from a patient, whether it’s blood or — in the case of COVID-19 — secretions collected during a deep nasal swab, said Marta Gaglia, an assistant professor of molecular biology and microbiology at the Tufts University School of Medicine in Boston.

“It’s a representation of how much virus is replicating in that tissue,” Gaglia told TODAY. “Think of a viral load about how much virus is on a person.”

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That’s different from how much virus someone needs to be exposed to in order to get sick, a concept known as the infectious dose.

“The viral load is a measure of how bright the fire is burning in an individual, whereas the infectious dose is the spark that gets that fire going,” Edward Parker, a research fellow at the London School of Hygiene and Tropical Medicine, told New Scientist.

When it comes to COVID-19, is a person with a high viral load more likely to infect other people?

It’s possible, but because the virus is so new, it’s not entirely clear, Gaglia said.

“In principle, you would think that they would shed more virus — there would be more virus coming out of that person and therefore it would be more likely for someone else to pick it up,” she said.

But different factors can influence transmission, so there's more to consider beyond the viral load.

How does viral load affect disease severity?

“If you get a big viral load — in other words, you’re sitting next to somebody for long periods of time and that virus just keeps hitting you — you’re going to get more sick,” said NBC News medical correspondent Dr. John Torres.

“It gets to the point where your body’s immune system can’t handle that overload of virus and that’s why were so concerned about people getting that huge viral load upfront.”

It’s one of the possibilities why some people have more severe disease than others: If you start off with a bigger dose, it might overwhelm your system more quickly, Gaglia added.

Why would some people produce and shed more virus than others?

If you assume a high viral load is connected with more severe disease, it could be a sign the person’s immune system isn’t fighting the virus very well, so the pathogen gets the chance to replicate better and in higher numbers, Gaglia said.

But again, a lot is still unknown for the new coronavirus.

Who is most exposed to people with high viral load?

Front-line health care workers who are constantly around people who are sick are most exposed. That’s why personal protective equipment like masks and face shields is so important.

People outside of a health care setting would be exposed to the highest viral load if they spent time in a room or another confined space with somebody who was sick, whether or not that person had symptoms, Gaglia said.

What does viral load mean for your household?

Is you have a loved one infected with the new coronavirus, it's important to take precautions to avoid getting sick yourself, including staying in a different room or at least sleeping in a separate bed from the patient.

But if both members of a couple become sick, they likely wouldn't keep re-infecting each other, Gaglia said. They probably would give each other the virus once, and then develop resistance for some time after recovering.

"It's not like you would get sick again, bouncing it back and forth," she noted.

Why is studying viral load important when it comes to COVID-19?

One of the great concerns with this disease is that people may be contagious before they show symptoms — when they’re out and about because don’t know they’re sick yet.

Studying their viral load could hold important answers.

“If an asymptomatic person has a low viral load, that’s better than if an asymptomatic person has the same viral load as somebody who is very sick,” Gaglia noted.

“Because in the second case, you would worry a lot more that they could transmit it to other people… the hope is we can figure out some of this stuff and then we don’t have to shut everything down to prevent this transmission.”

Another important aspect is predicting which patients with COVID-19 symptoms will have a mild case of the disease, and which will have the severe from, so doctors can target the care to people who are more likely to get worse, Gaglia said.

Viral load could provide important clues here, too.