Why It Matters How Much Virus Is in the Body

New data suggests that viral load might be an important factor for transmission and disease severity

Photo: Waldemar Brandt/Unsplash

Once a virus infects a body, it starts replicating. The amount of virus a person has inside of them — and the amount of virus that person is producing — is what’s referred to as viral load. For many viruses, the higher the viral load, the worse the disease. Scientists are still learning the implications of a high viral load of SARS-CoV-2, the virus that causes Covid-19, and new research is beginning to provide some insights.

Understanding viral load is important for a couple reasons, experts say. “The first is that presumably if your viral load in the upper respiratory tract is high, the combination of that and the amount and types of respiratory droplets that you are expelling is what we think contributes to the likelihood of transmission of Covid-19 from one person to another,” says Abraar Karan, MD, MPH, an internal medicine doctor at Brigham and Women’s Hospital and Harvard Medical School. “If you have a low viral load, presumably even if you are expelling many droplets, each droplet has a lower concentration of viral particles and is less likely to cause a clinical infection in someone else. We are still understanding what this threshold is, but the general concept holds.”

A recent study published in the journal JAMA Internal Medicine looked at over 300 people with Covid-19 in a community treatment center in South Korea and found that the people who were asymptomatic had viral loads (the amount of virus in their nose, throat, and lungs) that were similar to people with Covid-19 who had symptoms. This could mean that people with symptoms and people without symptoms may spread the virus similarly since they carry it at fairly equal levels. However, people without symptoms are less likely to cough and sneeze, and therefore it’s possible they don’t spread the virus as effectively even if they have high levels of SARS-CoV-2. More research is needed.

There’s also the question of whether a higher viral load of SARS-CoV-2 means a person could experience a more severe infection than someone with a lower amount of virus. “The more virus that has spread to different organs in different parts of the body, the more clinical manifestations that we see, such as blood clots, kidney failure, cardiac inflammation, cognitive issues, and more,” says Karan.

If that is indeed the case — and again, more research is needed — there’s been some suggestion that measures like masks would then potentially allow for less severe illness by reducing viral dose. In a July perspective published in the Journal of General Internal Medicine, three researchers suggest that by reducing the amount of virus-carrying droplets inhaled, mask wearing may lower the risk of developing a severe form of Covid-19.

“Masks can prevent many infections altogether, as was seen in health care workers when we moved to universal masking,” author Monica Gandhi, MD, an infectious disease specialist at the University of California, San Francisco, said in a statement about the perspective. “We’re also saying that masks, which filter out a majority of viral particles, can lead to a less severe infection if you do get one. If you get infected, but have no symptoms — that’s the best way you can ever get a virus.”

The idea that the viral dose, also known as viral inoculum, is important in determining the severity of disease is not a new concept, and Gandhi and her co-authors argue that mask wearers likely inhale fewer virus particles, which makes it easier for their immune systems to combat the infection. However, the connection between virus dose, viral load, and severity of infection for Covid-19 is not proven and remains under investigation.

Understanding the impact of viral load on infections is also important for drug testing and understanding at what point during infection a therapy might be most helpful, says Karan. “This is why we posit that remdesivir — which targets viral replication and reduces it — may have more of a clinical benefit/effect early on, although we need stronger clinical evidence still.” Remdesivir is an antiviral drug that’s being investigated as a possible treatment for severe Covid-19 infections.

But experts still do not have enough data to conclusively say how much the viral load of Covid-19 matters for disease spread and outcomes. And viral load is not a critically important measure for every kind of virus. For example, viral load for a virus like HIV has a clear prognostic purpose. Doctors want to keep HIV at undetectable levels in the body in order to prevent the virus from harming the body, says Amesh Adalja, MD, a senior scholar at the Johns Hopkins University Center for Health Security.

“The viral load of a respiratory virus doesn’t have the same type of importance as HIV, but it signifies how much viral replication has occurred,” he says.

Questions about the impact of viral load on Covid-19 transmission and severity will likely remain for some time, but the usual prevention measures still apply: Wear a mask, keep your distance, and wash your hands.

Health and science journalist. Former editor of Medium’s Covid-19 Blog and deputy editor at Elemental. TIME Magazine writer before that

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