Viral load is a puzzle in COVID-19

How sick a person gets from a virus can depend on how much of the pathogen that person was exposed to and how much virus is replicating in their body — questions that are still open for the novel coronavirus.

Why it matters: As people try to balance resuming parts of their daily lives with controlling their risk of COVID-19, understanding the role of viral load could help tailor public health measures and patient care.

Driving the news: An analysis of 5,000 genomic sequences of the coronavirus from patients found those infected with a now-dominant strain with a specific mutation "had higher loads of virus in their upper respiratory tracts, a potential factor in making the strain spread more effectively," the Washington Post reports.

• Viruses typically mutate as they spread through a population, and the mutation that is accumulating wasn't linked to the virus becoming deadlier, according to the study, which hasn't yet been peer-reviewed.
• But the research underscores open questions about COVID-19: How does the amount of virus in someone affect transmission to others, and the severity of the disease?

How it works: Viral dose is how much virus someone is exposed to when they are infected. Viral load is the amount of virus produced in someone's body after they are infected.

• A higher infectious dose of a virus and a higher viral load are linked to more severe disease from influenza, poxviruses and other viruses.

For SARS-CoV-2, the virus that causes COVID-19, "there's accumulating data on both sides of the equation," Monica Gandhi, a professor of medicine and an infectious disease doctor at UCSF, tells Axios.

• In a study of Syrian hamsters, for example, those infected with a higher dose of SARS-CoV-2 had worse outcomes than those with smaller amounts, supporting an earlier study on hamsters checking if "masks" helped prevent transmission, she points out.
• Outbreak investigations show that where there's universal masking of a population, the severity of disease goes down.
• Some researchers suggest a decline in death rate and the rise in asymptomatic cases in the U.S. this summer — both coming at a time when mask-wearing became more common — indicate reducing the dose of the virus may reduce the severity of disease.

"It's very interesting that it seems like it's correlating with masking, this lower rate of severe illness," says Gandhi, adding that the hypothesis remains unproven but has some growing evidence behind it.

• She co-authored a perspective piece in the NEJM this week that says universal adoption of masks could reduce infections and severity until an effective vaccine is distributed.
• Last week, CDC director Robert Redfield suggested face masks are "more guaranteed" to offer protection against the coronavirus than a potential vaccine.

What's happening: Evidence is emerging about the link between viral load and how severe COVID-19 is for a patient.

• In a study of more than 3,000 patients in three New York City hospitals, nearly 40% who tested positive for COVID-19 and had a high viral load died in the hospital.

• But the risk of dying was lower — dropping to about 15% — for patients with a low viral load, Michael Satlin, an infectious disease specialist at Weill Cornell Medicine and NewYork-Presbyterian, and colleagues reported last week in the journal Cancer Cell.
• Of note: The study was done at the height of the city's outbreak in the spring, and Satlin says that given improvements in caring for patients with COVID-19, he wouldn't expect to see such high mortality rates today.
• And the amount of virus the patients were exposed to or their viral load when symptoms started wasn't known — connecting those dots will require more testing, he adds.

Yes, but: Other studies have also found some people without symptoms can have viral loads similar to those with symptoms. And children, who tend to be spared severe COVID-19 complications, can carry as much or more of the virus in their upper respiratory tract.

• "Viral load is a part of the picture, but it's not the full picture," infectious disease specialist Ravina Kullar told MedPage Today, adding that disease severity depends on a person's immune system.

What to watch: Knowing a patient's viral load could be helpful to providers in determining how therapies should be directed.

• Viral load information is in the PCR tests done to confirm SARS-CoV-2 infection but the tests aren't approved by the FDA for that quantitative information, Satlin says.
• If it could be released to clinicians, "it could be very useful" for determining care for people who are hospitalized, he adds.

Go deeper: Where the science stands on using face masks against coronavirus