Science's COVID-19 reporting is supported by the Pulitzer Center and the Heising-Simons Foundation.
To the known risk factors for developing severe COVID-19—age, male sex, or any of a series of underlying conditions—a new study adds one more: high levels of the virus in your saliva. Standard COVID-19 tests sample the nasal passage. But several new tests look for SARS-CoV-2, the pandemic coronavirus, in saliva, and the new work finds a striking correlation between high virus levels there and later hospitalization or death. If the results are confirmed, saliva tests could help doctors prioritize which patients in the early stages of the disease should receive medicines that drive down levels of the virus.
"I thought it was pretty striking," says Shane Crotty, a virologist at the La Jolla Institute for Immunology, who was not involved with the research. Crotty notes the results suggest virus levels in saliva reflect viral load deep in the lungs, where the disease does much of its damage in severe cases. "That is a fundamentally valuable insight," Crotty says.
The new work isn't the first to link the body's coronavirus load and disease outcome. Several research groups have found a correlation between high viral levels in the nasal passages at the time of a patient's hospital admission and ultimate disease severity. But other groups have failed to find that same link.
The standard test to detect SARS-CoV-2 samples nasal mucus using nasopharyngeal (NP) swabs. The procedure is unpleasant, but it is the customary way to sample respiratory pathogens. In recent months, however, several research groups have developed and received emergency use authorization from the U.S. Food and Drug Administration for tests detecting SARS-CoV-2 in saliva.
Yale University researchers were among the first, and the university's hospitals have been using both saliva and NP swab tests. In both cases, labs analyze the samples using quantitative reverse transcription polymerase chain reaction tests, which can detect genetic material from SARS-CoV-2 and quantify the number of viral particles in each milliliter of sample.
Researchers led by Akiko Iwasaki, an immunologist at Yale, compared viral loads in saliva and NP swabs from 154 patients and 109 people without the virus. They divided the patients into groups that had low, medium, and high viral loads as determined by both types of test. Then they compared those results with the severity of symptoms the patients developed later.
They found that patients who developed severe disease, were hospitalized, or died were more likely to have had high virus loads in their saliva tests, but not in their NP swabs. Viral load in both saliva and nasal mucus declined over time in patients who recovered, but not in those who died.
When Iwasaki and her colleagues reviewed patients' electronic medical records for markers of disease in the blood, they found that high saliva viral loads correlated with high levels of immune signals such as cytokines and chemokines, nonspecific molecules that ramp up in response to viral infections and have been linked to tissue damage. People with more virus in their saliva also gradually lost certain cells that mount an immune response against viral targets, had lower levels of antibodies targeting the spike protein that the virus uses to enter cells, and were slower to develop the strong immune response needed to knock down the virus in cases where they recovered. The team's results appeared on 10 January in a preprint that has not been peer reviewed.
Iwasaki and her colleagues argue that saliva may be a better predictor of disease outcome than nasal mucus because the latter comes from the upper respiratory tract, whereas severe disease is associated with damage deep in the lungs. "Saliva may better represent what is going on in the lower respiratory tract," Iwasaki says, because cilia lining the respiratory tract naturally move mucus up from the lungs into the throat, where it mixes with saliva; coughs have the same effect.
The results don't have enough statistical power to reveal how much more likely a person with a high saliva viral load is to develop severe COVID-19, Iwasaki says. She is also eager for other groups to replicate the results, especially because efforts to link high NP swab viral loads with disease progression have had mixed results.
If other research confirms the finding, "it would clear away a lot of the fog" around this disease, Crotty says. Monica Gandhi, an infectious disease expert at the University of California, San Francisco, adds that if saliva tests are predictive, they could help doctors identify patients to treat early with either antibodies to reduce viral load or steroids to tamp down overactive nonspecific immune responses.
Saliva tests are cheaper and easier than NP tests, but much less widely available. So confirmation of the new results could bolster efforts to make saliva tests more readily available, says Sri Kosuri, CEO of Octant, Inc., a biotech company. "If this study happened in March, we'd be talking about whether we should be doing NP testing at all," Kosuri says.
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