COVID-19: Does the amount of virus one gets infected with affect severity of the disease?
Does the amount of the new coronavirus a person gets infected with decide whether a patient will develop a severe or mild disease? While scientists are divided on this subject currently, a new study has found a strong link between disease severity and the amount of virus present in the nose.
The study involved 76 patients admitted to the First Affiliated Hospital of Nanchang University, Nanchang, China, from January 21 to February 4, 2020. All patients were confirmed to have COVID-19 at the time of admission.
The researchers estimated the viral loads of their nasal swab samples. The amount of virus in the body is referred to as the viral load. The team from China found that the viral load of severe cases was around 60 times higher than that of mild cases. This suggests that “higher viral loads might be associated with severe clinical outcomes,” say the authors in their findings published in The Lancet.
While 23 (77%) of 30 severe cases received intensive care unit (ICU) treatment, none of the mild cases required ICU treatment, shows analysis.
“Mild cases were found to have an early viral clearance, with 90% of these patients repeatedly testing negative by day 10 post-onset. By contrast, all severe cases still tested positive at or beyond day 10 post-onset. Overall, our data indicate that, similar to SARS in 2002–03, patients with severe COVID-19 tend to have a high viral load and a long virus-shedding period,” say researchers.
Based on their findings, the team recommends that the viral load of COVID-19 might be a useful marker for assessing disease severity and prognosis.
In another study, published in The Lancet Infectious Diseases, researchers report the viral load and antibody profiles of 23 hospitalized COVID-19 patients. In these patients, the viral load peaked during the first week of illness and then gradually declined over the second week. Viral load was also shown to correlate with age.
“These findings have several practical implications. First, the high viral load during the early phase of illness suggests that patients could be most infectious during this period, and it might account for the high transmissibility of SARS-CoV-2. Second, age was associated with viral load in this study, which could explain the high degree of severe disease in older patients with SARS-CoV-2,” say experts. They, however, add that the relationship between viral load and disease severity needs to be further clarified.
In an interview with Intelligencer, Dr Ellen Foxman, assistant professor of laboratory medicine and immunobiology at Yale School of Medicine, said that according to some recent studies, there is a correlation between high viral load and more severe symptoms. “That really makes a lot of sense, because viral replication is needed for a virus to cause diseases. Usually, only a few viruses get in your body, but then they begin to make copies of themselves. That process of taking over cells and replicating is what leads to disease. If the viruses can’t replicate much, in general, they don’t cause as much disease,” she said.
“As far as transmitting from person to person, it makes sense that the amount of virus you are exposed to impacts how likely you are to become infected. If there’s one virus on a doorknob, you’re much less likely to get sick from touching the doorknob than if there’s 1,000 viruses on that doorknob. With a bigger exposure, some of the viruses are more likely to actually get into your airway and find a place to replicate,” Dr Foxman said in the interview.
However, other studies have not reached a similar conclusion. For instance, one analysis that looked at 94 COVID-19 patients, who were admitted to Guangzhou Eighth People’s Hospital, said there was “no obvious difference” in viral loads across sex, age groups, and disease severity.
In this study, 414 throat swabs were collected from these 94 patients, from the day of illness onset up to 32 days after onset. “We detected high viral loads soon after illness onset, which gradually decreased towards the detection limit at about 21 days after onset,” says the team from China.
The researchers observed the highest viral load in throat swabs at the time of symptom onset, and “inferred that infectiousness peaked on or before symptom onset.” They also found substantial transmission potential for patients who have not yet shown symptoms, with 44% of transmission before symptom onset.
In another study, the researchers did not observe significantly different viral loads in nasal swabs between symptomatic (those who show symptoms) and asymptomatic people, suggesting the same potential for transmitting the virus. The findings by researchers from Italy were based on 5,830 confirmed cases of the COVID-19 outbreak in Lombardy.
So far, over 861,300 cases have been reported globally, and over 42,350 have died in the COVID-19 pandemic.