As we make more progress in the fight against the novel coronavirus, findings are emerging about the viral load of SARS-CoV-2. Viral load refers to the amount of virus in the body of a person who is infected and could determine how ill a person can get from the disease or how infectious the person may be.
Identifying viral load patterns also helps researchers and scientists figure out the disease prognosis and assessing its severity.
In the case of COVID-19, however, it is not exactly clear if asymptomatic and symptomatic cases have different viral loads in their bodies. It also cannot be said certainly if a high viral load translates to more severe symptoms for a person or if they are more contagious.
Significantly, some recent research points out that the viral load patterns of SARS-CoV-2 differ to that of SARS-CoV. In the case of SARS-CoV-2, the viral loads are known to peak around the time of onset of symptoms, whereas in the case of SARS-CoV, the viral loads peaked about 10 days after the onset of symptoms. In the case of MERS, the viral loads are known to peak around 14 days after the onset of symptoms. The viral load profile of a particular pathogen can help in figuring out how to administer antiviral treatments to a patient, especially since there is no standard recommended treatment for COVID-19 as of now.
Here’s a look at some of the research that has been done on the subject.
What recent research has noted about viral loads of SARS-CoV-2
A study analysing the early phase of the outbreak in Lombardy, Italy (one of the worst-hit regions in the country) did not find a “significantly different” viral load in the nasal swabs of symptomatic and asymptomatic cases, which could mean that both have the same potential for transmitting the virus. The study also maintains that the “limited number” of asymptomatic infectious cases that were identified through contact tracing may suggest they play a “minor role” in the overall spread of infection.
Another study, which is yet to be peer-reviewed, conducted by researchers from the Guangzhou Eighth People’s Hospital, Guangzhou Medical University, Guangzhou, China and the World Health Organisation Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, University of Hong Kong, Hong Kong SAR, China, observed the highest viral load in throat swabs at the time of symptom onset, from which they infer that a person’s infectiousness may peak on or before symptom onset.
They note that the viral load gradually decreased towards the detection limit at about 21 days after onset. “There was no obvious difference in viral loads across sex, age groups and disease severity,” they say.
A study published in The Lancet journal corroborates the above finding that viral loads of SARS-CoV-2 were “very high” at the time of presentation of the patient and declined steadily thereafter. However, this study observed that peak viral loads correlated with a person’s age, which the previous study did not find.
They also note that the viral load profile of SARS-CoV-2 is similar to that of influenza, in the sense that it peaks at around the time of symptom onset. A higher viral load at the time of symptom onset can mean that the virus can be transmitted easily, “even when the symptoms are relatively mild”.
These researchers also note that a high initial viral load was associated with death in the case of SARS-CoV. However, in case of SARS-CoV-2, they did not notice a significant difference in the viral loads of patients with mild and severe symptoms.
In another study published in The Lancet, researchers note that the mean viral load in severe cases analysed by them and who were admitted to a hospital in China was over 60 times higher than that of mild cases, which suggests that higher viral loads may be associated with more severe outcomes.
What does this mean?
The research is still emerging and it cannot be said with certainty how viral loads of SARS-CoV-2 are associated with severe symptoms or how contagious the person is if they display higher viral loads. But the research done up to this point tells us that the viral loads start to peak within the first week of the onset of symptoms in case of COVID-19 and that viral loads gradually start to decrease after the onset of symptoms.
Broadly, a higher viral load may make the transmission of the virus easier and may mean more severe symptoms for the infected person. But research differs about the correlation of viral loads with age and it is also unclear if asymptomatic and symptomatic cases have different viral loads, since the Lombardy study noted no difference between the two.
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