Someone asked here a general/related (qualitative) question Does viral load of the initial infection affect its severity? but I'm interstested a more focused question regarding Covid-19:

How well does viral load (as measured "now") correlate with the severity of symptoms for this particular disease/virus?

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It appears to be more nuanced than just high viral load equates to more severe disease. We know that there are case reports of children with high viral loads yet were largely asymptomatic. It may also depend on how the viral load was assessed; whether by nasopharyngeal swabs, or by collecting deep sputum specimens. We know that viral load in the nasopharynx continues to diminish with time and does not necessarily reflect what is going on in the alveoli.

Fifteen studies measured the viral load of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) over the course of the coronavirus disease (COVID-19) using real-time reverse transcriptase polymerase chain reaction (rRT-PCR) testing. (5, 6, 8-10, 12, 15, 16, 18-21, 23, 24, 27) In general, the highest viral loads were observed at the time of symptom onset and for a few days after, with levels slowly decreasing over the next one to three weeks. Some studies observed increases in viral loads prior to clinical deterioration, particularly from lower respiratory tract specimens, and, similarly, decreases in viral load were observed prior to improvement of symptoms.(9, 16, 21) Another study reported an association between higher viral loads and more severe symptoms.(16)

Only one study measured viral load in the pre-symptomatic phase, and this was a case report of a child.(8) The child was asymptomatic on admission to hospital, with a relatively high viral load (nasopharyngeal sample Evidence summary for COVID-19 viral load over course of infection targeting ORF1ab-gene, cycle threshold value (Ct) = 13.73). The viral load decreased over the next nine days, although it raised slightly when the child experienced a fever on day two of admission, before falling again once the fever resolved.

It may be that if you have severe disease then you'll more likely have high viral load, but a high viral load doesn't automatically mean severe disease. And of course many deaths are by cytokine storm caused by an uncontrolled innate immune response where the adaptive immunity has yet to kick in to down regulate the inflammation generated by the innate immune response


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