As we expect, the values differ early and late in the infection and within different tissues. In addition, there is a wide variation in viral load, six orders of magnitude reflecting susceptibility and timing of sampling. Let’s get started by considering this graph depicting PCR and TCID 50 counts in the respiratory and GI tracts and the local lymph nodes and tonsils, the stand-in for our immune system. This is a far more accurate proxy for those “infectious units,” the virion A more accurate way to determine the presence of virions is to measure the number of cells killed by their presence in cell culture the TCID 50 . It is a proxy for virions, and large amounts strongly suggest the presence of current infection. PCR tests for COVID-19 identify the presence of “naked” RNA, not the capsule necessary for delivering the virus, not the virion. That information, coupled with our knowledge of coronaviruses in general and some mathematical manipulation, form the basis of their findings.Ī quick reminder.

The rhesus monkeys were infected with the virus, sacrificed on day 3 or 4 of infection, and quantitative measures were made of the virions present. The scientists used three studies of rhesus macaques infected by COVID-19 they were our closest relative, for which we had good data.

Why it may seem a bit like knowing the number of angels dancing on the head of a pin understanding the relative amounts of virions and viral particles responsible for COVID-19 will provide some insights into “pandemic dynamics” and our immune responses.

Virion turnover definition