October 28, 2020
Reviewed by Zeina A. Kanafani, M.D., MS, FIDSA
One of the important determinants of transmissibility of COVID-19 is the duration of infectiousness in symptomatic and asymptomatic individuals. A recently published study in Eurosurveillance evaluated the relationship between RT-PCR cycle threshold (Ct) values and the duration of infectiousness of COVID-19.
A total of 754 upper respiratory tract (URT) samples from 425 cases confirmed by RT-PCR were analyzed. The RT-PCR Ct values were used as a semiquantitative measure of viral load. The viral load decreased during the first 10 days after illness onset and then stabilized. The corresponding geometric mean Ct values were 28.18 in the first week, 30.65 in the second week (P < 0.001 compared to the first week), and 31.60 after 14 days (P = 0.01 compared to the first week; P = 0.49 compared to the second week).
Attempts were then made to culture SARS-CoV-2 from 324 URT samples from 253 patients, and the virus was isolated from 133 samples (41%). Median Ct values were similar across the spectrum of illness severity (31.23 in asymptomatic, 30.94 in mild-to-moderate, and 32.55 in severe infections; P = 0.79). The likelihood of recovering infectious virus was inversely correlated with the Ct value, with the odds ratio decreasing by 0.67 for each unit increase in Ct.
Viral isolation was highest around the time of symptom onset, with a median duration of viral shedding of 3 days. Viral detection decreased with time (74% in week 1 vs. 20% in week 2; P = 0.002). Patients who were asymptomatic at the time of testing were at least as likely to be culture-positive as those who were symptomatic at the time of testing.
The investigators recommend infection control measures in asymptomatic patients and in symptomatic patients until 10 days after symptom onset.