By Molly Walker

Viral loads were similar among asymptomatic and symptomatic COVID-19 patients and remained that way for weeks after diagnosis, according to South Korean researchers, who suggested that public health efforts to contain the pandemic should focus more on those without symptoms.

In a study of some 300 patients with positive PCR tests for SARS-CoV-2, asymptomatic individuals had a slightly shorter median time to negative conversion than symptomatic patients that trended towards significance (17 vs 19 days, respectively, P=0.07).

But cycle threshold values in PCR tests (a quantitative indicator of viral loads) for lower respiratory tract samples showed viral loads stayed about the same or even decreased more gradually in the asymptomatic patients from diagnosis to discharge from isolation, reported Eunjung Lee, MD, of Soonchunhyang University Seoul Hospital in Seoul, and colleagues.

The authors noted that, while asymptomatic transmission of COVID-19 is "an important factor" in controlling the spread of the virus, the clinical course of patients with asymptomatic infection remains somewhat mysterious.

"An important implication of our findings is there may be substantial underreporting of infected patients using the current symptom-based surveillance and screening," they wrote.

Lee and colleagues examined data from 303 patients with SARS-CoV-2 infection from March 6 to March 26 who were isolated at a community treatment center. Median patient age was 25, and two-thirds were women. Twelve patients had comorbidities, ten of whom had hypertension.

RT-PCR testing was performed on both the upper respiratory tract (from a nasopharynx and oropharynx swab) and lower-tract sputum specimens.

In the study sample, 193 patients were symptomatic at the time of isolation and of the 110 without symptoms, 21 developed them while in isolation. That left 89 who remained asymptomatic during follow-up (median 24 days).

Proportions of patients with negative conversion at days 14 and 21 were only slightly higher in asymptomatic patients (33.7% and 75.2%, respectively) compared to symptomatic patients (29.6% and 69.9%).

The authors also found that cycle threshold values of the env viral envelope gene in sputum dropped more gradually in asymptomatic versus symptomatic/presymptomatic patients, with a β value of -0.065 (P=0.005). Declines for other genetic segments did not drop markedly faster in the asymptomatic group.

"It appears that the env target signal was aberrant owing to fragmented or degraded genomes," Lee and colleagues wrote. "In a recent study, viral shedding from sputum has been shown to extend beyond symptom duration."

However, they added that detection of viral RNA does not equal detection of infectious virus, and larger epidemiological studies and experiments are needed to understand the link between viral shedding and transmissibility. They also noted they did not determine the role that viral molecular shedding played in transmission from asymptomatic patients. As well, the relatively young age of the study sample and other specifics (geography, timeframe) may limit generalizability.

Nevertheless, the findings don't support notions that asymptomatic patients are less likely to transmit the virus, or stop transmitting it sooner than those with clear symptoms. Lee and colleagues concluded that because asymptomatic transmission may play a key factor in community spread, "population-based surveillance and isolation of asymptomatic patients may be required."