Lymphocytes Play Crucial Role in Asymptomatic COVID-19 Infection

A retrospective study of 52 COVID-19 patients—published this week in mSphere—found that while asymptomatic COVID-19 patients hosted viral loads comparable to symptomatic patients, asymptomatic patients showed higher levels of lymphocytes. The researchers observed that asymptomatic patients also cleared the viral particles faster, and had lower risks of long-term complications. 

"Our findings suggested an important role for lymphocytes, especially T cells, in controlling virus shedding," said study leader Yuchen Xia, from Wuhan University's School of Basic Medical Sciences. 

Xia and his colleagues studied throat swabs and blood samples collected from patients at Renmin Hospital of Wuhan University, including 27 who had been admitted for complications related to COVID-19 and 25 asymptomatic patients who had been admitted for other reasons but tested positive for the SARS-CoV-2 virus upon arrival. The researchers used the throat swabs to assess the viral load, and on the blood samples, they ran tests to measure immunoglobins, cytokines, and immune cells.  

Although both groups of patients had comparable viral loads, asymptomatic patients showed a statistically significant increase in the number of CD4+ cells, compared to symptomatic patients. Like previous studies, the new analysis also showed that symptomatic patients were more likely to show impaired liver function than asymptomatic patients. In contrast to other work, however, the new research did not find significant differences in cytokine levels between the two groups.  

Xia's group recently began collaborating on a larger follow-up study with researchers in Germany, analyzing blood samples from more than 100 patients with severe COVID-19 symptoms and 30 patients with mild symptoms. They also plan to conduct animal studies to better understand the role of T cells in viral shedding.  

The team said they hope this study will bring attention to the importance of including transmission from asymptomatic people in widespread efforts to curb the pandemic. "More public health interventions and a broader range of testing may be necessary to control COVID-19," Xia concluded.