40% of COVID-19 patients may be asymptomatic as disease progresses

May 27 (UPI) -- As many as 40 percent of patients with COVID-19 may be asymptomatic, according to a small analysis of cases in China published Wednesday by JAMA Network Open.

Although patients with milder, asymptomatic COVID-19 -- the disease caused by the new coronavirus, SARS-CoV-2 -- may suffer less damage to their immune systems, they may still be contagious, but for less time than those with more serious illness, the authors concluded.

"As shown in prior studies, you can be infected with SARS-CoV-2 but not show symptoms," Dr. Daniel T. Leung, an associate professor of infectious diseases at the University of Utah School of Medicine, told UPI.

"Those who did not show symptoms were more likely to be younger and be women," added Leung, who was not involved in the JAMA study but has studied COVID-19 extensively.

Through Wednesday morning, there were nearly 1.7 million confirmed cases of COVID-19 across the United States, according to figures from Johns Hopkins University. However, it has been suggested that total cases may actually be much higher, as roughly 80 percent of those infected experience few if any symptoms, the World Health Organization has said.

For the study, the researchers from Zhongnan Hospital of Wuhan University in China, where the virus was first identified, analyzed data from 78 cases of confirmed COVID-19. The cases were linked with 26 people exposed to the Hunan seafood market -- where the outbreak is believed to have originated -- or close contact with others who had been infected.

All 78 patients were hospitalized in the same area of the hospital and given the same treatments by the same healthcare workers, the researchers said.

In all, 33, or 42 percent, of the patients were asymptomatic, even though they were monitored for symptoms and signs such as fever, fatigue and dry cough daily.

Further, all 78 were tested for COVID-19 every 24 to 48 hours, with throat or nasal swabs evaluated using RT-PCR. Patients in stable condition also had multiple chest CT scans -- for assessment of lung damage -- and their levels of CD4+ T lymphocyte were also measured routinely.

CD4+ T lymphocyte tests are used to assess immune system function.

The researchers found that COVID-19 patients who were asymptomatic tended to be younger, on average 37 years old, compared to those who had evident symptoms, who were roughly 56 years old. In addition, two thirds of the asymptomatic patients were women, they observed.

Overall, asymptomatic patients had "less consumption" of CD4+T lymphocytes and had faster lung recovery in CT scans, showing improvement in nine days versus 15 days in those with symptoms, the researchers noted. Asymptomatic patients also shed virus -- were contagious -- for eight days, compared to 19 days in patients with outward symptoms, they said.

"The shorter window means we probably have to do more asymptomatic testing relative to what would be the case if their duration of shedding was just as long as symptomatic individuals -- that is, they're a bit harder to catch," Matthew Ferrari, an associate professor of biology at Pennsylvania State University's Center for Infectious Disease Dynamics, told UPI.

Ferrari was not part of the study.

Asymptomatic patients had fewer "fluctuations" in virus testing findings over the study period than those with symptoms.

This, researchers said, suggests those with milder illness have more "stable" levels of virus while they're sick, whereas more seriously ill patients see higher or lower amounts of the virus in their system as the disease progresses.

"The findings show the importance of test-and-trace strategy -- identifying those who are infected through making testing available to anyone with symptoms, testing all their contacts whether they have symptoms or not and isolating anyone who tests positive, regardless of symptoms," Leung said.

"Currently, it is impossible to test every asymptomatic person, so the test-and-trace strategy is one way to identify and isolate those who are infected but asymptomatic," he added.