COVID-19 Asymptomatic Infection Rate Remains High

Heidi Splete

December 14, 2021

Editor's note: Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

Based on data from a meta-analysis of 95 studies that included nearly 30,000,000 individuals, the pooled percentage of asymptomatic COVID-19 infections was 0.25% in the tested population and 40.5% among confirmed cases.

Asymptomatic infections remain potential sources of transmission for COVID-19, especially as communities reopen and public life resumes, but the percentage of these infections among those tested and among those diagnosed with COVID-19 has not been examined, wrote Qiuyue Ma, PhD, and colleagues of Peking University, Beijing, China.

In a study published in JAMA Network Open  the researchers identified 44 cross-sectional studies, 41 cohort studies, seven case series, and three case series on transmission studies. A total of 74 studies were conducted in developed countries, including those in Europe, North America, and Asia. Approximately one third (37) of the studies were conducted among healthcare workers or in-hospital patients, 17 among nursing home staff or residents, and 14 among community residents. In addition, 13 studies involved pregnant women, eight involved air or cruise ship travelers, and six involved close contacts of individuals with confirmed infections.

The meta-analysis included 29,776,306 tested individuals; 11,516 of them had asymptomatic infections.

Overall, the pooled percentage of asymptomatic infections among the tested population was 0.25%. In an analysis of different study populations, the percentage was higher in nursing home residents or staff (4.52%), air or cruise ship travelers (2.02%), and pregnant women (2.34%), compared against the pooled percentage.

The pooled percentage of asymptomatic infections among the confirmed population was 40.50%, and this percentage was higher in pregnant women (54.11%), air or cruise ship travelers (52.91%), and nursing home residents or staff (47.53%).

The pooled percentage in the tested population was higher than the overall percentage when the mean age of the study population was 60 years or older (3.69%). By contrast, in the confirmed population, the pooled percentage was higher than the overall percentage when the study population was younger than 20 years (60.2%) or aged 20 to 39 years (49.5%).

The researchers noted in their discussion that the varying percentage of asymptomatic individuals according to community prevalence might impact the heterogeneity of the included studies. They also noted the high number of studies conducted in nursing home populations, groups in which asymptomatic individuals were more likely to be tested.

The study findings were limited by several factors, including the potential for missed studies that were not published at the time of the meta-analysis, as well as the exclusion of studies written in Chinese, the researchers noted. Other limitations included lack of follow-up on presymptomatic and covert infections, and the focus on specific populations, factors that may limit the degree to which the results can be generalized.

However, the results highlight the need to screen for asymptomatic infections, especially in countries where COVID-19 has been better controlled, the researchers said. Management strategies for asymptomatic infections, when identified, should include isolation and contact tracing similar to strategies used with confirmed cases, they added. 

More Testing Needed to Catch Cases Early

"During the initial phase of [the] COVID-19 pandemic, testing was not widely available in the United States or the rest of the world," Setu Patolia, MD, of Saint Louis University School of Medicine, St. Louis, Missouri, said in an interview. Much of the world still lacks access to COVID-19 testing, and early in the pandemic only severely symptomatic patients were tested, he said. "With new variants, particularly the Omicron variant, which may have mild or minimally symptomatic disease, asymptomatic carriers play an important role in propagation of the pandemic," he explained. "It is important to know the asymptomatic carrier rate among the general population for the future control of [the] pandemic," he added.

Patolia said he was surprised by the study finding that one in 400 people in the general population could be asymptomatic carriers of COVID-19.

"Also, nursing home patients are more at risk of complications of COVID, and I expected that they would have a higher rate of symptomatic disease as compared to [the] general population," said Patolia. He was also surprised by the high rate of asymptomatic infections in travelers.

"Physicians should be more aware about the asymptomatic carrier rate, particularly in travelers and nursing home patients," he noted. "Travelers carry high risk of transferring infection from one region to another region of the world, and physicians should advise them to get tested despite the absence of symptoms," Patolia emphasized. "Similarly, once any nursing home patient has been diagnosed with COVID-19, physicians should be more careful with the rest of the nursing home patients, and test them despite the absence of the symptoms," he added.

Patolia also recommended that pregnant women wear masks to help prevent disease transmission when visiting a doctor’s office or labor unit.

Looking ahead, there is a need for cheaper at-home testing kits, so that all vulnerable populations can be tested fast and frequently, Patolia said.

The study was supported by the National Natural Science Foundation of China. The researchers had no financial conflicts to disclose. Patolia has disclosed no relevant financial relationships.

JAMA. Published online December 14, 2021. Abstract

Heidi Splete is a freelance medical journalist with 20 years of experience.

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