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.
, wrote Qiuyue Ma, PhD, and colleagues of Peking University, Beijing.
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 health care 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, 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.