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BEIJING, China — Four in 10 COVID-19 patients are asymptomatic carriers of the virus responsible for over 800,000 deaths in the United States, a new study warns. Researchers from China say a global study of almost 30 million people found “silent” cases of the virus  are twice as prevalent than previous estimates feared.

The results show 40.5 percent of the confirmed cases of the illness are among people who show no symptoms of the infection at all. Rates rose among certain groups including pregnant women (54%), air and cruise travelers (53%), and care home residents or staff (48%).

“The high percentage of asymptomatic infections highlights the potential transmission risk of asymptomatic infections in communities,” corresponding author Professor Min Liu of Peking University writes in the journal JAMA Network Open.

The data shows 4.5 percent of undiagnosed care home residents or staff had the coronavirus without displaying symptoms. Typical signs of infection include fever, cough, loss of taste or smell, fatigue, and shortness of breath. The findings also apply to over one in 50 pregnant women and air or cruise travelers.

“This finding of a high percentage of asymptomatic infections among air or cruise travelers suggests that screening and quarantine on airport arrival is important for reducing community transmissions, especially in countries without local transmission,” Prof. Liu’s team writes.

COVID spreading silently in hospitals?

The results come from a review of 95 studies involving 29,776,306 individuals from across the world. Overall, the number of asymptomatic infections among tested populations was relatively low — 0.25 percent or one in every 400 people.

However, this rate soared to nearly 40 percent among those testing positive for the virus. Earlier evidence suggested one in five infected people experience no symptoms and are less likely to spread COVID than their visibly sick peers.

“These findings suggest that asymptomatic infections might contribute to the transmission of SARS-CoV-2 within the community,” Prof Liu says.

Study authors are calling for increased and regular testing, especially in specific industries such as air travel.

“In addition, we found that approximately one-third of individuals with confirmed COVID-19 were asymptomatic among health care workers or in-hospital patients,” Prof. Liu continues.

“Because asymptomatic health care workers might contribute to disease spread in and out of hospitals, surveillance of asymptomatic individuals is important for infection control and transmission reduction in health care settings and community. Meanwhile, hand hygiene and personal protective equipment were necessary for hospital visitors.”

Younger patients more likely to be asymptomatic

The meta-analysis also found those under the age of 39 were most likely to be asymptomatic, confirming previous research.

“This indicated that young adults who often presented mild or no symptoms were a potential source of transmission in the community,” study authors write.

The study is the biggest and most updated of its kind, covering Europe, the U.S., Africa, Asia, and South America. It included 19,884 people with a confirmed COVID-19 infection, with 11,069 being asymptomatic.

“Our results could raise awareness among the public and policy makers and provide evidence for prevention strategies,” Prof. Liu says.

“Screening for asymptomatic infection is required, especially for countries and regions that have successfully controlled SARS-CoV-2. Asymptomatic infections should be under management similar to that for confirmed infections, including isolating and contact tracing.”

South West News Service writer Mark Waghorn contributed to this report.

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