Up to 40% of people who are infected with Covid will suffer an asymptomatic case, study finds

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About four in ten COVID-19 cases are asymptomatic, according to a new meta-analysis.

The research performed by a team from Beijing’s Peking University included 95 studies conducted on three different continents, representing almost 30 million people who were tested for Covid.

Overall, about 40 percent of Covid patients in the analysis had asymptomatic cases. For some sub-groups, like pregnant women and travelers on airplanes and cruises, the figure was about 50 percent.

The study shows that many people who may feel fine are still at risk of spreading the virus, and demonstrates the importance of widely available Covid testing, particularly for travelers.

A new study finding that 40% of Covid cases are asymptomatic demonstrates the need for widely available testing. Pictured: Covid testing in Houston, Texas, August 2021

The study found that certain populations, such as air and cruise travelers and pregnant women, were more likely to be asymptomatic than the overall population

The study found that certain populations, such as air and cruise travelers and pregnant women, were more likely to be asymptomatic than the overall population

Covid cases can range from severe symptoms that land patients in the hospital – to mild, cold-like symptoms, or no symptoms at all. 

Asymptomatic cases may help the coronavirus spread from person to person, because people can carry it without realizing they’re sick.

Such cases distinguish the novel coronavirus from past SARS viruses, which typically don’t spread without symptoms.

The new study on asymptomatic Covid infection suggests that they may be even more common than previously thought.

Researchers compiled and analyzed data from a number of studies focused on asymptomatic cases.

The analysis was published Tuesday in JAMA Network Open.

It included 95 scientific studies in total, including 35 done in Europe, 32 in North America, and 25 in Asia. 

The researchers defined an asymptomatic case as a person who tested positive on a PCR test – considered to be the gold standard for Covid testing – while not feeling any symptoms at the time.

Overall, the 95 studies included 29.8 million individuals who were tested for the virus.

Among the population, 11,500 had asymptomatic Covid infections – about 0.25 percent.

The researchers also used 77 out of the 95 studies to estimate the number of Covid cases that occurred without symptoms.

Those studies included about 20,000 Covid cases in total, of which 40.5 percent were asymptomatic.

Certain populations had higher numbers of asymptomatic Covid cases, the researchers found.

Younger people are more likely to be asymptomatic, the researchers found, with 60% of Covid patients classified as asymptomatic in studies with a median age under 20

Younger people are more likely to be asymptomatic, the researchers found, with 60% of Covid patients classified as asymptomatic in studies with a median age under 20

For example, among pregnant women, 54.1 percent of Covid infections were asymptomatic.

Among travelers on airplanes or cruises, 52.9 percent of infections were asymptomatic.

Among nursing home residents and staff, 39.7 of infections were asymptomatic.

Among healthcare workers and hospital patients, the share of asymptomatic infections was lower: 30 percent.

Studies in North America and Europe tended to have higher shares of asymptomatic infections – 46.3 percent and 44.2 percent, respectively – compared to 27.6 percent in Asia.

‘This lower percentage in Asia might be related to the large city-wide SARS-CoV-2 nucleic acid screening program in China,’ the researchers wrote.

China’s large-scale testing program leads to a far higher number of people getting tested than surveillance programs in other countries.

In addition, the researchers found that younger Covid patients were more likely to be asymptomatic.

When the mean age of a study was under 20, they found, 60 percent of patients were asymptomatic.

When the mean age was between 20 and 39, 50 percent of patients were asymptomatic.

Rapid testing and regular testing for certain occupations, such as airplane workers, can help identify more asymptomatic Covid cases. Pictured: People wait in line for a pop-up testing site in New York City, December 2021

Rapid testing and regular testing for certain occupations, such as airplane workers, can help identify more asymptomatic Covid cases. Pictured: People wait in line for a pop-up testing site in New York City, December 2021

While the papers included in this meta-analysis study varied greatly in their locations, populations, and sample sizes, the study’s overall findings align with past research suggesting that a high number of Covid cases occur without symptoms.

‘This is a highly important finding because this substantial group of people can transmit the virus to others,’ physician Dr Eric Topol, who has worked on similar research but wasn’t involved with the new study, told UPI.

‘It is essential that we have pervasive rapid testing to determine a person’s infectiousness to limit transmission for people without symptoms,’ he said.

In addition to demonstrating the need for more testing of people who don’t have Covid symptoms, the study shows the importance of testing before travel.

The study found that a high share of air and cruise travelers infected with Covid may be asymptomatic, which ‘suggests that screening and quarantine on airport arrival is important for reducing community transmissions, especially in countries without local transmission,’ the researchers wrote.

‘To prevent further transmission in communities, asymptomatic individuals among the general population should be tested,’ they said.

‘If resources are limited, workers in specific industries such as air transportation should be routinely tested.’

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