Why Are Some People With the Coronavirus Asymptomatic?

The spread of the coronavirus around the world has prompted myriad questions, including why some people don't show any symptoms when they catch the bug.

One report published in the BMJ earlier this month based on figures from China found four-fifths of coronavirus cases may be asymptomatic.

According to Johns Hopkins University, more than 2.9 million people have been diagnosed with COVID-19, and 207,446 have died. Almost 875,500 people are known to have survived. The U.S. is the country with the most known cases of COVID-19, as shown in the Statista graph below.

So why is the virus deadly for some while others don't even know they have it?

Professor Brendan Wren, Dean of the Faculty of Infectious and Tropical Diseases at the London School of Hygiene and Tropical Medicine, told Newsweek there are a variety of reasons why a person may be infected by coronavirus but not show any symptoms.

Such a person may have gotten a low dose of the virus when they were infected, which may be more likely as people pay greater attention to their hygiene, he said. "Their overall immune system keeps it in check, and the individual has a sub [clinical]-infection, rather like a vaccine," said Wren.

"In addition, individuals are genetically different and some have a more efficient immune response to initially counteract the virus," he added.

In an article published in The Conversation, experts from the University of Technology Sydney, the University of Sydney, and Sydney Children's Hospitals Network in Australia explained that the coronavirus invades the body by latching on to a receptor mostly found in the lungs, kidneys, heart, and gut.

"Having a strong immune response during the incubation period can prevent the infection taking hold, reduce the actual quantity of virus in the body and prevent it from getting to the lungs," they said.

It is thought that children, for instance, have not been hit as hard by COVID-19 cases because what is known as their innate immunity "is greater than in adults," according to the authors. Innate immunity is not specific to particular infections, and includes parts of the body like the skin, mucous membrane, and some white blood cells, they said.

Others are thought to have specific genetic variations which mean they put up what is known as an adaptive immune response quicker than others, the team wrote. This is built up by the body against specific infections.

"By generating an early adaptive immune response, the body seems to recognize the virus during the incubation period and fight it off," they said.

Asked how important asymptomatic spread is thought to be in the COVID-19 pandemic, Wren said: "We don't know for sure, but it appears that there are many individuals who are asymptomatic, or have mild symptoms. These individuals can inadvertently spread the virus without realizing, although not as efficiently as a person exhibiting full symptoms who would likely be coughing."

A substantial proportion of the population would need to be assessed and diagnosed with COVID-19, either while they're ill or after with an antibody test, to gain a clearer idea of how important asymptomatic spread is for the coronavirus, Wren said.

Last week, scientists wrote in a New England Journal of Medicine editorial that asymptomatic spread, including where people initially show no symptoms but then go on to develop them, "is the Achilles' heel of Covid-19 pandemic control through the public health strategies we have currently deployed."