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?

Statista Worldwide confirmed cases covid-19
According to the graphic, the U.S. has the most cases worldwide, making up nearly a third. Statista.com

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."

Centers for Disease Control and Prevention Advice on Using Face Coverings to Slow Spread of COVID-19

  • CDC recommends wearing a cloth face covering in public where social distancing measures are difficult to maintain.
  • A simple cloth face covering can help slow the spread of the virus by those infected and by those who do not exhibit symptoms.
  • Cloth face coverings can be fashioned from household items. Guides are offered by the CDC. (https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/diy-cloth-face-coverings.html)
  • Cloth face coverings should be washed regularly. A washing machine will suffice.
  • Practice safe removal of face coverings by not touching eyes, nose, and mouth, and wash hands immediately after removing the covering.

World Health Organization advice for avoiding spread of coronavirus disease (COVID-19)

Hygiene advice

  • Clean hands frequently with soap and water, or alcohol-based hand rub.
  • Wash hands after coughing or sneezing; when caring for the sick; before, during and after food preparation; before eating; after using the toilet; when hands are visibly dirty; and after handling animals or waste.
  • Maintain at least 1 meter (3 feet) distance from anyone who is coughing or sneezing.
  • Avoid touching your hands, nose and mouth. Do not spit in public.
  • Cover your mouth and nose with a tissue or bent elbow when coughing or sneezing. Discard the tissue immediately and clean your hands.

Medical advice

  • Avoid close contact with others if you have any symptoms.
  • Stay at home if you feel unwell, even with mild symptoms such as headache and runny nose, to avoid potential spread of the disease to medical facilities and other people.
  • If you develop serious symptoms (fever, cough, difficulty breathing) seek medical care early and contact local health authorities in advance.
  • Note any recent contact with others and travel details to provide to authorities who can trace and prevent spread of the disease.
  • Stay up to date on COVID-19 developments issued by health authorities and follow their guidance.

Mask and glove usage

  • Healthy individuals only need to wear a mask if taking care of a sick person.
  • Wear a mask if you are coughing or sneezing.
  • Masks are effective when used in combination with frequent hand cleaning.
  • Do not touch the mask while wearing it. Clean hands if you touch the mask.
  • Learn how to properly put on, remove and dispose of masks. Clean hands after disposing of the mask.
  • Do not reuse single-use masks.
  • Regularly washing bare hands is more effective against catching COVID-19 than wearing rubber gloves.
  • The COVID-19 virus can still be picked up on rubber gloves and transmitted by touching your face.
covid19, coronavirus, getty
A woman wears a face mask to prevent the spread of the novel coronavirus as she walks in the streets of La Romana in Dominican Republic, on April 15, 2020. ERIKA SANTELICES/afp/AFP via Getty Images