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20% of Coronavirus Infections Are Asymptomatic but Still Contagious

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If you have SARS-CoV-2 you can spread the disease even without symptoms. Getty Images
  • Researchers found an estimated 20 percent of people with an infection with the new coronavirus remain symptom-free.
  • Even people who are truly asymptomatic are able to spread the virus.
  • Experts say it’s critical to continue practicing good hygiene, physical distancing, and mask-wearing to reduce the spread of the virus.

Even people who don’t show symptoms of COVID-19 should be tested if they’ve been exposed to the new coronavirus, SARS-CoV-2, according to guidance issued by the Centers for Disease Control and Prevention (CDC) last Friday.

The guidance comes as new research finds that up to 1 in 5 coronavirus infections present with no symptoms but are still contagious.

According to the CDC, you should be tested for the virus if you believe you’ve “been in close contact, such as within 6 feet of a person with documented SARS-CoV-2 infection for at least 15 minutes and do not have symptoms.”

The new study, published today in PLOS Medicine, reviewed information gathered early in the pandemic to find that most coronavirus infections will show symptoms at some point during infection.

The researchers from the University of Bern in Switzerland reviewed studies conducted in the early weeks of the pandemic using a database of SARS-CoV-2 evidence from March and June of this year.

Their findings suggest only an estimated 20 percent of infections remained symptom-free.

They specifically analyzed 79 studies containing data on more than 6,000 people, with about 1,300 defined as asymptomatic, to determine the proportion of people with an infection who never developed symptoms.

But this didn’t mean the person couldn’t spread the virus.

“Keep in mind that a person with either asymptomatic or presymptomatic SARS-COV-2 infection can still transmit the virus,” study authors Diana Buitrago-Garcia, a PhD student, and Nicola Low, a professor of epidemiology and public health at the Institute of Social and Preventive Medicine at the University of Bern in Switzerland, told Healthline in a joint emailed statement.

Experts say some people may actually have symptoms but not realize they are signs of COVID-19.

“There is some indication that many asymptomatic people actually suffer occult [hidden] disease, which affects their actual physical abilities during the infection and for some time thereafter,” William A. Haseltine, PhD, a former professor at Harvard Medical School and Harvard School of Public Health and the founder and chair of the division of biochemical pharmacology and the division of human retrovirology, told Healthline.

Haseltine, who’s not associated with the study, emphasizes that a definitive answer to the question is still under investigation and will require long-term follow-up.

“The findings of this systematic review of publications early in the pandemic suggests that most SARS-CoV-2 infections are not asymptomatic throughout the course of infection,” the study authors concluded.

The researchers emphasized that these “presymptomatic” cases and the risk of disease spread from those infections means that a combination of preventive measures, including “enhanced hand and respiratory hygiene, testing and tracing, and isolation strategies and social distancing,” will remain critical.

Dr. Hana Hakim, an associate member of the department of infectious diseases at St. Jude Children’s Research Hospital in Memphis, Tennessee, says the results of contact tracing on cruises and in nursing homes, households, and similar settings show that 30 to 40 percent of COVID-19 cases don’t show symptoms at the time of testing, although symptoms may develop soon after testing.

“There have been reports of COVID-19 cases that were diagnosed after close contact with persons who were asymptomatic but subsequently tested positive for SARS-CoV-2,” she said.

Hakim confirmed that this indicates “that transmission of SARS-CoV-2 from asymptomatic or presymptomatic people happens, is significant, and has contributed to the global spread of the virus.”

She cautions that, because people in close contact with one another (within about 6 feet) might have an asymptomatic or presymptomatic infection, it’s critical to follow precautions that interrupt the chain of transmission, “like maintaining at least 6-feet distance from others, wearing face masks, practicing hand hygiene, and surface disinfection.”

Haseltine warns that asymptomatic infections could be as infectious as serious cases of disease.

“The reason being is that the concentration of virus in oral nasal secretions peaks early in the infection process and can reach as many as a billion virus particles per milliliter,” he said.

Counterintuitively, the longer people are seriously ill, the concentration of virus drops by “many orders of magnitude.” In fact, oftentimes infectious viruses aren’t detectable.

“Therefore, asymptomatic people or people in early stages of disease that may be unaware that they are infected are the most contagious,” Haseltine explained.

“While the reported relative percent of new COVID-19 cases who acquired infection from an individual with asymptomatic COVID-19 infection is variable, that such transmission occurs is well accepted,” said Dr. Aditya H. Gaur, director of clinical research and infectious diseases at St. Jude Children’s Research Hospital.

Gaur also said modeling data suggests that “such transmission is a major driver of the spread of SARS-CoV-2 infection.”

However, the combined use of physical distancing, mask-wearing, and hygiene measures are powerful tools to reduce risk.

“These measures reduce the risk of transmission, and it is the bundling of all of these prevention measures which makes for the most effective risk mitigation approach against SARS-CoV-2 infection,” he said.

New research finds that about 20 percent of people with a coronavirus infection don’t experience any symptoms.

But many who don’t have symptoms when they’re tested may likely develop them later. This means people who have no COVID-19 symptoms should still be tested.

Experts emphasize that even people who never develop symptoms of COVID-19 can still spread the disease, and that it’s critical to continue practicing good hygiene, physical distancing, and mask-wearing to reduce the spread of the virus.