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More Than One-Third of People With COVID-19 May Not Have Symptoms

More Than One-Third of People With COVID-19 May Not Have Symptoms

So-called silent spreaders can unknowingly pass the novel coronavirus on to others.

Updated on May 29, 2020 at 4:00pm EST.

As the global tally of confirmed COVID-19 cases continues to climb, it’s likely that the actual number of people with the disease around the world is still much higher.

This discrepancy is partly the result of insufficient testing and possibly the fact that some patients develop symptoms that are not usually associated with a respiratory disease, such as diarrhea. Mounting research suggests some people with COVID-19 who don’t develop the most commonly reported signs of the disease—including fever, coughing, shortness of breath and other respiratory symptoms—may be spreading the virus unknowingly.

But it’s becoming increasingly clear that others who are carrying SARS-CoV-2—the coronavirus that causes COVID-19—may not develop symptoms for several days or not have any symptoms at all. (Yes, you could be infected and not even know it.)

On May 20, the U.S. Centers for Disease Control and Prevention (CDC) updated its guidance. Based on the latest available data, the CDC now estimates that some 35 percent of people with COVID-19 are asymptomatic, or do not have any symptoms of the disease.

The CDC also estimates that 40 percent of SARS-CoV-2 spread is occurring before people develop symptoms.

Experts caution that these “silent spreaders” are making the pandemic much more difficult to contain.

Some COVID-19 symptoms may be missed
People infected with COVID-19 have developed a range of symptoms associated with a respiratory infection, including:

  • Fever
  • Dry cough
  • Fatigue
  • Loss of sense of smell
  • Phlegm or a productive cough
  • Shortness of breath
  • Sore throat
  • Headache
  • Muscle and joint pain
  • Chills

But digestive symptoms—diarrhea, loss of appetite, nausea and vomiting—are also commonly reported among COVID-19 patients. Some evidence suggests that certain COVID-19 patients may only develop these digestive symptoms and not experience fever or any respiratory symptoms. Researchers in China also found that those with digestive symptoms took longer to clear the virus from their systems.

Exactly how COVID-19 affects the digestive system is still under investigation but these findings suggest that many people with mild infections and digestive symptoms, which they may not readily associate with COVID-19, could be unknowingly spreading the novel coronavirus to others.

People can spread COVID-19 before they develop symptoms
Symptoms of COVID-19 may appear anywhere from 2 to 14 days after exposure to the novel coronavirus, according to the Centers for Disease Control and Prevention (CDC).

The World Health Organization reports that on average, symptoms appear 5 to 6 days after exposure. In some cases, the SARS-CoV-2 incubation period—the amount of time between exposure to an infection and when symptoms begin—may be even longer than 14 days, according to a March 2020 study published in the Annals of Internal Medicine.

The WHO adds that some people have tested positive for COVID-19 up to 3 days before they developed symptoms, supporting the idea that people can be infectious and shed the virus before they realize they are sick.

The CDC also reported the findings of a study conducted by researchers in Singapore. The study, which examined 243 cases reported between January 23 and March 16, found that pre-symptomatic spread of COVID-19 was likely to blame for seven clusters of COVID-19 cases.

‘Stealth transmission’ may be underestimated
Large-scale testing for the novel coronavirus being conducted in Iceland suggests asymptomatic spread of COVID-19 may be more of an issue than previously thought.

In addition to testing being done in hospitals among symptomatic patients, the biopharma company deCODE Genetics is reportedly conducting additional widespread testing for COVID-19 among the general population. Some 5 percent of Iceland’s population has been tested, including those who are not under quarantine or exhibiting symptoms of the infection.

Overall, aside from the Faroe Islands, the rate of testing in Iceland is greater than any other country—including South Korea and Germany, which have been hailed for their aggressive testing.

Although fewer than 1 percent of the tests results were positive, deCODE Genetics founder Kári Stefánsson, MD, told CNN that around 50 percent of those who tested positive reported having no symptoms.

"What it means in my mind, is that because we are screening the general population, we are catching people early in the infection before they start showing symptoms," Stefánsson said.

"The results of the additional tests performed by deCODE have given an indication that efforts to limit the spread of the virus have been effective so far," Iceland’s government reported in a March 21 statement, adding, "testing in the general population will continue to elicit a much clearer picture of the actual spread of the SARS-CoV-2 virus in Iceland."

What you can do to help
At this point, everyone should assume that they may be a carrier of the new coronavirus. 

As scientists continue to learn more about COVID-19 and its effects on the body, one of the best ways to slow the spread of the pandemic is to continue to practice social distancing and wear a mask or nonmedical cloth face covering when going out in public— even as stay-at-home or lockdown orders are gradually lifted in your area. You should wear a facemask over your nose and mouth whether you have symptoms or not.

This measure is intended to protect others from you—not just protect you from the virus.

Other preventive measures you should also continue to take:

  • Wash your hands well and often with soap and water for at least 20 seconds.
  • Use an alcohol-based hand sanitizer (if soap and water aren’t available).
  • Avoid touching your face with unwashed hands.
  • Stay home if you are sick or have symptoms.
  • Cover coughs and sneezes with tissues that you then put in the trash.
  • Frequently disinfect surfaces with cleaning spray or wipes.

If you think you’re infected
Anyone who thinks they may have COVID-19 or has been in contact with anyone who’s been diagnosed should isolate themselves immediately and take steps to ensure they don’t spread the infection to others, including:

  • Being monitored at home for 14 days
  • Having their temperature taken daily
  • Being on the lookout for symptoms, such as fever, cough or trouble breathing

Call your healthcare provider (HCP) for instructions. Do not go to your doctor’s office without calling ahead first and letting the office staff know that you suspect you’ve been exposed to COVID-19.

Your doctor should determine if you can be treated at home, if you should be tested for the coronavirus and where that should be done.

If you develop serious warning signs of COVID-19, however, you need to seek immediate medical attention. These reg flags may include:

  • Difficulty breathing or shortness of breath
  • Persistent pain or pressure in the chest
  • Feeling confused
  • Bluish lips or face

Call 911 and let the operator know that you have or think you may have COVID-19. If you have a medical mask, put it on before help arrives.

Medically reviewed in May 2020.

Sources:
National Institutes of Health. “To Beat COVID-19, Social Distancing is a Must.”
Centers for Disease Control and Prevention. “Symptoms of Coronavirus.”
World Health Organization. “Coronavirus disease 2019 (COVID-19) Situation Report – 73.”
SA Lauer, KH Grantz, Q Bi, et al. “The Incubation Period of Coronavirus Disease 2019 (COVID-19) From Publicly Reported Confirmed Cases: Estimation and Application.” Annals of Internal Medicine. 2020;M20-0504. doi:10.7326/M20-0504.
Centers for Disease Control and Prevention. “Presymptomatic Transmission of SARS-CoV-2 — Singapore, January 23–March 16, 2020.”
Government of Iceland. “Large scale testing of general population in Iceland underway.”
Centers for Disease Control and Prevention. “COVID-19 Pandemic Planning Scenarios.”

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