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Study: Only 20% of COVID-19 Cases Are Asymptomatic

woman wearing mask getting her temperature checked

 

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Key Takeaways

  • Some people with SARS-CoV-2, the virus that causes COVID-19, do not show any symptoms.
  • New research found about 20% of people with the virus are asymptomatic.
  • Earlier estimates said a larger percentage of people were asymptomatic.
  • Being asymptomatic is different than being presymptomatic.

Not as many people with SARS-CoV-2, the virus that causes COVID-19, are truly asymptomatic compared to earlier estimates, a new study finds.

A September study published in PLOS Medicine indicates that only about 20% of people infected do not show any symptoms. Earlier research from the Centers for Disease Control and Prevention (CDC) indicated about 40% of people were asymptomatic. But this decreased percentage doesn't change the fact that asymptomatic carriers are still contagious. In the same report, the CDC says the chance of transmission from asymptomatic people is 75%.

The new study highlights research from March to June. The data included 79 studies with details on more than 6,500 people. Of this group, 1,287 people were classified as asymptomatic.

Researchers made a clear distinction between asymptomatic and presymptomatic carriers. "Asymptomatic" means a person never shows symptoms, while "presymptomatic" means they did not show symptoms when diagnosed, but later developed them. Every COVID-19 patient begins presymptomatic, and about 80% will go on to develop symptoms. Presymptomatic transmission may significantly contribute to the epidemic, the study authors say.

“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 authors wrote.

Amesh Adalja, MD, an infectious disease senior scholar at the Johns Hopkins Bloomberg School of Public Health in Maryland, tells Verywell it’s not surprising to see varying results from studies in terms of the percentage of asymptomatic COVID-19 patients.

“It will take some time to develop a full understanding of all the nuances of COVID-19,” he says. “The important point to remember is that there are some contexts in which patients who do not know they’re infected are contagious to others, prompting recommendations for social distancing and face coverings."

Asymptomatic Carriers Still Pose Big Risk

John Bower, MD, a pediatric infectious disease specialist at Akron Children’s Hospital in Ohio, tells Verywell the data is fairly consistent with estimates from other reports.

What makes this study different from earlier estimates of asymptomatic infection is its distinction between asymptomatic cases and presymptomatic ones, Bower says.

“Consequently, the percentage of infected individuals who are truly asymptomatic are expected to be lower,” he says. 

Newer studies have included better methods so we can distinguish asymptomatic from presymptomatic cases and gauge their relative roles in spreading infection. As these improved studies begin to appear, the error gap in our estimates will decrease, he explains.

People who are asymptomatic pose “a significant ongoing source” for new infections, he says. In fact, one study on COVID-19 patients in South Korea found that the viral load in the upper respiratory tract wasn’t significantly different in asymptomatic and symptomatic patients.

Every infected person goes through a presymptomatic phase where the viral organism is incubating, Bower says. Everyone in that phase poses a risk of transmission for at least two days prior to symptom onset.

“The results of this study should make us, if anything, more vigilant to maintain social distance and funnel resources into contact tracing,” Bower says. “It is essential that we maintain all our current infection prevention practices: distancing, masking, hand hygiene, and contact tracing.

What This Means For You

While there may be fewer asymptomatic carriers than researchers initially estimated, asymptomatic COVID-19 patients can still spread the virus. You should still practice hand washing, social distancing, and mask-wearing even if you don't have symptoms.

Keep Being Safe, Regardless of Symptom Status

Some people may think that if they feel fine, they may be less likely to spread the virus. That’s not true.

Even 20% is a fairly sizable proportion of asymptomatic people that potentially could transmit,” Beth Thielen, MD, an assistant professor and pediatric infectious disease physician at the University of Minnesota Medical School, tells Verywell.

Asymptomatic or presymptomatic carriers may still be unknowingly spreading the virus before they test positive.

Thielen notes that the study authors mentioned age was an important factor in the data. The authors say children appear more likely than adults to be asymptomatic, and are likely large drivers of COVID-19 transmission.

“At that time, you have to make a decision about whether or not to go out around other people if you do not have symptoms," she says. "It makes a bigger difference for public health surveillance because of how many rounds of viral transmission could take place before infection is detected by symptoms."

Even if you're asymptomatic, you should still be taking the same safety precautions to protect others.

“The rate of asymptomatic transmission is not low enough that asymptomatic people do not need to take precautions to prevent infection,” Thielen says. “Everyone should still use face coverings, avoid large groups and maintain physical distancing even if they are not symptomatic."

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  1. Buitrago-Garcia, D, et al. Occurrence and transmission potential of asymptomatic and presymptomatic SARS-CoV-2 infections: A living systematic review and meta-analysis. PLOS Medicine. Sept. 22, 2020. doi:10.1371/journal.pmed.1003346

  2. U.S. Centers for Disease Control and Prevention. COVID-19 pandemic planning scenarios. Updated September 10, 2020.

  3. Ra SH, Lim JS, Kim G, et al. Upper respiratory viral load in asymptomatic individuals and mildly symptomatic patients with SARS-CoV-2 infection.Thorax. 22 September 2020. doi:10.1136/thoraxjnl-2020-215042