Study: COVID-19 Spreads Quickly, Sometimes Before Symptoms Appear


More than 10% of COVID-19 patients were infected by an asymptomatic person, according to new research.

Researchers from the University of Texas (UT) at Austin have found that the time between cases in a chain of novel coronavirus (COVID-19) transmission is less than a week and that more than 10% of patients were infected by an asymptomatic person.1

In a press release from UT, the authors noted that the speed of an epidemic depends firstly on how many people each case infects (the reproduction number) and secondly on how long it takes for infections to spread between people (the serial interval).1

The researchers analyzed the time it takes for symptoms to appear in first the person who infected another, and then in the infected second person. They discovered that the average serial interval for COVID-19 in China was approximately 4 days.1

Because the serial interval of COVID-19 is short, the researchers said emerging outbreaks will grow quickly.1

“The data suggest that this coronavirus may spread like the flu,” said Lauren Ancel Meyers, a professor at UT Austin and a researcher on the study, in a press release. “That means we need to move quickly and aggressively to curb the emerging threat.”1

The team examined more than 450 infection case reports from 93 cities in China and were among the first to estimate the rate of asymptomatic transmission. The findings could clarify the rate of asymptomatic transmission and help public health officials determine how to best contain the spread of the disease.1

In the news release, Meyers pointed out that the data may change over time as more cases emerge.

“Our findings are corroborated by instances of silent transmission and rising case counts in hundreds of cities worldwide,” Meyers said in the news release. “This tells us that COVID-19 outbreaks can be elusive and require extreme measures.”1

Those measures include social distancing, which has been enforced in several states facing large outbreaks. Six counties in the San Francisco Bay Area have announced a “shelter in place” order, which is expected to last until at least April 7, according to the San Francisco Chronicle.2 The order directs everyone to stay home and avoid others as much as possible.2

In a similar measure, New Jersey Governor Phil Murphy (D) has announced a curfew from 8 p.m. until 5 a.m.3

“The time for us to take our strongest and most direct actions to date to slow the spread of the coronavirus is now,” Murphy said in a video announcing the measures.3


  • Coronavirus Spreads Quickly and Sometimes before People Have Symptoms, Study Finds [news release]. University of Texas at Austin; March 16, 2020. Accessed March 17, 2020.
  • Allday, E. Bay Area orders ‘shelter in place,’ only essentially businesses open in 6 counties. San Francisco Chronicle; March 16, 2020. Accessed March 17, 2020.
  • Governor Murphy Announces Social Distancing Measures to Mitigate Further Spread of COVID-19. New Jersey Office of the Governor on YouTube; posted March 16, 2020. Accessed March 17, 2020.

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