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Although some key aspects of how COVID-19 spreads are beginning to be determined, unanswered questions regarding transmission dynamics remain. One key question has been whether the virus can be spread by people before they start exhibiting symptoms. A new paper addresses this question and, although the conclusions are based on early data, the findings indicate the presence of pre-symptomatic transmission in more than 10% of cases.

The speed of an epidemic depends on two things—how many people each case infects (the reproduction number) and how long it takes cases to spread (the serial interval). Short serial intervals mean emerging outbreaks will grow quickly and could be difficult to stop.

A team of scientists from the University of Texas (UT) at Austin collaborated with researchers at Dalian Minzu University and Beijing Normal University in China, the University of Hong Kong, and the Institute Pasteur to identify how quickly COVID-19 can spread from person to person. They found that time between cases in a chain of transmission is less than a week.

The data are presented in a paper in press with the journal Emerging Infectious Diseases.Although the final version has not been released, the preprint can be viewed on under the title, “The serial interval of COVID-19 from publicly reported confirmed cases.

“Ebola, with a serial interval of several weeks, is much easier to contain than influenza, with a serial interval of only a few days. Public health responders to Ebola outbreaks have much more time to identify and isolate cases before they infect others,” said Lauren Ancel Meyers, PhD, a professor of integrative biology at UT Austin and senior author on the study. “The data suggest that this coronavirus may spread like the flu. That means we need to move quickly and aggressively to curb the emerging threat.”

In order to draw their conclusions, the team calculated what’s called the serial interval of the virus. When the serial interval is shorter than the incubation period of a virus, pre-symptomatic transmission is likely to have taken place.

More specifically, the serial interval of COVID-19 is defined as the time duration between a primary case (infector) developing symptoms and secondary case (infectee).​ The authors note that the serial interval is a critical piece of information to determine the reproduction number—which can indicate the extent of interventions required to control an epidemic.

The team estimated the distribution of serial intervals for 468 confirmed cases of COVID-19 reported in 93 Chinese cities by February 8, 2020. The mean and standard deviation are 3.96 (95% CI 3.53–4.39) and 4.75 (95% CI 4.46–5.07) days, respectively.

Notably, 59 of the 468 (12.6%) reports indicate that the infectee developed symptoms earlier than the infector. Thus, pre-symptomatic transmission may be occurring meaning that the infected persons may be infectious before their symptoms appear.

Researchers have had some uncertainty about asymptomatic transmission with SARS-CoV-2. These data provide the strongest evidence yet that people without symptoms are transmitting the virus.

“This provides evidence that extensive control measures including isolation, quarantine, school closures, travel restrictions, and cancellation of mass gatherings may be warranted,” Meyers said. “Asymptomatic transmission definitely makes containment more difficult.”

Meyers pointed out that with hundreds of new cases emerging around the world every day, the data may offer a different picture over time. Infection case reports are based on people’s memories of where they went and whom they had contact with. If health officials move quickly to isolate patients, that may also skew the data.

The authors caution that their findings should be “interpreted as working hypotheses regarding the infectiousness of COVID-19” and require further validation as more data become available.

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

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