Serial Interval of COVID-19 Estimated at 3.96 Days

Reports show infectee can develop symptoms before infector, indicating presymptomatic transmission.

HealthDay News – The serial interval of coronavirus 2019 (COVID-19), defined as the time between a primary case and secondary case developing symptoms, is 3.96 days, according to a study published online March 19 in Emerging Infectious Diseases, a publication of the US Centers for Disease Control and Prevention.

Zhanwei Du, PhD, from the University of Texas at Austin, and colleagues estimated the distribution of serial intervals using data on 468 COVID-19 transmission events reported in 93 Chinese cities between January 21, 2020, and February 8, 2020.

The researchers note that 59 of the 468 reports show that the infectee developed symptoms before the infector, indicating that presymptomatic transmission may be occurring. The mean serial interval for COVID-19 was estimated at 3.96 days, with a standard deviation of 4.75; this is considerably lower than the reported serial intervals for severe acute respiratory syndrome (8.4 days) and Middle East respiratory syndrome (14.6 days). When the index case was imported versus locally infected, the mean serial interval was slightly but not significantly longer (4.06 vs 3.66 days); the secondary transmission was slightly shorter when it occurred within a household versus outside the household (4.03 vs 4.56 days). Based on these findings combined with published estimates for the early exponential growth rate in Wuhan, the basic reproduction number was estimated at 1.32, which is lower than estimates assuming a mean serial interval of more than 7 days.

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

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