March 27, 2020

Serial Interval of COVID-19

Stephen G. Baum, MD reviewing Du Z et al. Emerg Infect Dis 2020 Mar 19

Among confirmed cases of COVID-19 in China outside of Hubei province, the mean period between symptom onset in an infector and an infectee was 3.96 days, much shorter than that of SARS and MERS.

Data are accumulating daily on transmission of novel SARS-CoV-2, the causative agent of COVID-19. These data are vital in controlling the spread of this virus and terminating the current pandemic. Determination of the serial interval, the time between the start of symptoms in the primary patient (infector) and onset of symptoms in the patient receiving that infection from the infector (the infectee) is important in calculating the reproduction number (R0), the number of infectees resulting from one infector. Published retrospective data from 468 reports on transmission of confirmed cases of COVID-19 in China occurring outside Hubei province provide probable date of symptom onset for both infector and infectee. Researchers have now used these dates to calculate the mean serial interval of SARS-CoV-2 infection.

The mean serial interval was calculated as 3.96 days, considerably shorter than the mean serial interval calculated for SARS (8.4 days) or MERS (14.6 days). Using these numbers, the calculated R0 is 1.32, lower than previous estimates. Of the 468 reports, 59 indicated that the infectee had symptoms earlier than the infector, indicating presymptomatic infection from either the putative infector or another asymptomatic infector.

COMMENT

Serial interval is related to but somewhat different from incubation period (NEJM JW Infect Dis May 2020 and Ann Intern Med 2020 Mar 10; [e-pub]). Incubation period does not account for time of symptom onset in the infector. Despite several confounding issues with these new data, on which the authors elaborate, the good news is the lower estimated R0 indicating more gradual spread and possible easier containment. The bad news is probable confirmation of significant asymptomatic contagion.

EDITOR DISCLOSURES AT TIME OF PUBLICATION

Disclosures for Stephen G. Baum, MD at time of publication

Consultant/Advisory Board	Clinical Infectious Diseases
Equity	Select Medical Holdings Corp. (no annual income is derived from this equity holding)
Editorial Boards	Medical Letter

CITATION(S):

Du Z et al. Serial interval of COVID-19 among publicly reported confirmed cases. Emerg Infect Dis 2020 Mar 19; [e-pub]. (https://doi.org/10.3201/eid2606.200357)