What are the transmission dynamics of COVID-19 before and after symptom onset?

Bottom Line:

Transmission of COVID-19 is highest before and soon after symptom onset, and that isolation of symptomatic patients is not a sufficient mitigation strategy when used alone.


Cheng, H. et al. Contact Tracing Assessment of COVID-19 Transmission Dynamics in Taiwan and Risk at Different Exposure Periods Before and After Symptom Onset. JAMA Internal Medicine (2020). DOI: 10.1001/jamainternmed.2020.2020

Date Published:

1 May 2020


This study examined the transmission dynamics of COVID-19 among confirmed cases (n=100) and contacts (n=2,761) in Taiwan from January 15 to March 18, 2020. The authors categorized close contacts by exposure: household (5.5%), nonhousehold family (2.8%), health care (25.3%) and other (66.5%). All secondary cases’ initial exposure occurred within six days of symptom onset in the index cases, compared to only 68% of all non-case contacts. The secondary clinical attack rates were similar in several exposure windows from symptom onset: <0 days: 1.0%; 0-3 days: 0.9%; 4-5 days: 1.4%. Therefore, this study suggests aggressive contact tracing (starting at 4 days prior to symptom onset) and social distancing are vital to interrupting the COVID-19 transmission chain.

Summary by: David Lee