High transmissibility of COVID-19 near symptom onset.

High transmissibility of COVID-19 near symptom onset. Spencer EA, Heneghan C


Published on June 27, 2020

Reference Hao-Yuan Cheng, Shu-Wan Jian, Ding-Ping Liu, et al. High transmissibility of COVID-19 near symptom onset medRxiv 2020.03.18.20034561; doi: https://doi.org/10.1101/2020.03.18.20034561
Study type
Country Taiwan, China
Setting Community and Hospital
Funding Details Non Reported
Transmission mode Person to person
Exposures Close contact

Bottom Line

The transmission was highest around the time of symptom onset and reduced quickly over subsequent days. Secondary infection was higher among household contacts than that in healthcare or other settings.

Evidence Summary

Thirty-two confirmed cases were enrolled, and 1,043 close contacts identified, and among these, 12 index-secondary cases were identified. All close contacts were quarantined at home for 14 days after their latest exposure to the index case. The secondary clinical attack rate was 0.9% (95% CI 0.5 to 1.7%).

No cases were identified among 605 contacts with exposure to a case later than 5 days after symptom onset. Within 5 days of symptom onset, the attack rate was 2.4% (95%CI 1.1 to 4.5%). Among 36 household contacts, there were 7 secondary cases, 2 of which were asymptomatic.

The attack rate was higher among household contacts (13.6%, 95% CI 4.7 to 29.5%) and non-household family contacts (8.5%, 95% CI 2.4 to 20.3%) than that in healthcare or other settings.

Among 47 nonhousehold family contacts, there were 5 secondary cases, 1 of which was asymptomatic. The higher secondary clinical attack rate for contacts near symptom onset remained when the analysis was restricted to household and family contacts.

Among 301 healthcare workers and 659 others, there were no secondary cases. No nosocomial infection was observed.

What did they do?

This was a prospective case-ascertained study on laboratory-confirmed COVID-19 cases and their contacts. The secondary clinical attack rate (considering symptomatic cases only) was analyzed for different exposure windows after symptom onset of index cases and for different exposure settings.

Study reliability

This is a small study. Contacts were not examined prior to symptom onset of the index cases. Therefore early transmission could be much higher than these estimates.

Clearly defined setting Demographic characteristics described Follow-up length was sufficient Transmission outcomes assessed Main biases are taken into consideration
Yes No Unclear Yes Unclear

What else should I consider?

It is unclear what protective measures were being undertaken by cases, contacts or healthcare workers during the relevant time periods, other than quarantine of the identified close contact.

About the authors

Carl Heneghan

Carl is Professor of EBM & Director of CEBM at the University of Oxford. He is also a GP and tweets @carlheneghan. He has an active interest in discovering the truth behind health research findings

Elizabeth Spencer

Dr Elizabeth Spencer; MMedSci, PhD. Epidemiologist, Nuffield Department for Primary Care Health Sciences, University of Oxford.