Contact Tracing Assessment of COVID-19 Transmission Dynamics in Taiwan and Risk at Different Exposure Periods Before and After Symptom Onset

Epidemiology Transversal
Cheng HY et al
JAMA Intern Med

Main result

Of the 2,761 people who were in close contact with 100 patients with COVID-19 (primary subjects), 23 cases of transmission were reported (secondary subjects). Based on these cases, the authors estimate:
- a median virus incubation period of 4.1 days (95% [CrI], 0.4 - 15.8)
- an interval between the contamination of a primary subject and the time to the contamination of a secondary subject of 4.1 days (95% [CrI], 0.1 - 27.8). All cases of secondary infection have occurred before the 6th day after the onset of symptoms in primary subjects. Close contact with a subject before the onset of symptoms are not without risk. The risk of contamination for household contacts (Secondary Clinical Attack Rate = 4.6% [2.3 - 9.3]) and in the family outside the household (TACS = 5.3% [2.1 - 12.8]) is much larger than for caregivers and other contacts. The TACS is greater in elderly patients and in cases with severe symptoms. For the 9 primary asymptomatic subjects, no case of secondary transmission was observed.


Given the contagiousness of the virus before the onset of the first symptoms and the small difference in the TACS between patients without post-symptomatic contact and those having had one, the authors conclude that the quarantine measures, even coupled with contact tracing, will not be sufficient to stem the epidemic and that social distancing measures will be necessary.

Strength of evidence Moderate

- Contact tracing by investigation, in particular before the onset of symptoms is, by nature, not exhaustive. 
- In addition, the choice of 4 days before the onset of symptoms, although in agreement with the recommendations of the WHO, is arbitrary and limits the observation of previous potential contaminations.
- Furthermore, it is questionable whether the large number of secondary infections occurring quickly is not strongly linked to the fact that these contacts are also in close contact with each other within the household.
- Finally, the fact that among people in close contact, only those with symptoms were subjected to a PCR test limits the interpretation of the number of secondary infection numbers


Delineate the dynamics of COVID-19 transmission, assess the risk of infection at different exposure windows, and estimate the infectious period.



A prospective study on confirmed cases. The authors recruited 100 sick patients and followed the progress of their condition for 2 months. The cases were confirmed by PCR test and their severity was determined according to WHO recommendations. After confirming the infection in the laboratory, an epidemiological investigation including contact tracing was carried out by the research team of the Taiwan CDC (Taiwan Centers for Disease Control) and the local health authorities. The period of the investigation went from 4 days before the onset of symptoms until the date of confirmation by testing. For asymptomatic patients, an estimate of the date of contamination was made.
In this study, the definition of “close contact” concerns a person who has been face to face with a COVID-19 positive patient, without PPE, and for a total duration of more than 15 min during the investigation period. The 2,761 people who were in “close contact” with primary subjects were all followed and quarantined during the 14 days following their contact. For each subject, PCR testing was conditioned by the appearance of symptoms, with the exception of healthcare personnel who provided care to the subjects and people at risk, who were systematically tested.

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