The accuracy of symptom onset dates may be affected by recall bias, which is when participants cannot recall accurately the details of an event. This is especially the case if the symptoms were mild, which can lead to uncertainty as to when the symptoms actually started

Smith College

- Possibility of an unknown source might have initiated the seven clusters
- recall bias
- lack of ability to detect asymptomatic illness may lead to bias

Limitations of the study

Wei WE, Li Z, Chiew CJ, Yong SE, Toh MP, Lee VJ. Presymptomatic Transmission of SARS-CoV-2 — Singapore, January 23–March 16, 2020. MMWR Morb Mortal Wkly Rep 2020;69:411–415. DOI: http://dx.doi.org/10.15585/mmwr.mm6914e1

Presymptomatic Transmission of SARS-CoV-2 — Singapore, January 23–March 16, 2020

Recall Bias

The majority of detection and surveillance activities are focused on symptomatic people, therefore an under-detection of asymptomatic illness is more than probable, which may be contributed by recall bias and interview bias (an expectation that n matter how mild, there is going to be a symptom)

Lack of ability to detect asymptomatic illness

There is a possibility of an unknown source might have initiated the seven clusters, though presymptomatic transmission is estimated to be more likely as Covid-19 wasn't widespread in Singapore during the investigated time period and strong surveillance systems were in place to detect cases

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