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

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

Possibility of Unknown Source

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

Smith College

A study investigated all the clinical and epidemiologic data of the 243 confirmed Covid-19 cases in Singapore between January 23 and March 16, 2020. Clinical data included clinical histories before, during, and after the first positive SARS-CoV-2 test. Epidemiologic data included the activity history during the two weeks preceding symptom onset to ascertain possible sources of infection and contact tracing records. Instances of likely presymptomatic transmission were identified by making sure cases that were epidemiologically linked (clusters) had clear contact between a source patient and a patient infected by the source (a secondary patient), had no other likely explanations for infection, and had the source patient’s date of symptom onset occurring after the date of exposure to the secondary patient who was subsequently infected. Seven clusters were identified as likely presymptomatic transmission occurences, with a total of 10 cases attributed to presymptomatic transmission.

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

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

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