Case investigation for communicable disease control routinely involves contact tracing to: 1) discover the person(s) or event(s) that could be a potential source of infection, 2) identify people (contacts) with whom the person diagnosed with the communicable disease (case) may have interacted when they were infectious (potential exposure), and 3) prevent further disease transmission.
COVID-19 case investigation and contact tracing efforts provide support to people who have been diagnosed with COVID-19 (cases) and exposed to SARS-CoV-2 (contacts) to facilitate prompt isolation and quarantine in order to prevent further spread of SARS-CoV-2. The contact elicitation window is defined according to the infectious period. The greatest impact in preventing the further spread of COVID-19 occurs when contacts begin quarantine within 6 days of exposure to a person with COVID-19, regardless of the presence of symptoms.
Source investigation is another valuable approach to contact tracing. It involves looking back over the 14 days prior to symptom onset or specimen collection date (for asymptomatic cases) and identifying interactions with people (close contacts), places, and events that may have been the source of the infection for the person with COVID-19 (see Figure 1). To identify person(s), places(s), or event(s) most likely to be the source of exposure to the index case, the timeframe of 2–7 days1,2 prior to symptom onset or specimen collection date can be considered. Source investigation follows the path from one case to another, providing opportunities to identify additional (secondary) cases who may be undiagnosed. In this context, a secondary case is another case potentially infected by or linked to the potential source case. (See Figure 2). It also facilitates opportunities for testing, isolation, and quarantine for their contacts. Source investigation is often referred to as “backward, “retrospective,” “bidirectional,” or “reverse” contact tracing.
Figure 1: