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COVID-19 Source Investigation

COVID-19 Source Investigation
Updated Feb. 23, 2021

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:


What is the value of source investigation in responding to COVID-19?

  • Source investigation facilitates case finding by working through the network of interactions and activities of a person diagnosed with COVID-19 to identify potential sources (e.g., people, places, events), often uncovering clusters and identifying source and secondary cases who may be undiagnosed (see Figure 2). Source investigation is applicable throughout the pandemic. It is valuable when health departments are facing a high burden of COVID-19 cases and have to scale back on individual case investigation and contact tracing. It is also useful when cases decline significantly and the focus shifts to identifying the remaining cases or sources of group transmission.

Figure 2:

Figure 2 COVID-19 Source Investigation: Potential to Interrupt Ongoing Transmission
  • Source investigation enhances detection of clusters and outbreaks. When two or more people diagnosed with COVID-19 are epidemiologically linked, this typically indicates widespread transmission and is a priority for public health follow-up. Figure 2 depicts a network of cases and contacts, highlighting opportunities to identify clusters, potential source(s), secondary cases, and contacts, to limit transmission. Even as health departments struggle to balance vaccine distribution with case investigation, contact tracing, testing, and quarantine, tools such as source investigation are important in identifying and containing clusters and outbreaks.
  • Source investigation offers opportunities to reinforce messages regarding isolation or quarantine for participants of events or gatherings where transmission may have occurred. In some instances, multiple cases may have been identified among persons who attended a particular sporting event (e.g., hockey, wrestling), recreational or social event (e.g., choir practice, wedding), or gathering (e.g., extended family gathering), indicating this setting as a potential source of infection (see Figure 3).  It offers the opportunity to identify others who attended the same event or gathering, notify them of their potential exposure, ask them about recent COVID-19 vaccination and diagnosis history, and provide recommendations for testing, quarantine, or isolation as appropriate. This is sometimes referred to as “location-based” contact tracing.

Figure 3:

COVID-19 Contact Tracing
  • Source investigation offers opportunities to assess and enhance prevention and mitigation strategies in congregate living settings, high-density workplaces, and other facilities or settings where transmission may have occurred. Given the potential for the rapid and extensive transmission of SARS-CoV-2 in congregate living settings and high-density workplaces, when a setting is identified as a potential source of infection, follow-up with the facility can help identify potential ongoing transmission, implement testing strategies (e.g., high density workplaces, institutions of higher education, nursing homes), facilitate isolation and quarantine (e.g., of close contacts or second-generation contacts), and promote additional preventive measures within the facility to limit future transmission.
  • Source investigation helps public health officials better understand transmission factors (e.g., spread, virulence), elicit travel history, and acquire potential risk information associated with the emergence of cases caused by viral variants. This information may help ascertain whether the person with COVID-19 was exposed during travel, providing opportunities to identify and notify other travelers who may also have been exposed, and identify cases that may have been imported from another geographic location, informing other prevention and community mitigation measures.
  • Source investigation offers the opportunity to emphasize the importance of testing, isolation, quarantine, mask use, and identification of contacts (some of whom may be unaware of their exposure and/or beginning to develop signs and symptoms) to interrupt further transmission.