Prioritizing Case Investigations and Contact Tracing for COVID-19 in High Burden Jurisdictions
Case investigation and contact tracing are an essential part of the COVID-19 response pdf icon[62 pages]. Prompt isolation of people diagnosed with COVID-19, and identification and quarantine of close contacts, can effectively interrupt disease transmission and reduce spread of SARS CoV-2. When implemented quickly, contact tracing, along with other community mitigation efforts work together to reduce transmission. Where increases in COVID-19 cases strain resources, health departments may need to prioritize case investigation and contact tracing.
Health departments experiencing surge or crisis situations around COVID-19 should prioritize case investigation interviews of people who tested positive for or were diagnosed with COVID-19 in the past 6 days (based on specimen collection date or symptom onset, if known). They should also focus contact tracing efforts on:
- household contacts exposed in the past 6 days, and
- people living, working, or visiting congregate living facilities, high density workplaces or other settings (or events) with potential extensive transmission.
In some instances, further prioritization may be necessary.
People diagnosed with COVID-19 should be strongly encouraged to notify all their household contacts:
- to immediately self-quarantine; and
- to seek additional guidance from their health department or CDC’s COVID-19 website.
As resources allow, health departments can expand case investigation and contact tracing for people outside the household who:
- are at increased risk for severe illness (older adults, people with certain medical conditions, and people who may need to take extra precautions against COVID-19); or
- are part of a cluster; or
- were exposed to COVID-19 within the past 6 days.
As resources allow or COVID-19 case rates decline, health departments should:
- expand case investigation interviews to people with positive COVID-19 test specimens collected in the past 14 days; and
- expand contact tracing to all contacts exposed within the past 14 days.
- expand prioritization to include source investigation.
If more than 14 days have elapsed since specimen collection, case investigation and contact tracing should not be pursued unless there are unique circumstances associated with the person tested (e.g., part of large outbreak associated with congregate living or high density workplace or work in a healthcare setting).
At this time, if you have been vaccinated for COVID-19, you should follow existing CDC guidance on when and how long to self-isolate if you have COVID-19 or self-quarantine if you are a close contact. CDC continues to closely monitor the evolving science for information that would warrant changes to the recommendations for quarantine of close contacts.
These prioritization recommendations are intended to assist state, tribal, local, and territorial health departments, particularly those with escalating COVID-19 case counts, delays in receipt of laboratory test results or healthcare provider reports, or with insufficient capacity to conduct case investigation and contact tracing for all newly reported COVID-19 cases in a timely manner.
When health departments are facing a high burden of COVID-19 cases, they may not have the resources to complete timely case investigation and contact tracing activities for all reported cases of COVID-19. In these situations, prioritization of public health activities may become necessary. CDC’s Case Investigation & Contact Tracing Guidance suggests that when resources are limited, health departments should prioritize the most impactful public health activities. CDC’s COVID -19 Case Investigation and Contact Tracing Hierarchies outline priority populations for case investigation and contact tracing. These new recommendations are intended to provide additional information and an overview of prioritization strategies for consideration. Implementation of these recommendations should be guided by what is feasible, practical, and acceptable for each community.
As the burden of COVID-19 worsens in an area, and the capacity to investigate new cases in a timely manner becomes more difficult or is not feasible, health departments should prioritize which cases to investigate and which contacts to trace. In addition, health departments should emphasize broader community mitigation measures. Determining when burden is high will vary by area. Feasibility of case investigation and contact tracing is determined by the volume and trajectory of new cases, as well as staff resources and capacity. Below are examples of how to assess an area’s capacity to conduct case investigation and contact tracing.
- High burden could be defined as a backlog of cases for each interviewer that is at least twice the number (100% more) they are able to interview each day;
- Medium burden could be defined as a backlog of cases for each interviewer that is 50% more than the number of cases they are able to interview each day;
- Low burden could be defined as a reasonable number of cases for each interviewer to call each day.
When experiencing a high burden of cases, Figure 1 is a summary of recommendations that health departments could use to prioritize case investigation and contact tracing activities.
Highest Priority Activities
For cases, prioritize case investigations for people diagnosed with COVID-19 in the past 6 days
(based on specimen collection date or symptoms onset, if known)
For contacts, prioritize:
- Household contacts exposed in the past 6 days
- People living, working, or visiting congregate living facilities, high density workplaces or other settings (or events) with potential extensive transmission
Next Level Priority Activities
As resources allow, expand case investigations and contact tracing to people outside the household who are:
- At increased risk for severe illness
- Part of a cluster
- Exposed within the past 6 days
Low Priority Activities
- As resources allow or COVID-19 case rates decline, expand case investigation interviews to people with positive COVID-19 test specimens collected in the past 14 days, and expand contact tracing to all contacts exposed within the past 14 days
- If more than 14 days have elapsed since specimen collection, case investigation and contact tracing should generally not be pursued
As shown in Figure 2 below, prioritization of case investigation and contact tracing should focus on activities that have the greatest impact on slowing the spread of infection and protecting populations at risk.
Maximize Effectiveness (Time-based Factors)
- People testing positive for or diagnosed with COVID-19 (cases) within the past 6 days, based on specimen collection or symptom onset date
- People exposed to COVID-19 (contacts) within the past 6 days
Protect Household Contacts
People (contacts) who live with, provide care for, or visit the person diagnosed with COVID-19
Prevent Outbreaks & Clusters
- Known clusters or outbreaks
- People living, working, or visiting congregate living facilities, high-density workplaces, other settings (or events) with potential for extensive transmission
Protect People at Increased Risk
- People at increased risk for severe illness
- People who may need additional precautions against COVID-19
- People living with, working with, visiting or providing services to other people at increased risk for severe illness from COVID-19
Why prioritize cases and contacts using a time-based strategy?
To identify and quarantine contacts early enough to prevent additional transmission, case investigations should be conducted as close to the infected person’s test date and symptom onset as possible. Recent mathematical modeling shows that the greatest impact in preventing the further spread of COVID-19 is when contacts begin quarantine within 6 days of exposure to a person with COVID-19, regardless of the presence of symptoms. This 6-day window can be applied to prioritize high-impact public health action for both cases and contacts:
- People testing positive for or diagnosed with COVID-19 (cases) within the past 6 days should be given the highest priority for a case investigation interview. This makes it more likely that their contacts will be reached within this 6-day time period.
- When determining which cases to interview, health departments often only have either the date the specimen was collected (laboratory reports) or the date of symptom onset (case reports of symptomatic diagnosis by healthcare providers). To determine the 6-day interval for the highest priority cases to interview, health departments should start with the specimen collection or symptom onset date (if known).
- When laboratory or case reports contain additional information regarding occupation, certain medical conditions, or other risks, this information may be used to inform interview prioritization based upon health department priorities.
- People exposed to COVID-19 (contacts) within the past 6 days, especially household contacts, should be given the highest priority for public health follow-up, to initiate quarantine and limit further transmission.
Why prioritize household contacts?
Household transmission of SARS-CoV-2 is common. Most close contact exposures are with family members and other household contacts. A prospective study found that transmission rates of SARS-CoV-2 and secondary infections among household members were higher than have been previously reported, at 53 percent. Another study suggests that extended family gatherings (e.g., funeral, birthday party) may also facilitate transmission of SARS-CoV-2. Extensive transmission is a particular concern in households living in close quarters, where multiple people share the same rooms and bathrooms, and households that are multigenerational.
When at crisis levels, household contacts may be the easiest among high-risk contacts to reach. Even when case levels decrease, household contacts will remain at high risk for acquisition of COVID-19. For prioritization purposes, household contacts include those who live with, provide care for, or visit the person diagnosed with COVID-19.
Why prioritize settings (or events) with potential for extensive transmission?
Cases and contacts related to congregate living facilities, high density workplaces and other settings (or events) with potential extensive transmission should be prioritized to prevent large-scale community transmission. Clusters and outbreaks may occur in various settings, including hospitals and other healthcare settings, nursing homes, correctional and detention facilities, childcare facilities, K-12 schools, and institutions of higher education (IHEs), high-density worksites, and events or large gatherings, among others. For additional information about non-healthcare worksites clusters see the CDC website. Tools to assess and respond to COVID-19 in healthcare settings and other non-healthcare workplaces pdf icon[23 pages] are also available.
Some sites and exposures may call for a different strategy for investigation of an outbreak or cluster, including a combination of quarantine and testing strategies for contacts. Information regarding testing strategies in specific locations can be found on CDC COVID-19 websites, and include: high-density critical infrastructure workplaces, institutions of higher education, K-12 schools, and homeless shelters and encampments.
Why prioritize people at increased risk?
People at increased risk for severe illness should be prioritized for a case investigation interview and contact tracing. This is an important step to assess health status, facilitate medical monitoring, and connect people to healthcare, should emergency warning signs or other serious complications arise. This is particularly important since many testing sites use self-administered risk questionnaires and have minimal interaction between the person (case or contact) being tested and a healthcare professional. Interaction with the health department during the case investigation interview or notification of exposure (contact tracing) may be the only opportunity for some people at risk for severe illness from COVID-19 to become connected to healthcare and medical monitoring.
Persons who should be prioritized for case investigation and contact tracing include:
- People at increased risk for severe illness from COVID-19. This category includes older adults, and people with certain medical conditions.
- People who may need to take extra precautions against COVID-19, to protect their health. This category includes include racial and ethnic minority populations, people who are pregnant or breastfeeding, people with disabilities, people with developmental and behavioral disorders, people with drug use or substance use disorder, people living in rural communities, people experiencing homelessness, newly resettled refugee populations, and people who live in nursing home and longer-term care facilities or group homes for people with disabilities.
- People who live with, work with, visit, or provide services (e.g., healthcare providers, emergency medical services personnel, firefighters, other first responders) to people at increased risk for severe illness from COVID-19 should also be prioritized for contact tracing.
Please note that the list of underlying medical conditions and populations at risk is continuously updated on the CDC website as new data emerges. Programs are encouraged to check the website frequently and update protocols accordingly.