Updated: February 2022
A COVID-19 cluster is commonly defined as when 2 or more people from different households share the same exposure source within a 14 day period. This definition may vary based on cluster type or jurisdiction. We define an outbreak as one or more related COVID-19 clusters where 4+ households share an exposure source at an organized social or community event, business, educational institution, or congregate setting.
For the purposes of this chapter, we discuss the application of outbreak investigation and response strategies regularly used in congregate settings in the context of non-congregate, community-based settings to reduce community transmission of COVID-19.
An outbreak response comprises all the steps necessary to investigate and contain an outbreak. This comprehensive strategy is designed to stop transmission by focusing on COVID-19 clusters.
- Outbreak investigation refers to uncovering when and where a case was infected and who else may have been exposed there; mapping out the extent of the cluster(s) including all cases, close contacts, and casual contacts; identifying key areas for risk mitigation
- Outbreak containment includes all the interventions (e.g. notifications, testing, vaccination, social support, preventive measures, referrals for treatments) put into action to suppress the cluster(s) and prevent future ones
Outbreak response addresses both close contacts and casual contacts of infected individuals. True close contacts are at highest risk for COVID-19 infection, but casual contacts can also be at risk for infection and therefore can propagate transmission if not tested following a possible exposure. Outbreak response is conducted regardless of vaccination status.
- Close contacts are those with likely exposure that meet the criteria of ‘close contact’ (see exposure definition here). They typically need to be quarantined.
- Casual contacts are people who do not meet the strict definition of close contact and those who do not need to quarantine but may have been exposed through sharing common spaces. For example, coworkers in a workplace outbreak who likely share airspace, people worshiping at the same church, people who attended the same party, etc.
Core Elements: Effective outbreak responses require a full range of COVID-19 public health interventions including: widespread notifications, contact tracing for cases, care resource coordination for cases and their close contacts in order to support isolation and quarantine, testing, vaccination, and treatment. The process as a whole is described in more detail in the Core Elements section.
Leadership: Ideally local health departments in conjunction with a well-trained contact tracing program lead outbreak response. (See Governance and Partnerships and Workforce and Training). This could mean the outbreak response team is integrated with the contact tracing workforce, or it could mean they communicate and collaborate with a separate contact tracing team.
An appropriate and comprehensive outbreak response strategy is:
- Equitable: COVID-19 finds and thrives in environments with high social vulnerability. By bringing essential resources and outreach to high-risk settings where cases are already occurring, the strategy invests in marginalized communities to ensure equity in response.
- Comprehensiveness and coordination: It unifies key pillars of response (testing, tracing, vaccines, treatment, supported isolation/quarantine), and the same team oversees performance of, or linkage to, each.
- Efficient: It identifies locations that have cases for a reason—low vaccination rates, facility features, activities taking place—and targets interventions to them.
- Impact: By working to identify and contain clusters early, interventions prevent spread in both the immediate setting and in the wider community. It uses existing public health infrastructure and workforce and unifies the outbreak control strategies already in place across the US. As testing volume drops, investigating clusters and intervening is even more critical to identify undiagnosed infections.
- Flexible and Adaptable: Easily integrating into varied public health structures, outbreak response teams are part of the solution to the transition from emergency response to long-term systems improvement.
Updated: February 2022
COVID-19 is a clustering disease, and people are believed to be most infectious prior to symptom onset. This results in exposures often occurring in workplaces, schools, recreational activities, and other public-facing locations prior to individuals knowing that they are sick.
There are generally two types of COVID-19 outbreaks:
- Slower, ongoing chains of transmission
- These are seen frequently in workplaces where extended exposures result in many people being exposed, sometimes even despite efforts to implement preventive measures. These outbreaks can produce several cases each week and span over the course of weeks or months.
- Larger, more explosive outbreak events
- These can also occur in workplaces, but are frequently seen in public-facing locations, such as restaurants or nightclubs, places of worship, and high-risk exposure activities, such as youth sports and recreational activities where exertion and heavy breathing may increase exposure risk or where preventive measures are more difficult to implement.
COVID-19 has and will continue to exploit vulnerability within communities, exacerbating existing inequities. These vulnerabilities are often due to geographic, demographic, or socioeconomic characteristics of affected communities, but also extend into populations who are vulnerable to COVID-19 due to high risk behaviors. By following COVID in these vulnerable populations and focusing the public health response on outbreaks, departments can tailor their response to meet the needs of these specific communities, increasing access to resources essential for treatment of COVID-19 and minimizing the risk of ongoing transmission in the community. The vulnerability that rendered these populations susceptible to an outbreak in the first place is likely to render them susceptible to another outbreak in the future; acting quickly to provide education, tests, vaccines, and other resources reduces these chances while the community is most likely to heed public health advice.
Unvaccinated people are at highest risk of both contracting COVID and suffering negative health outcomes. Additionally, people who are unvaccinated may belong to social networks who share their hesitations about vaccination. Another strength of outbreak response is identifying pockets of unvaccinated people such that public health officials have an opportunity to engage, build trust, educate, and provide resources to this community and minimize their risks for subsequent outbreaks or exposures. Core Elements
Updated: December, 2021
There are five core elements of outbreak response:
- Widespread Notification: Notification to individuals extending farther than the close contact definition is critical in stopping the spread of COVID-19. Close contacts are those at highest risk for contracting COVID-19; however, exposures frequently occur over extended periods of time or involve activities that can increase the risk of transmission of COVID-19, despite those individuals not being strictly considered a ‘close contact’. Ensuring there is a system in place to notify these casual contacts is another critical piece to outbreak response. Widespread notifications are important in both high and low transmission settings, but the process by which people are notified of their potential exposure may change from setting to setting. For example, it may involve notifying employees at a workplace, members of a church, notices at a gym, or patrons at a restaurant, etc., and may employ phone calls, email, social media, or group text messages.. These widespread notifications help people make informed decisions about their health, symptom monitoring, and test seeking behaviors following a possible exposure.
- Testing: COVID transmits rapidly, and people are infectious prior to the development of symptoms. To get ahead of secondary COVID cases and ongoing transmission, a wide net should be cast when recommending and facilitating testing for those linked to a cluster, including close and casual contacts and household members. Testing should be recommended regardless of vaccination status. Those who test positive will enter the contact tracing program, where source investigation can take place. A process should be in place to be able to record, and include for investigations, home-based rapid test results. Mobile testing units, knowledge of nearby testing locations, and education for organization leaders are all good cluster-based testing strategies. When possible, sequencing the virus from positive tests can yield additional crucial information (see Sequencing below).
- Contact Tracing: Close contacts discovered through cluster investigation should be quarantined and entered into the contact tracing program. If resources are limited, high-risk close contacts can be prioritized to facilitate resource provision and treatment referrals.
- Tactical vaccination: Tactical vaccination involves deploying vaccination efforts specifically to people and communities who are part of an outbreak (close and casual contacts, or other connected people who may be at risk due to the outbreak, e.g. place of employment, place of worship, other places with indoor shared airspace, etc). Tactical vaccination often focuses education and outreach efforts on people who are unvaccinated or under-vaccinated, as they are most likely to become infected and infectious in the short- and long-term. It can include entire social networks who need access to vaccination. Tactical vaccination can be supported through mobile vaccination units, helping people register for local vaccine appointments, and educating individuals and trusted authorities linked to the cluster.
- Social support services: Those infected with or exposed to COVID should be connected to support services if they need help with accessing testing, resources for safely isolating/quarantining, vaccines, or treatment referrals. Because outbreaks tend to occur in vulnerable populations, incorporating resource coordination can connect people to the social safety net and – for example – reduce food insecurity by enrolling families in SNAP (previously called food stamps) and connecting them to food banks.
Updated: February 2022
A comprehensive outbreak response will include additional epidemiological, clinical, and public health measures to ensure the full scope of the outbreak is understood, contained, and future outbreaks are mitigated.
- Sequencing: Where possible, all positive test samples should be sequenced for variants. This is especially important for clusters in communities with known or suspected variants or unusual outcomes. Sequencing can also be used to associate additional cases with clusters or even form the basis of detection of clusters when sequencing data is available in real time. When used in conjunction with information from source investigation, sequencing can be a powerful tool for understanding where transmission is occurring, which in turn can lead to more appropriate mitigation recommendations.
- Therapeutics: Link people infected with or exposed to COVID, especially those at risk of severe outcomes, to monoclonal antibody therapy (as therapy or prophylaxis) and other COVID-19 therapeutics (paxlovid and malnupirovir). Treatment options continue to be developed and are becoming much more widely available. Outbreak response is an opportunity for early identification of contacts who may benefit from monoclonal antibody therapy and could be directly linked to providers for evaluation.
- Location-specific guidance: Guidance tailored to the outbreak location should be given, including for ventilation, collection of patron lists, linkage with testing and vaccination services, and other measures that help with cluster prevention and investigation.
- Community outreach: The outbreak response team should proactively conduct community outreach based on lessons from outbreak investigation and response. This can range from contacting similar locations to recommending preventive measures, talking to trusted authorities in the local community, providing educational materials, and local testing or vaccination resources directly to the locations.
Updated: December, 2021
Coordination and communication
- Outbreak response may involve coordination amongst a wide group of individuals and groups, including the outbreak team, local health departments (LHDs), state-level Department of Public Health (DPH) and those involved in liaising with the communities, businesses, mayoral offices, and others who are involved in the response to the outbreak.
- Although LHDs and state-level DPHs are jurisdictional, COVID-19 is not. Communication across LHDs, and between LHDs and DPHs, is critical in responding to clusters.
- Outbreak specialists should be in constant communication with all departments involved in a cluster; the jurisdiction of the cluster event itself and the jurisdictions of residence for each individual exposed or possibly exposed in the cluster event. Depending on the size and scope of the cluster, frequent communication with the DPH is recommended as well.
- The ability to communicate clearly with business owners and community members is necessary to ensure swift exposure notifications and recommendations for testing and vaccination.
Investigation and response
- During the cluster response, outbreak specialists can work with the LHDs to collect close contacts, casual contacts, and provide appropriate guidance to the business owners or hosts of social events throughout the cluster.
- LHDs can deploy their testing and vaccination resources to identified cluster locations to mitigate spread and future outbreaks.
- Following the investigation and completion of the cluster response, outbreak specialists can present a summary of the cluster, including a case map if available, to local officials involved in the response. These presentations and resulting discussion can help to glean lessons learned, prevent clusters in the same location or similar sectors, and can help guide policy decisions.
- In addition to outbreak specialist-identified clusters, LHDs and other state officials can share tips with the cluster team to get ahead of transmission before significant transmission occurs in high-risk locations.
Updated: February 2022
Outbreak Response teams can be as small as a few people or inclusive of complex team/unit structures with sector-specific focuses and a variety of workflows. Some settings will identify ‘flex’ team members from a regionally based pool that can be activated to support local outbreaks when needed.
The key roles of the outbreak investigation team itself include:
- Outbreak Specialists:
- Identify and investigate potential outbreaks: review exposure sources provided by COVID-19 cases; triage exposure sources and identify clusters; collaborate with specialized case investigators to conduct location outreach and exposure notifications;
- Respond to outbreaks: collaborate with Care Resource Coordinators to identify and address social support needs and bottlenecks to testing, treatment, vaccination, and isolation/quarantine; connect cluster locations with testing and vaccination resources to prevent future outbreaks; provide guidance and technical support to the location in accordance with local guidelines
- Document and report: map and document cluster across all cases and contacts; record data and metrics around cluster investigation; report to local and state governments swiftly and accurately;
- Train: teach and mentor contact tracing staff on source investigation, epidemiology of clusters. The Outbreak Specialists often rely on the larger contact tracing workforce. It is critical that the bulk of the contact tracing workforce be trained at least on the basics of source investigation, epidemiological principles of clustering, and basic cluster investigation. The more well trained the contact tracing workforce is, the smaller and more agile the outbreak investigation team itself can be.
- Specialized Case Investigators: Collaborate with outbreak specialists to support cluster investigations; conduct location outreach for exposure notifications; assist with widespread notification of individuals possibly exposed during outbreaks; explain risk of infection and transmitting COVID-19; connect the location and individuals with testing and vaccination resources; refer cases and contacts to Care Resource Coordinators if more complex resource needs are identified
- Management and Leadership Team: Responsible for oversight and accountability of the team; review metrics to ensure appropriate prioritization; encourage collaborative workflow development and team building; adapt to evolving epidemics; HR and administrative support
These additional teams can be integrated within the outbreak response team, or work in partnership with them:
- Contact Tracing Workforce: The outbreak-specific workforce outlined above relies on a strong contact tracing workforce, including contact tracers, case investigators, and care resource coordination. Communication pathways between these two teams are critical to identify outbreaks and provide continual training on epidemiology, clear data priorities, and standards on how data is recorded.
- Care Resource Coordinators: Care Resource Coordinators (CRCs) provide social support and are often embedded within the contact tracing workforce to ensure cases and contacts have the material support they need for isolation/quarantine. Integrating resource coordination within outbreak response is important given the high number of cases/contacts that may be identified, often with a high degree of vulnerability. In addition, specifically putting resource coordination within the outbreak team means that these personnel can assist with community partnerships, navigate barriers to testing, vaccination, and treatment, and work with community partners on culturally appropriate communication strategies (including specific language needs).
Updated: December, 2021
- Diverse workforce: Outbreaks occur in high-risk groups; these groups tend to be demographically, geographically, or behaviorally similar. An outbreak response workforce that is representative of the community will understand the cultural and societal contexts of the outbreaks. Language skills are critical for tracing and outbreak response; hiring a team that speaks the languages in the community is necessary.
- Varied backgrounds: Knowledge of public health systems and disease control measures are beneficial in outbreak response programs, but are not the sole areas of expertise valuable to the response. Many other skillsets, such as communication, public relations, health education, and a variety of employment backgrounds are beneficial to outbreak investigation and response as these outbreaks are not restricted to typical public health environments. The ability to communicate and build trusting relationships with community members, businesses, and health departments is often a skill that can’t be easily taught, and is incredibly valuable in relationship building and mitigating transmission.
Updated: February 2022
In ideal circumstances, a specialized outbreak response team can be built from the existing contact tracing workforce, but this may vary depending on the context. Outbreak specialists and specialized case investigators should have a basic understanding of case investigation and contact tracing. While not necessary, a moderate understanding of epidemiological and clinical principles is helpful for a fast-paced and quickly evolving outbreak response team.
Extensive academic backgrounds or first-hand knowledge of epidemiology beyond contact tracing experience is not required for most roles in the outbreak response team, however a leadership team that is able to guide new team members through necessary concepts is beneficial to the growth of the team.
Once the team has received the baseline COVID-19 contact tracing training modules (advanced case investigation, source investigation, retrospective contact tracing, basic COVID-19 epidemiology, introduction to clusters, methods for location outreach, etc.; view Training on ‘Introduction to COVID-19 source investigation & clusters for examples of training modules); strategic development should be ongoing for both the outbreak specialists and specialized case investigators, as disease and transmission dynamics evolve frequently, and should include:
- Trend assessment (geographical, sector-specific, demographic, etc.)
- Development of sector-specific strategies for communicating with businesses and other public locations
- COVID-19 guidelines as they evolve for tracing, testing, and vaccination
- Basics on COVID-19 testing options (pooled testing, outbreak testing, individual testing, PCR, antigen, etc.)
- Basics on COVID-19 treatment options (monoclonal antibody therapies, COVID-19 antivirals, etc.)
Training for the outbreak response team should be ongoing. While formal initial trainings can be helpful to get the team started, informal and conversational seminars can be effective in facilitating ongoing training and in-depth understanding of public health and COVID-19 transmission dynamics
Updated: February 2022
Outbreak response workflows are generally dependent on the level of transmission within the community. Source investigation is critical at all stages of an epidemic, but the specific outbreak response can be adapted based on high and low levels of COVID-19 community transmission and local prioritization.
At any time, the core workflow elements of outbreak response rely on:
- Source Investigation: Understanding where COVID-19 comes from is critical to interrupting ongoing chains of transmission and preventing future outbreaks. Source investigation should be conducted with each confirmed, probable, or suspected case of COVID-19, and includes baseline questions case investigators and contact tracers use to assess possible high-risk exposure sources within the 14 days prior to symptom onset. As COVID-19 is a clustering disease, it’s likely that more than one person was exposed at that same exposure event. Source investigation is necessary in both high and low transmission settings. An approach for source investigation is included in Asking about and documenting COVID-19 exposure sources.
- Location Outreach: Once an exposure source has been identified, the location needs to be informed of the potential exposure as quickly as possible. Conducting outreach to these locations serves as a touch point with management, the business owner, or other point person for the organization where preventive measures can be reinforced and any misunderstandings can be addressed. Asking general questions, such as “How many people have been out sick recently?” can expand the conversation and may provide information more than just the exposure the outbreak team has identified. The importance of widespread notification, testing, and vaccination can also be discussed with the point of contact. Rosters of employees, guests, patrons, or congregants can be collected so the outbreak team can assist in widespread and anonymous notifications of possible exposures and recommendations for testing. Guidance for making these notifications and starting investigations is given in Working with COVID-19 exposures in public or crowded locations. Should the point of contact prefer to do these notifications internally, the lead outbreak investigator can provide the necessary information for them to do so, examples provided in Widespread notification following COVID-19 exposures
- Cluster Mapping: Mapping chains of transmission and outbreak events can often highlight transmission dynamics and help to visualize where preventive measures may have broken down. Maps can be simple, but are invaluable tools to help share the lessons learned with community partners and local or state government officials who may not be as closely involved in COVID-19 transmission dynamics as public health officials are. Cluster maps are valuable in both high and low transmission settings. Examples and instructions on cluster mapping can be found in How to make a case map for outbreak investigations
If a high percentage of cases can be thoroughly and completely interviewed by available staff, workflow modifications can include:
- Individual Case-Linkages: Large clusters may not be as frequently identified in areas or seasons with low levels of community transmission, but cases are either imported into or spread within a community. Strategies should be developed to understand COVID-19 case demographic and geographic data to identify hidden linkages between cases. This strategy should be coupled with widespread testing and vaccination campaigns to protect these social groups and geographical areas from outbreaks in the event of an increase of COVID-19 community transmission. Real-time genomic sequencing, when available, can be very effective at uncovering some of these linkages if available.
Additionally, some elements can be added to these core workflows as resources allow and when needed based on the types of transmission that are most common in the area:
- Community Outreach: Increases in transmission can be anticipated ahead of social events, holidays, and religious celebrations. Sector-specific community outreach is an effective tool for increasing messaging and education related to preventive measures. For example, if a church leader were inclined to send a COVID-19 prevention message to their congregation ahead of a large religious holiday, their community is likely to heed their advice much more seriously than a public health official. Every opportunity to engage community leaders and leverage their channels of communication should be used to mitigate outbreak events, in both high and low transmission settings.
- Super-Spreading Event Response: COVID-19 is a clustering disease, and in areas with high levels of community transmission, larger outbreaks are common in workplaces and public-facing locations. Quick identification of these outbreaks is crucial, as workplaces and public-facing locations act as amplifiers for COVID-19 transmission, creating an opportunity for extensive household and community spread. Strategies should be developed to quickly identify these mega-clusters, coupled with swift sector-specific mitigation techniques and widespread exposure notifications. Testing and vaccination should be encouraged for staff and any community member associated with the outbreak location.
Updated: December, 2021
A core element of effective outbreak response is to remain dynamic and flexible, and to use the information that is being gathered in ongoing outbreaks to better discover and respond to future outbreaks. Some elements of this include:
- Shifting trends: Outbreak response teams should continuously be monitoring for changes in disease transmission, severity, and affected populations. These shifts in COVID-19 trends should be monitored and reported to local and state public health authorities, and prioritization adjusted to accommodate as necessary. For example, shifts may occur in age patterns, types of exposure settings, vaccine breakthroughs, geography of outbreaks, etc.
- Prioritization: As transmission dynamics shift, prioritization should also shift to ensure outbreak response is being targeted to the highest-risk populations. These priorities could be demographic, geographic, or sector-specific.
- Surges: Numerous outbreaks are occurring during COVID-19 surges when community transmission is at its highest, but the types and size of outbreaks shift with transmission levels. Outbreak response teams should be dynamic and able to identify high priority outbreaks, defined by extent or risk for poor clinical outcome. These outbreaks should have resources made available as fast as possible for widespread testing and vaccination.
Updated: December, 2021
Tool: Proposed Data Queries to Assist in Outbreak Investigations (provides a list of standard data queries that can be useful in cluster detection and identification)
Sophisticated tools like algorithmic cluster detection and text-based data mining can be helpful in cluster detection, but they should not form the basis of investigation. Standard methods of manual data review by outbreak specialists and case investigators, as well as clear communication across the teams and high-quality data collection, are a much more important starting point.
One of the key tools to detecting clusters is a data system that allows all users to both record key individual-level information related to clusters (e.g. source investigation results) and to ‘flag’ cases that are potentially part of cluster events. This can be done with the use of specific variables in the data system, or through external methods such as spreadsheets to track potential cluster leads. This allows cases of interest to be reviewed by the outbreak team, without requiring all contact tracing staff to be fully trained on investigation techniques.
Another key tool is the ability to generate data queries or perform complex searches in your data system. At minimum, outbreak specialists should be able to generate data queries to view the demographics (date of birth, language, gender, address, city of residence), basic clinical information (vaccination status, presence of symptoms, symptom onset date), and contact tracing data (exposure source, household connections, employer name/address). Additionally, the ability to perform keyword searches or filter these fields is required (e.g., searching for all cases in the data query among a given employer).
Updated: December, 2021
Outbreak response metrics and evaluation should be conducted in tandem with evaluation of the standard contact tracing system. Much of the work of the outbreak response team will be dependent on the quality and availability of data collected through normal contact tracing. In particular, exposure sources and employer information are critical foundations of an outbreak response team.
Much like building the outbreak investigation team and outbreak investigation itself, monitoring of the program should be flexible and adapt to what is possible with the current team. Metrics and outcomes can range from simple (eg. number of clusters created, number of cases associated) to more complex (eg. percent of transmission explained, average cluster attack rates, and vaccine effectiveness).
Examples of metrics:
- Number of clusters created in a time period
- Number of clusters per transmission sector
- Number of cases and contacts associated with clusters
- Percent of contacts and casual contacts reached during cluster investigation
- Numbers of people connected to testing, vaccination, and other social support services
Examples of exploratory data analysis to identify trends, confirm or readdress priorities, and identify gaps:
- Proportion of cases coming from each language group
- Proportion of cases coming from each age group
- Rates of cases over time and by geography
- Searching for zip codes, streets, or specific addresses with high case rates
The process of cluster identification, investigation, and analysis is a cycle where any one step should be informing the others. At a minimum, the following should be assessed regularly:
- Proportion of an age group’s cases that are connected to clusters
- Proportion of cases reporting a given primary language that are connected to clusters
- Vaccination status of cluster identified and all cases identified
- Attack rates and total sizes of clusters per sector and over time
Updated: February 2022
A fully integrated and comprehensive outbreak response team helps to target resources (human, time, and material) to populations that are the most vulnerable to COVID-19. Whether by geographic or demographic characteristic, or shared social beliefs on COVID-19 and/or vaccination, COVID-19 identifies individuals and locations that need the resources the most.
People are often more willing to discuss testing and vaccination following an exposure or outbreak to COVID-19, even if previously hesitant. Mobilizing education campaigns, testing, vaccination, treatment, and social support resources available to populations as they are personally impacted by COVID-19 increases their interest in using these resources. Improving vaccination in these populations prevents severe disease and future outbreaks, lessening the burden on the healthcare infrastructure and allowing Health Departments to prioritize other public health interventions in addition to COVID-19 response. Those who choose to get tested and vaccinated through an outbreak response can also in turn be trained to become vaccination ambassadors for their family, friends, wider social networks, and communities.
Community-wide mitigation measures are highly effective when the baseline knowledge in the community about disease prevention is low. As the community learns how to live with COVID-19, shifting the public health response to focus on those most vulnerable, or locations at highest risk for outbreaks, narrows the scope of the public health response while simultaneously focusing where it is most needed.