Cluster Detection

HIV cluster detection identifies communities affected by rapid HIV transmission. Health departments can identify HIV clusters through analyzing surveillance data. Additionally, HIV clusters can be identified by providers, partner services and frontline staff, and community members. These methods complement each other.

Health departments should analyze HIV surveillance data monthly to quickly identify and monitor HIV clusters. In most cases, state health departments should conduct these analyses on behalf of local health departments. Analyses should include time-space and molecular cluster detection. Low morbidity jurisdictions (areas that have low numbers of HIV diagnoses) may conduct analyses as needed when new diagnoses or molecular sequences are reported.

Time-space analysis

Time-space analysis identifies increases in HIV diagnoses in a particular geographic area or population. CDC provides software programs that health departments can use or adapt for routine analyses. Health departments can also develop their own approaches to time-space analysis.

Increases in new diagnoses may or may not reflect rapid HIV transmission. Time-space increases need further investigation because they may indicate one large cluster, many small clusters, or an increase in HIV testing that has led to new diagnoses of older infections.

People within the network experiencing rapid transmission might not be part of the identified time-space cluster. This can occur if they have undiagnosed HIV or the network expands beyond the identified geographic area.

Molecular cluster analysis

Health departments conduct molecular analysis using a portion of the genetic sequence of HIV that comes from HIV drug resistance testing. These data are used to monitor HIV drug resistance and detect rapid HIV transmission. Rapid HIV transmission is evident when molecular data show groups of extremely similar HIV genetic sequences.

People with HIV in the network experiencing rapid transmission might not be included in the identified molecular cluster because:

  • They haven’t received a diagnosis yet.
  • They didn’t receive an HIV drug resistance test, or the test did not produce a result.
  • Their molecular test result wasn’t sent to the health department.

Molecular analysis examines the genetic sequence of the virus, not the person. Many people may have similar HIV genetic sequences. Therefore, when two people have similar HIV genetic sequences, it doesn’t mean one person passed HIV to the other.

There are different approaches to analyzing molecular data to identify clusters, and not all focus on identifying rapid transmission. CDC’s approach identifies clusters of rapid transmission, and the most concerning of these are known as “national priority clusters.” Health departments should respond to all national priority clusters. Health departments may also respond to clusters meeting local criteria, depending on their capacity.

CDC has provided Secure HIV-TRACE, a secure, web-based application for health departments to use for molecular cluster detection. Secure HIV-TRACE identifies national priority clusters and other concerning clusters. Information on how health department staff may access Secure HIV-TRACE can be found in this video.

Molecular Analysis Considerations

HIV surveillance programs should collect and store HIV sequence data only in the form of Sanger sequences or, when next generation sequencing (NGS) has been conducted, consensus sequences. Currently, there is no documented public health benefit to collecting and storing raw NGS data through HIV surveillance. Therefore, the risks of collecting these data outweigh any potential benefits.

Analyses of HIV sequence data reported to HIV surveillance programs shouldn’t be interpreted as determining transmission direction or showing direct transmission between individuals. Analyses also shouldn’t attempt to determine the direction of transmission between individuals.

See Data Protections for CDR Activities for information on protecting molecular data.

HIV and STI partner services

Partner services staff interview people newly diagnosed with HIV or other STIs to connect them and their partners with services. Staff can notify partners of possible exposure, get them tested, and link them to prevention and care. Because partner services staff work closely with local communities, they may notice patterns or increases in HIV diagnoses.

Health care providers and community members

Clinical providers or community members may help identify HIV clusters. Observed increases in HIV diagnoses require further investigation to determine whether they reflect transmission increases in a community.

Detecting clusters across jurisdictions

CDC conducts routine analyses of national HIV surveillance data to identify clusters of rapid transmission. While health departments can identify clusters within your jurisdiction, CDC can find clusters affecting multiple jurisdictions. See Considerations for Multijurisdictional Clusters for more information.

The importance of complete, timely, and accurate data

Health departments can work to increase the percentage of providers ordering HIV drug resistance tests and laboratories reporting sequence results. These tests are important for individual health and can also help detect and improve community response to HIV clusters. Increasing reporting speed and accuracy can improve cluster detection capabilities.

Health departments can contact your HIV Surveillance Branch Project Officer for technical assistance related to data completeness, timeliness, and accuracy.

Detected clusters are only a part of the network

Many factors affect how much of the larger network public health partners detect. These include:

  • Cluster detection method
  • Availability of HIV testing programs
  • Access to HIV care
  • Data completeness

Once a cluster is detected, collecting additional information can help to identify more of the network and highlight service gaps. This information can help improve prevention and care services and better meet the needs of people in the affected network. It can also help reduce HIV transmission.

Health department surveillance staff should refer to the “Technical Guidance for HIV Surveillance Programs” document provided by the HIV Surveillance Branch for more information on HIV cluster detection.