HIV Cluster Detection and Response – Frequently Asked Questions

HIV cluster detection and response (CDR) is a strategy to help public health agencies identify communities affected by rapid HIV transmission. To conduct the strategy, agencies first use data routinely reported to health departments to identify where HIV prevention and treatment services and other interventions are urgently needed. Follow-up services may include linking people in the community to HIV testing, pre-exposure prophylaxis (PrEP), syringe services programs (SSPs), and HIV medical care – all of which help prevent new HIV infections in the communities.

There are several ways an HIV cluster can be identified. For example, a healthcare provider might notice a sharp increase in new HIV diagnoses in a community or their practice and notify the health department. In other cases, public health staff conducting partner services interviews might find an unusual number of new diagnoses among a specific group of people in their community.

Sometimes, clusters might not be apparent until public health staff review and analyze information about new diagnoses from across a city, county, or state. Public health staff may also identify a cluster by using molecular data analysis. This technique involves using information from molecular tests used in routine clinical care that determine the sequences of HIV viral genes to help clinicians pick the best HIV treatment regimen for their patient. These sequences are then reported to the health department and may be used to identify similar sequences circulating in their jurisdiction. HIV changes over time in different ways in each person. When a group of people have similar molecular HIV sequences, it suggests that little time has passed between when HIV was acquired and transmitted. This is often referred to as rapid transmission.

In public health, the term “cluster” refers to a group of health events that are somehow related or have some characteristic in common. An “outbreak” is when the number of these events is higher than what would routinely be expected in an area or among a specific group in a given time period. Often, a cluster involves just a few people and an outbreak involves a larger number. Much of the time, there is no clear line between a cluster and an outbreak and either term could apply.

For CDC’s purposes, HIV clusters or outbreaks refer to groups of people that are experiencing rapid HIV transmission. The terms are sometimes used interchangeably, and CDC typically defers to state and local health departments on how they characterize a given situation. Using proven response strategies helps jurisdictions respond to HIV clusters or outbreaks quickly and appropriately.

Advances in HIV diagnosis, treatment, and prevention have made it possible to envision an end to the HIV epidemic. New tools can identify rapid HIV transmission, enabling public health staff to respond by identifying and addressing gaps in important treatment and prevention resources. Examples of gaps that may allow for rapid HIV transmission include:

• Missed opportunities for testing;
• Low linkage to care;
• Inadequate access to PrEP;
• Limited access to sterile injection equipment; and
• Other service delivery strategies that do not meet clients’ needs.

Addressing these gaps can help stop further HIV transmission by quickly expanding access to HIV prevention and care. For these reasons, responding quickly to outbreaks is a key strategy of the federal Ending the HIV Epidemic in the U.S. (EHE) initiative.

A cluster indicates a gap in HIV treatment and prevention services. Cluster detection and response does not just identify transmission, it is an opportunity to curate highly focused and tailored interventions, such as specialized care coordination, where general interventions have failed. Cluster detection and response uses epidemiology to alert health departments of the need to tailor HIV resources to people who need them, rather than expecting people to navigate difficult-to-access resources in a system that is not meeting their needs.

The Respond strategy is one of the four pillars of the Ending the HIV Epidemic in the U.S. (EHE) initiative, along with Diagnose, Treat, and Prevent. The Respond strategy focuses on responding quickly to HIV clusters and outbreaks to get vital prevention and treatment services to people who need them. Through EHE, CDC supports state and local health departments in implementing several cluster detection and response strategies to address growing clusters of HIV infections. Many health departments now also use molecular data analysis to detect clusters more precisely and completely, which enables them to better address and prevent further transmission.

Cluster detection and response strategies, including molecular data analysis, have identified hundreds of previously unrecognized HIV clusters. On average, HIV transmission in these clusters is 11 times as fast as the national average, and some clusters have transmission rates up to 30 times the national average, or even higher. Prompt recognition of these clusters enables more rapid and effective responses that direct resources and prevention services where they are needed most (see the next question for specific examples.)

Many communities have successfully used cluster detection and response strategies to reduce HIV transmission and stop HIV outbreaks by:

• improving HIV care and viral suppression outcomes
• increasing testing and use of prevention services
• identifying where additional HIV services are most urgently needed
• enhancing engagement with communities experiencing rapid HIV transmission to improve prevention and care services

Since 2015, cluster detection and response activities, including molecular data analysis, have identified hundreds of HIV clusters and outbreaks across the United States. Examples include:

• In Lawrence and Lowell, MA, an HIV outbreak among people who inject drugs, many experiencing homelessness, was first noticed in 2016 by a community health provider. Further investigation by health officials resulted in response efforts, including expanded syringe service programs (SSPs) across the state.
• In San Antonio, TX, an HIV cluster among young Latino gay and bisexual men was first identified in 2017 using molecular data analysis. This discovery inspired a community-wide collaboration resulting in a coalition with leadership from people with HIV to address gaps in prevention and treatment services in the community.
• In Minneapolis, MN, an HIV outbreak was identified in 2019 among people who inject drugs and were experiencing homelessness, the majority of whom were from tribal communities. Initially observed by community health workers and confirmed by state health officials, identification of this cluster allowed a clinic already practicing culturally competent care to provide HIV prevention and care services directly to its clients instead of referring them to other facilities.
• For more information and new stories, see the Community Spotlights page.

Cluster detection and response (CDR) activities are a vital part of ending HIV transmission, and this work requires thoughtful engagement with communities across the nation. Because of this, CDC implementation guidance for CDR efforts requires health departments receiving CDC funding to engage with their local communities. This engagement helps ensure that implementation of these strategies is responsive to the local context. The guidance encourages health departments to effectively communicate about the important role of CDR in HIV prevention programs.

CDC also values direct engagement on this issue. During the past several years, CDC has held many discussions with HIV community members, community-based organizations, and health departments through multiple forums designed to address evolving questions and concerns about CDR strategies, including molecular data analysis. CDC continues to expand community and partner engagement at the federal, state, and local levels.

CDR provides opportunities to curate highly focused and tailored interventions where general population interventions have failed. Community and partner engagement can provide important guidance to inform and strengthen these interventions. CDC supports health departments in the development of inclusive and non-stigmatizing messages about HIV transmission clusters.

Molecular HIV sequence analysis cannot determine if a specific person transmitted HIV to another person. Molecular data analysis examines the genetics of the virus, not the person — and only a fraction of the total HIV genome is reported. These partial sequences are the result of a routine test that helps a healthcare provider know which HIV treatment regimen will be most effective for their patients. They are also used to help identify where rapid HIV transmission is occurring. Viral genetic sequences are not unique to individuals, and dozens of individuals may share very similar viral sequences.

CDC has issued guidance that health departments should not attempt to determine transmission direction or prove direct transmission between individuals. Doing so has little public health benefit.

CDC understands the importance of protecting individuals’ privacy and has put strong security measures in place to ensure that HIV data are protected at CDC and in health departments. HIV data held at CDC are strongly protected by an Assurance of Confidentiality [PDF – 137 KB] under Section 308(d) of the Public Health Service Act. The Assurance of Confidentiality guarantees that all information is held in strict confidence; used only for the purposes stated in the Assurance of Confidentiality; will not otherwise be disclosed or released without the consent of the individual or institution; and that this protection lasts forever, including after death.

As a condition of receiving CDC HIV prevention funding, state and local health departments must comply with strict standards for data security. Those standards require that before any HIV data are sent to CDC, personally identifiable information such as names must be removed, and all data must be encrypted during transfer and when stored.

CDC also provides guidance to state and local health departments on the collection and use of HIV sequence data. To ensure this guidance is followed, CDC requires states to specifically assess protections in place to prevent the release of public health data for any non-public health purpose (including for use in criminal cases), while also requiring states to develop action plans to address any gaps in data protection.

To proactively ensure the privacy of people with HIV, CDC has provided guidance about HIV sequences collected in health departments and CDC. Laboratories conducting genotyping have traditionally used a method called Sanger sequencing. Many labs are now moving to what is called “Next Generation Sequencing” also called “Next-Gen” or NGS.

“Raw” NGS sequences contain more information than Sanger sequences or consensus NGS sequences – like a high-definition version compared with a low-definition version – and could potentially provide more information on direction of transmission compared to Sanger sequences. In the case of HIV surveillance, CDC has decided that this level of detail is not needed for public health purposes, and that Sanger or consensus sequences (which contain the most common base pairs in a given position on a sequence) provide an adequate level of detail for public health purposes without increasing risks of these data being used inappropriately to try and determine direction of transmission. Current HIV data systems only allow for submission of Sanger or consensus sequences; the guidance is intended to address potential future changes in HIV reporting systems.

Furthermore, CDC has provided guidance to health departments that HIV sequence data reported to HIV surveillance programs should not be released to GenBank or other public repositories by health departments or other academic partners without individual consent.

State and local health departments can protect the data of people with HIV, regardless of their jurisdiction’s HIV criminalization laws and practices. CDC guidance states that CDC-funded HIV programs should not release personally identifiable information to anyone outside of public health except in circumstances involving significant risk of harm to the public or if required by law. Even when required, only the minimum information should be released.

Ending the HIV Epidemic in the U.S. requires addressing structural barriers to HIV prevention and care. After more than 30 years of HIV research and significant biomedical advancements to treat and prevent HIV transmission, some state laws and practices are now outdated and do not reflect our current understanding of HIV. In many cases, these laws have been shown to discourage HIV testing, increase stigma, and exacerbate disparities. Current scientific and medical evidence should inform state laws and practices that criminalize behaviors by people with HIV. States should consider updating or repealing outdated laws and practices. To end the HIV epidemic, public health, criminal justice, and legislative systems must work together to ensure that laws protect the community, are evidence-based and just, and support public health efforts.

Healthcare providers and clinical laboratories are required by law to report certain types of infections to their local or state health departments. Separate consent (beyond basic medical consent for testing or care) is not required for reportable conditions mandated by state laws or regulations.

HIV is a reportable condition by law/regulation in all 50 states and Washington, D.C., as well as six U.S. dependent areas. CDC funds state and territorial health departments to collect data on persons diagnosed with HIV; all personal identifiers are removed from the data before being transmitted to CDC via a secure data network. Most states also require HIV-related lab data (including all CD4 and viral load data) to be reported to state health departments. These lab data are an essential component of HIV surveillance systems. They provide information that helps direct HIV prevention resources to jurisdictions where they are needed most and inform community-based HIV prevention and treatment efforts.

No. HIV surveillance data stored at CDC do not contain any personally identifiable information, including individuals’ names, addresses, or social security numbers. Individual-level HIV surveillance data that are stored at CDC are strongly protected from release by legal agreements with states and through a federal Assurance of Confidentiality. CDC does not share data that contain individual-level HIV surveillance data. In fact, the Assurance of Confidentiality prohibits CDC from sharing any information that might allow a person to be identified with the public, with family members, for civil or criminal litigation purposes, or with other non-health agencies of the federal, state, or local governments, including immigration-related agencies.

Public health staff have long used cluster detection and response strategies successfully for many diseases.

Although the use of molecular data analysis is relatively new for HIV, public health staff have used it for many years to track other disease outbreaks. For example, molecular data have also been critical to identifying outbreaks of tuberculosis for more than 15 years. More than three-quarters of recent TB outbreaks were first identified using molecular data.

In addition, molecular data have been critical to identify foodborne illness outbreaks, which may be geographically dispersed. PulseNet, CDC’s molecular system for detecting foodborne outbreaks, which started more than 20 years ago, is estimated to avert more than 250,000 illnesses from Salmonella annually.