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HIV Cluster and Outbreak Detection and Response





HIV cluster detection and response (CDR) identifies communities affected by
rapid HIV transmission so that public health agencies can identify where HIV
prevention and treatment services and programs are urgently needed and
provide them. These services may include linking people in the community
to HIV testing, medical care, pre-exposure prophylaxis, and syringe services
programs. Real-time HIV cluster detection and response is key to stopping
HIV transmission and is a pillar of the federal Ending the HIV Epidemic
in the U.S. initiative.


The Benefits of HIV Cluster Detection and Response

Many communities have successfully used CDR strategies to improve HIV care and
viral suppression outcomes, to increase HIV testing and use of prevention services,
and to reduce HIV transmission. CDR can reveal gaps in HIV prevention and treatment
services, which can then be addressed through collaboration and partnership by state
and local health departments, medical and social service providers, CBOs, and others.

The presence of an HIV cluster or outbreak is a sign of increased HIV transmission
among a group of people in an area or in a sexual or social network. An HIV cluster
or outbreak can indicate gaps in HIV prevention or care for the people in the network.
Tailoring and focusing services to the people in the network to address these gaps in
services helps bring HIV prevention and care to people who need it and helps prevent
transmission to others.



Identifying HIV Clusters

HIV clusters or outbreaks refer to groups of people that are experiencing rapid HIV
transmission. CDC and health departments can identify HIV clusters or outbreaks a
few different ways. In some cases, public health agencies use multiple ways to
understand HIV transmission in an area at the same time. This can better inform
ways in which public health departments and communities can respond to help people
with newly acquired HIV and prevent transmission to others.

One way clusters or outbreaks can be identified is when medical providers, public
health staff, or others in the community notice an increase in HIV diagnoses among
a specific group of people in their community, such as people who inject drugs or men
who have sex with men. Another way is when CDC and health departments analyze
HIV surveillance data to identify areas where HIV diagnoses are increasing.

A process known as molecular data analysis can help identify HIV clusters or outbreaks.
Health care providers routinely conduct drug resistance testing to find the best HIV
medication for their patients. This process generates genetic sequences from the virus
(not the person); these sequences are a portion of the larger HIV genome. CDC and
health departments can then analyze these sequences to identify groups, or clusters,
of similar HIV sequences. Because HIV mutates quickly in each person’s body, similar
genetic sequences in the virus indicate that rapid transmission is occurring. Molecular
data analysis can help detect HIV clusters and outbreaks more rapidly and comprehensively
than had previously been possible.

Molecular data analysis has helped detect nearly 300 growing HIV transmission clusters
across the United States, the majority of which had not been previously recognized.
In other cases, clusters were initially identified through increased diagnoses in an area,
then molecular data analysis showed that the cluster was much larger than initially thought.
In addition, comparing molecular data across geographic areas (counties, states, or multiple states)
can help determine if a cluster or outbreak is contained to a single community or if
multiple areas are affected.


Responding to HIV Clusters

When an HIV cluster or outbreak is identified, public health agencies can engage a wide range of
health care providers, advocates, and other community leaders who work together to design
interventions that address the community’s specific needs. HIV responses seek to reach people
in affected networks, including people with undiagnosed HIV, people with diagnosed HIV who
might not be accessing HIV care or other services, and people who do not have HIV but would
benefit from prevention services.

Response efforts can be scaled to the size and characteristics of the cluster or outbreak and
the needs of the people affected by it. CDR includes routine efforts to identify and prioritize
clusters, initial response to small clusters, and expanded or escalated response to more
concerning clusters and outbreaks. These public health response efforts can have long-lasting
benefits that extend beyond the individuals affected by the initial cluster, for instance by
working to expand services like testing or PrEP in an area, or by forging new community
collaborations dedicated to HIV prevention.



CDC’s Role in HIV Cluster Detection and Response

CDC provides funding to health departments to support their CDR work and helps state and
local public health staff identify and respond to clusters or outbreaks. CDC also provides
guidance to help state and local health department staff be prepared to detect and respond
to clusters and outbreaks. In addition, CDC provides assistance and advice to public health
staff who are responding to a cluster or outbreak, and at times will provide direct staffing
to support response needs.


Page last reviewed:
March 2, 2022