HIV Cluster and Outbreak Detection and Response (CDR)

What We Do and Why

Our work is an important part of ending the epidemic of HIV in Washington.

  • We use data to find communities affected by rapid HIV spread.
  • We help public health agencies reach communities and people who urgently need HIV prevention and treatment services.
  • We partner with local organizations to provide needed services in order to stop the spread of HIV and meet people’s needs.

What does CDR stand for?

  • Cluster: a group of people with new and existing HIV diagnoses who may be part of a network of HIV spread.
  • Outbreak: when the number of new HIV diagnoses is higher than what would be normally expected in an area or among a specific group of people in a given time period.
  • Detection: using data to understand where HIV transmission, or spread, is happening.
  • Response: moving resources where they are needed to provide prevention, treatment and testing, and other services people may need like housing and food.

How do we find clusters and outbreaks of HIV? 

Field Tools


Health care providers or community partners might notice an unusual increase in diagnoses in a certain area or group of people, and report that to the health department.

Partner Services

Partner services field staff are health department workers who reach out to people newly diagnosed with HIV to offer support with partner notification and help get people linked to services. Because they are talking directly to people newly diagnosed with HIV, they can make certain connections, like if a group of people with new diagnoses may be part of the same social network.

Data Tools


Time-space cluster analysis uses a computer program to track patterns in HIV diagnoses within certain timeframes and geographical areas. For example, the computer program may track a certain number of new diagnoses during the month of November (time) within a certain county (space).

Molecular analysis

Molecular analysis uses information from blood tests used in routine clinical care to track transmission patterns.

When someone is newly diagnosed with HIV, or when they need to change their treatment, their doctor will take a sample of their blood and send it to the laboratory. The laboratory sequences a portion of the genetic material of the HIV  to see if the virus is resistant to any common HIV medications. The doctor can use the results from this test to pick the best HIV treatment for their patient. The sequence is then reported to the health department, where it is stored in a secure database. 

HIV changes over time in different ways in each person. When a group of people have viruses with similar molecular sequences, it suggests that little time has passed between when HIV was acquired and transmitted. This is often referred to as rapid transmission. When we see a larger than usual number of people newly diagnosed with HIV whose HIV is molecularly similar, it tells us that HIV is being transmitted rapidly in that network – which is a sign to us that people in that network aren’t able to access the services they need.

Molecular analysis has been used in other diseases and conditions – for example, it’s how we track different variants of COVID-19. However, we have only recently started using molecular analysis in HIV. People have raised concerns about the use of molecular analysis for HIV, particularly regarding privacy and confidentiality. Here in Washington, we’ve started to engage community members on this topic to share information, learn about their concerns, and see what we can do to respond. Read more about our community engagement process below.

How do we respond to clusters and outbreaks of HIV? 

Our Usual Process

When a person is newly diagnosed with HIV in Washington state, we contact that person individually to help link them to services they need, like medical care or housing. We also make sure any of their recent partners, including sex and needle sharing partners, get tested for HIV.

Most of the people living with HIV in Washington have access to antiretroviral medications, so they are undetectable and are not transmitting HIV to sex or needle sharing partners. And many people living without HIV in Washington, who may become exposed to it, have access to other prevention methods they need, like condoms and PrEP. So, usually, HIV spreads pretty slowly in Washington state.

Responding to Clusters of HIV

When rapid transmission of HIV is happening, it’s a sign to us that our usual system is not meeting the needs of the people involved. In these cases, we might decide a special response is needed. If so, public health field staff work together with community-based organizations, healthcare providers, and other partners to implement the response. This could include doing field outreach to do testing and education; making clinical services such as HIV treatment and PrEP more accessible; offering free and accessible prevention services such as condoms, PrEP, and safer injection supplies; and more, depending on what people linked to the cluster need most.

That’s why clusters are so important. If we notice the cluster and find the people involved quickly enough, we can provide services, interrupt transmission, and prevent new cases of HIV. ​

The response ends when all the people linked to the cluster have accessed HIV care and other services they need. Afterwards, we debrief to talk about how it went and what we can do better next time. ​

What’s happening in Washington?

Outbreak Response Plan

Response plans are an important way to make sure that if an outbreak occurs, we know what to do and who to contact. Of course, response activities will depend on who is part of the cluster and what they need; where the cluster is taking place and what resources are available; and what partners are in the area and need to be involved. A single response plan for the entire state can’t take into account all of those different factors. Our office has developed a statewide HIV Cluster and Outbreak Response Plan that outlines who in our office is involved in finding and monitoring clusters, and how we would plan a response. It is updated regularly, and we’re working with partners in local health jurisdictions and community-based contracted partners to help them develop the portion of the response plan that outlines specific community needs, resources, and partners.

Community Engagement

In 2022, we initiated a community engagement process to conduct outreach to and engagement of communities disproportionately impacted by HIV for the purpose of delivering information on HIV surveillance activities, gathering input and feedback on the state’s HIV outbreak response plan, responding to questions and concerns, and developing recommendations for future engagement of these communities. We worked with an external contractor, Brian Minalga, who facilitated eight virtual discussions with participants from across the state. The findings and recommendations from these sessions are shared in Brian’s final report (Washington State HIV Response Plan: 2022 Community Engagement Report (PDF)).

These sessions were just a first step in what we know will be an ongoing process of community engagement around these topics. Participants offered valuable feedback and suggestions on how to continue to engage community members and reach those most impacted by HIV in Washington state. We will draw on these recommendations as we continue to partner with community towards our goal to end the epidemic of HIV in Washington.



Chelsey Kaasa, Supplemental HIV Surveillance Supervisor