Clusters of Invasive <i>Haemophilus influenzae</i> Type b Disease Among Adults Using Substances or Experiencing Homelessness or Housing Instability — Alaska, Oregon, and Washington, 2023─2025

Since the introduction and widespread use of Haemophilus influenzae type b (Hib) conjugate vaccines, invasive Hib disease has become rare in the United States, and outbreaks are uncommon.During April 2023-December 2025, two genetically distinct clusters comprising 44 cases of invasive Hib disease in adults were identified: one in Alaska (14 cases) and a second spanning Washington (23) and Oregon (seven).Cases were identified through routine surveillance or notification from a hospital, and clusters were identified via whole-genome sequencing.The median patient age was 53.5 years; 43 (98%) persons had bacteremia and 42 (95%) had pneumonia.Among the 44 patients, 34 (77%) smoked one or more substances, 34 (77%) used illicit substances, and 30 (68%) were experiencing homelessness or housing instability.Overall, 40 (91%) patients did not have documentation of receipt of Hib vaccination; 35 (88%) of those would not have been eligible for routine Hib conjugate vaccination as children because they would have been older than the recommended age range for vaccination at the time the vaccine was introduced.These emerging Hib clusters reveal that adults, particularly those using substances or experiencing homelessness or housing instability, are at risk for this otherwise rare vaccine-preventable disease.Data to guide an optimal public health response to these clusters are limited.Expanding surveillance for invasive H. influenzae disease in adults could help assess the scope of this problem, identify future outbreaks, and guide the development and implementation of strategies for prevention.