Hib Disease Clusters Among Adults Who Use Drugs or Lack Stable Housing, 2023–2025

In brief

• Since Haemophilus influenzae type b (Hib) conjugate vaccines arrived in the U.S. in 1987, outbreaks of invasive Hib disease have become uncommon.
• From April 2023 to December 2025, two clusters totaling 44 cases were found among adults in Alaska, Oregon and Washington. Most patients were too old to have been vaccinated routinely as children.
• Smoking (77%), illicit drug use (77%) and homelessness or housing instability (68%) were common.
• Better surveillance of invasive H. influenzae disease in adults could show how big the problem is, find future clusters, and guide protection for people at risk.

Background

Before the vaccines, Hib was a leading cause of meningitis, epiglottitis, pneumonia and septic arthritis in children under 5. Routine childhood vaccination made invasive Hib rare in children and adults, though American Indian and Alaska Native people remain disproportionately affected. Recently, British Columbia reported a rise among adults experiencing homelessness or housing instability and using substances, including excessive alcohol.

Invasive H. influenzae disease is nationally notifiable, but what states report varies. In Alaska, adult Hib cases are found through routine statewide surveillance, and a rise was reported to CDC in March 2025. In Washington, invasive Hib was reportable only in children under 5; an academic hospital noticed more adult cases and alerted the local health department. In Oregon, all ages are reportable, through CDC's Active Bacterial Core surveillance. A cluster was two or more confirmed cases — Hib grown from a normally sterile site — whose isolates were closely related by whole-genome sequencing. Patient details came mainly from medical records.

The clusters

The two clusters of invasive Hib disease among adults, Alaska, Washington and Oregon, 2023–2025. CDC, MMWR.

All isolates but one were sequence type 6 (ST-6); one from Washington was ST-3134. The two clusters differed by more than 200 single-nucleotide polymorphisms. (One more adult case in Oregon did not fit either cluster and was left out.)

The patients

Only 4 (9%) had any record of Hib vaccination. Of the other 40, 35 (88%) were born before 1986 — too old for routine childhood vaccination when the vaccine was introduced. (Marijuana, legal for adults in all three states, is not counted as illicit here.)

What it means

These clusters show that adults — especially those using substances or without stable housing — are at risk from a disease that is otherwise rare and preventable by vaccine. Most were never vaccinated, and any immunity from exposure or vaccination may have waned.

• Substance use raises infection risk in many ways, including by altering immune responses; few patients had a documented immune condition, but most used drugs.
• Smoking may be an important contributor: a 2019 meta-analysis linked tobacco smoking to community-acquired pneumonia, and 95% of these patients had pneumonia — against 37 (50%) of 74 adults with invasive Hib in surveillance data for 2014–2023.
• The strain alone probably isn't the driver: two distinct ST-6 clusters here, and several sequence types in British Columbia. Both ST-6 and ST-3134 belong to clonal complex 6, which an older, small study found responsible for most invasive Hib; CDC did not routinely sequence Hib isolates before 2026.
• Investigating is hard: patients can be hard to reach, drug-use networks are shifting and hidden, Hib may spread through people without symptoms, and clusters may go unseen where adult Hib isn't reportable.
• Vaccination: small studies suggest one dose of Hib conjugate vaccine is immunogenic and well tolerated in adults, including some with immune conditions. A vaccination campaign in British Columbia was followed by fewer cases, and Anchorage is offering Hib vaccine to adults experiencing homelessness. Larger cities would face resource limits and might need to target those at highest risk.

Limitations: cases may have been missed, especially in Washington; substance use and housing instability, taken from medical records, may be underestimated or misclassified; and these regional clusters may not represent the nation.

State health departments are asked to notify CDC of adult cases of invasive Hib among people using substances or experiencing homelessness, and of any adult clusters.

Sources

Based on Jennifer P. Collins and others, "Clusters of Invasive Haemophilus influenzae Type b Disease Among Adults Using Substances or Experiencing Homelessness or Housing Instability — Alaska, Oregon, and Washington, 2023–2025," MMWR Morbidity and Mortality Weekly Report, volume 75, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The sequencing methods and references are left out.