Epidemiology of invasive Haemophilus influenzae disease in northwestern Ontario: comparison of invasive and noninvasive H. influenzae clinical isolates.

In the post-Haemophilus influenzae type b (Hib) vaccine era, invasive H. influenzae type a (Hia) disease emerged in North American Indigenous populations. The role of Hia in noninvasive disease is uncertain; it is unknown whether noninvasive Hia infections are prevalent in populations with a high incidence of invasive disease, and whether invasive and noninvasive Hia isolates have different characteristics. We analyzed all invasive and noninvasive clinical H. influenzae isolates collected in a northwestern Ontario hospital serving 82% Indigenous population over 5.5 years. Serotyping, clonal analysis, and antimicrobial sensitivity testing were conducted on 233 noninvasive and 20 invasive isolates. Among noninvasive isolates, 91% were nontypeable (NTHi) and 3% were Hia; Hia was the most frequent invasive isolate (60%). Incidence rates of invasive H. influenzae disease (12.5/100 000/year) greatly exceeded average provincial data, with the highest found in <6-year-old children (63.9/100 000/year); the proportion of Hia among invasive isolates was seven times larger than in Ontario. No difference in clonal characteristics between invasive and noninvasive Hia isolates was found. Antibiotic resistance was more common among NTHi than among encapsulated isolates, without differences between invasive and noninvasive isolates. Considering the significance of Hia in Indigenous populations, pediatric immunization against Hia will be useful to prevent serious infections in young Indigenous children.