The Health and Social Care Act 2001 authorizes the HPA to process confidential patient information for public health purposes (www.legislation.hmso.gov.uk/si/si2002/20021438.htm). HPA provides a national identification and serotyping service for invasive clinical H. influenzae isolates for all National Health Service hospital microbiology laboratories in England and Wales through its Haemophilus Reference Unit and conducts enhanced national surveillance through a combination of isolate submission, routine laboratory reporting, and clinical reporting schemes (3,18). Clinicians, microbiologists, and public health physicians are encouraged to report invasive H. influenzae cases to the HPA and submit isolates for species confirmation and serotyping (19). The HPA also receives electronic reports of clinically significant invasive pathogens from microbiologists in ≈400 National Health Service laboratories who are routinely contacted to submit isolates to the HPA if not already done, as well as notifications of all H. influenzae–related deaths from the Office for National Statistics (ONS) (www.statistics.gov.uk) (18).

We defined invasive H. influenzae disease as isolation of the organism from a normally sterile site. Localized infections such as pneumonia were included if accompanied by a sterile site isolate. Isolates were confirmed as H. influenzae by growth requirement for X and V factors (20) and ompP2-specific PCR positivity (21). Capsulation status was determined by PCR by using bexA-specific primers (22). Capsular type was confirmed by capsule-specific PCR by using primers for types Hia–Hif (23) and slide agglutination (20).

Laboratory-confirmed Hif and Hie infections diagnosed during 2009–2010 were followed up 3 months after diagnosis by sending a questionnaire to the patient’s general practitioner (www.hpa.org.uk/Topics/InfectiousDiseases/InfectionsAZ/HaemophilusInfluenzaeTypeB/EnhancedSurveillanceHaemophiliusInfluenzae) and requesting a copy of the hospital discharge summary. A reminder letter was sent to nonresponders after 8 weeks; missing or outstanding information was obtained 1 month later by telephone. Postmortem reports were obtained for all fatal cases that were referred to a coroner. In July 2011, the final outcomes of all patients were confirmed, and causes of death were obtained from the ONS death registrations dataset. Hie/Hif–attributable death was defined as isolation of a pure growth of Hie/Hif from a normally sterile site with clear clinical, radiologic, or histopathologic evidence of invasive bacterial infection and appropriate exclusion of other diagnoses.

Multilocus sequence typing (MLST) was performed according to methods previously described (24). Sequence types (STs) were assigned by using an online reference database (http://haemophilus.mlst.net). Minimum-spanning trees were constructed by using Bionumerics version 6.1 software (Advanced Maths, Sint-Martins-Latem, Belgium) as described (25).

Data were analyzed by using Stata version 11.0 (StataCorp LP, College Station, TX, USA). Categorical variables were expressed as proportions and compared by using the χ² or Fisher exact test. Data that did not follow a normal distribution were presented as medians with interquartile ranges (IQR). Annual population estimates were obtained from ONS. The binomial method was used to calculate 95% CIs for incidence rates. To estimate any increase in Hie/Hif incidence over time, an overdispersed Poisson regression model was fitted for the annual number of Hie/Hif cases. Yearly changes in population and proportion of total H. influenzae isolates serotyped were included in the model as offsets.

In England and Wales, encapsulated H. influenzae other than Hib rarely cause invasive disease; among those, Hie and Hif are most common. These serotypes are similar to ncHi in that they cause invasive disease mainly in older adults, who often have comorbidities and present with pneumonia. However, although Hie is 3× less common than Hif, it appears to be more virulent, with more complications of meningitis and higher infection-attributable CFR, even after adjustment for age and comorbidities.

The HPA national reference service for H. influenzae species confirmation and serotyping provides a rich source of epidemiologic and molecular surveillance data, and the consistently high proportion of serotyped isolates with data enhancement from multiple sources further strengthens the quality and completeness of the surveillance program (18). This data source enables more accurate interpretation of trends in disease rates and pathogens over time, as demonstrated by the relatively stable trends in the rare H. influenzae serotypes over the past decade. Despite the small number of cases, our results show small but gradual annual increases in invasive Hie and Hif disease. These increases are most likely to be a consequence of 1) the expansion of at-risk populations in industrialized countries resulting from increased survival rates among those at the extremes of age, who often have long-term comorbidities; 2) better survival prognosis among patients with malignancy; and 3) increased use of immunosuppressive therapy for immunologic and rheumatologic conditions.

Other countries have also reported an increase in invasive Hif disease since routine Hib vaccination was introduced. In 1 surveillance study involving multiple states in the United States, Hif incidence increased from 0.14/100,000 population in 1989 to 1.9/100,000 in 1994 (13); in Utah, Hif incidence increased from 0.14/100,000 person-years during 1998–2008 to 0.48/100,000 during 2007–2008 (8). The US Active Bacterial Core surveillance also recently reported an increase in invasive Hif disease, from 0.06 cases/100,000 population in 1989 to 0.25/100,000 in 2008 (16). In Sweden, invasive Hif disease increased 2.3% annually during 1997–2009 (10); however, other studies, including a recent study in Europe involving national surveillance data from 14 countries over 11 years, did not show any changes in Hif incidence (7,26). In relation to Hie, perhaps because of the small number of invasive cases caused by this serotype, we identified only the recent US Active Bacterial Core surveillance study that reported a small increase in disease incidence over 2 decades (16).

In addition to national epidemiologic surveillance, we prospectively collected clinical information for all laboratory-confirmed infections. Most published reports of Hie and Hif infection are in the form of individual cases, with few studies reporting a sufficient number of cases focusing on clinical aspects of specific H. influenzae serotypes (10,13,27). Our results indicate that Hie and Hif share many features characteristic of invasive ncHi disease, such as patient age distribution, higher risk for infection among vulnerable populations, and higher CFR compared with Hib (7). Certain features that were previously attributed primarily to ncHi, such as causing hepatobiliary disease (28,29) and disease during pregnancy (30), were also observed among Hie and Hif case-patients, respectively, in our study. The proportion of case-patients with comorbidities in our cohort was similar to previous smaller studies (60%–80%) (13,27,31–34) and to that among ncHi case-patients (13,26,33,35–37). Several Hif case-patients had disorders of B-cell immunity, such as chronic lymphocytic leukemia and multiple myeloma, which have also been reported among persons with invasive ncHi; this finding suggests a major role for humoral immunity (10). Declining B-cell function has also been speculated to explain the increasing incidence with age (10). The finding that none of the infants in this study had any reported comorbidities at infection, however, is intriguing. Most children and adults outside this age group had comorbidities, which suggests that some of these infants might have as-yet-undetected subtle immunologic abnormalities that made them vulnerable to opportunistic infections.

Although Hie and Hif should be compared with caution because of the relatively small number of cases, our results suggest that Hie causes more severe clinical disease. This observation is supported by recent population-based studies that have reported a worse outcome for patients with Hie than for those with Hif, including a CFR of 24% versus 10.0% (p = 0.003) in a study in Europe (7) and 32% versus 6% (p = 0.002) in a US study (38). One possible reason that Hie has a lower incidence but it is more fatal than Hif could be that this serotype is less pathogenic and infects persons who are older and/or in much poorer health and who, therefore, are more likely to die. This hypothesis, however, is not supported by our study because the median age at disease onset and the prevalence of any or multiple comorbidities were similar, and the 7-day CFR remained significant even after adjusting for these risk factors. However, the 6-month CFR for the 2 serotypes was similar, which suggests that these pathogens can infect highly vulnerable persons who, even if they survived the infection, subsequently succumbed to their underlying illness; this finding has been also observed for invasive ncHi disease (10,35). For example, a recent study in Sweden reported that the 28-day CFR for invasive ncHi disease was 8%, but 1-year CFR increased to 29% (10).

MLST analysis of the clinical isolates revealed dominant Hie (ST18) and Hif (ST124) types that have been reported among bacterial strains from Europe and North America (http://haemophilus.mlst.net) (24). In 2 recent independent studies in Canada, ST124 was also the dominant Hif type, and the Hie types detected differed from ST18 at only a single MLST locus (9,39). None of the Hie or Hif MLST types in our study have been reported outside of their respective serotypes. These observations, combined with phylogenetic analysis of MLST sequences across capsulated and noncapsulated H. influenzae isolates in our and other studies (24,40), suggest that any increase in Hie and Hif disease is not due to genetic capsule switching events between these serotypes and Hib.

In conclusion, invasive Hie and Hif infections are rare in England and Wales, but their incidence is increasing slowly. The clinical features are similar to ncHi, although Hie appears to be associated with more severe disease and worse infection-attributable outcome. Given that most infections occur among persons with comorbidities, it would be prudent to investigate all previously healthy persons in whom Hie/Hif infections develop for possible underlying immune deficiency, malignancy, or other undiagnosed conditions. Phylogenetic analysis of clinical isolates showed no evidence of serotype replacement by capsule switching between Hie or Hif and Hib and no association between sequence types and clinical disease or outcome.