Reemergence of InvasiveHaemophilus influenzaeType b Disease in a Well‐Vaccinated Population in Remote Alaska

Abstract

Before vaccination, Alaska Natives experienced very high rates of invasive Haemophilus influenzae type b (Hib) disease and carriage. Vaccination with Hib conjugate vaccine PRP-OMP (polyribosylribitol phosphate Neisseria meningitidis outer membrane protein) began in 1991 and resulted in a sharp decline in cases. In 1996, after switching to a different Hib conjugate vaccine, DTP-HbOC (which combines diphtheria-tetanus-whole cell pertussis vaccines with HbOC [Hib oligosaccharide CRM197]), cases of invasive Hib disease increased, suggesting ongoing Hib transmission despite widespread vaccination. To determine the prevalence of and risk factors for carriage, a cross-sectional study of oropharyngeal Hib carriage was conducted among Alaska Native children aged 1-5 years in remote southwestern Alaska. Of 496 children with swabs taken, 46 (9.3%) were colonized with Hib. Carriage rates varied by village from 2.2% to 13.2% and by age from 6.1% in 1-year-olds to 14.7% in 5-year-olds. Crowding was associated with Hib carriage. Widespread vaccination with PRP-OMP Hib conjugate vaccine did not eliminate carriage in this population of Alaska Natives, and ongoing carriage contributed to disease resurgence.

The continuing role ofHaemophilus influenzaetype b carriage surveillance as a mechanism for early detection of invasive disease activity

“…56,57 A resurgence of Hib disease among Alaskan children in the 1990s, lead authors to suggest that special attention is required to eliminate Hib disease and carriage in high-risk populations. 11,53 As carriage surveillance informs of the Hib environment within a community, acting also as a surrogate for vaccine uptake in immunity studies. The presence of Hib nasopharyngeal carriage can indicate disease transmission dynamics.…”

“…19,20,59 Thus, to combat resurgence occurring despite active surveillance of invasive disease, some authors have suggested a change in approach. 11,53 This new approach identifies strategies for eliminating Hib disease focusing on the reservoirs of colonization within families and communities, and considering each case of active disease as a sentinel event for Hib transmission.…”

“…25 However, despite the marked disappearance of invasive Hib disease, some studies encompassing mostly Indigenous populations report persisting Hib carriage, albeit at lower levels than pre-vaccination carriage. 2,11,21,23,50,53 The issue of persistent Hib carriage in vaccinated populations occurs globally, mostly in minority populations, often despite high levels of vaccine uptake. 2,12 A study of Native Alaskans reported Hib oropharyngeal carriage as high as 9.3% in fully vaccinated under five year old children.…”

“…54 Other studies in Alaska found carriage rates up to 13.2% in a 97% vaccinated population, with authors concluding that carriage is the driving force behind Hib disease resurgence in remote Alaska. 53 More recently health researchers have reported resurgence of Hib carriage UK, in fully vaccinated school children. They hypothesize that high rates of carriage (4.8% in 6-12 y olds) during 2005 was due to a waning of antibody levels in the absence of a catch-up vaccine program.…”

“…[89][90][91] This paper presents summary information on additional medical or immunological risks based on the few publications available containing detailed risk information, from the UK, Canada and Alaska (US) ( Table 1). 20,53,57,92,93 Associated medical conditions reported for children experiencing true Hib country which does not offer routine Hib vaccination, indicated that Hib carriage commenced at age 3-5 mo, peaking at 18-21 mo. 58 Nasopharangeal carriage prior to three months is indicative of high risk or active infection, thus Hib carriage rates can serve as surrogate markers for disease occurrence and risk in some populations.…”