Immunogenicity of H. Influenzae Type b PRP-OMP Vaccines in American Indian and Alaska Native Children (the HibVax Study)

Historically, American Indian and Alaska Native (AI/AN) children aged \<5 years have experienced invasive H. influenzae type b (Hib) disease at a rate that is at least 5 times higher than the general U.S. population. In the pre-vaccine era, the incidence of Hib disease peaked earlier for AI/AN children at 4-5 months than general US children at 6-9 months. Therefore, prevention efforts for AI/AN populations focused on identifying a vaccine that would protect against disease in early infancy. Studies in AI/AN children revealed that the Hib conjugate vaccine with the capsular polysaccharide (polyribosylribitol phosphate polysaccharide \[PRP\]) coupled to the outer membrane protein complex of Neisseria meningitidis (OMP) induced anti-PRP IgG titers that correlated with protection (GMC ≥0.15 μg/mL) and demonstrated high efficacy after a single dose in infancy. Hib PRP-OMP was licensed in 1991 as PedvaxHIB; following introduction of a two-dose primary series and a booster dose, the rate of Hib disease decreased substantially among AI/AN children. The importance of PRP-OMP vaccine to disease control was highlighted in the 1990s in Alaska when use of a non-PRP OMP Hib vaccine was associated with an increase in disease incidence in AN children. In 1999, the American Academy of Pediatrics Committee for Native American Child Health released its official preference for Hib PRP-OMP for use in AI/AN populations.

In spite of Hib vaccine coverage similar to or greater than the national average, AI/AN populations periodically experience pediatric cases of invasive Hib disease. In contrast to the pre-Hib vaccine era, a majority of these cases occur in fully vaccinated children beyond the first year of life. This epidemiologic shift in the age at which disease occurs may indicate ongoing transmission in the presence of waning immunity following vaccination.

Vaxelis is a licensed hexavalent combination vaccine that contains Hib PRP-OMP and antigens (Hepatitis B surface antigen, Diphtheria Toxoid, Tetanus Toxoid, Acellular Pertussis \[DTaP\], and Inactivated Polio Virus \[IPV\]) to protect against diseases caused by 5 other organisms. Vaxelis contains 3.0 µg/mL of PRP-OMP antigen, compared to 7.5 µg/mL in PedvaxHIB. This lower concentration of PRP-OMP has been shown to be less reactogenic and similarly immunogenic to higher doses. Vaxelis is approved for administration as a 3-dose primary series at 2, 4, and 6 months of age. A booster dose with a different licensed Hib vaccine (e.g., PedvaxHIB, ActHib) is required at 12-15 months. Vaxelis Hib PRP-OMP was found to be highly immunogenic post-dose 2 at 4 months and post-dose 3 at 6 months. However, immunogenicity post-dose 1 was not measured in the phase 3 clinical trials of this vaccine.