Economisation of vaccination against Haemophilus influenzae type b: a randomised trial of immunogenicity of fractional-dose and two-dose regimens

Background The cost of Haemophilus influenzae type b (Hib) conjugate vaccines has limited their use in non-industrialised countries. To identify more economical vaccination schedules, we carried out a randomised trial of the immunogenicity of alternative regimens to the standard three-dose series. Methods 627 Chilean infants were randomly allocated to one of four regimens with either Hib polysaccharide-tetanus toxoid conjugate vaccine (PRP-T) or Hib oligosaccharide-diphtheria mutant toxoid conjugate vaccine (PRP-CRM197), for a total of eight groups. All infants receive diphtheria-tetanus-pertussis (DTP) vacine at ages 2, 4, and 6 months. The regimens included three full doses, three functional doses consisting of one half or one third of the full dose, and a regimen of two full doses (at age 4 and 6 months). The primary outcome was the proportion of infants with serum anti-polyribosylribitol phosphate (PRP, the type b capsular polysaccharide) concentrations of 0•15 μg/mL or more at age 8 months. Findings 93% (95% CI 85–98) of infants vaccinated with three full doses of PRP-T or PRP-CRM197 (95% CI 84–98) achieved anti-PRP concentrations of 0•15 μg/mL or more at age 8 months, compared with 91% (83–96) to 100% (95–100) of infants immunised with any fractional-dose regimen. Of the infants vaccinated with two doses of PRP-T or PRP-CRM197, 99% (93–100) and 87% (77–93) developed anti-PRP concentrations of 0•15 μg/mL or more, respectively. Interpretation 91% (83–96) to 100% (95–100) of infants immunised with one-half or one-third of a full dose of Hib conjugate developed protective antibody concentrations. Carrier priming with DTP may make two-dose schedules an option in some places. These alternative regimens could bring the cost of Hib vacines within reach of countries that currently cannot afford them.