Most cases occur in children who are unvaccinated or incompletely vaccinated.
Risk to Travelers Invasive Hib disease occurs throughout the world. Few countries routinely use Hib vaccine, so invasive Hib disease remains common in many countries of the world. However, as of February 1999, TriHIBit is licensed only for use as the fourth dose of the Hib and DTaP series. It should not be given for the first, second, or third doses of the Hib series. The number of doses in the primary series depends on the type of vaccine used.