Before the vaccine apparently 1 in 200 children under the age of five developed invasive hib disease. 15-30 percent of those became hearing impaired and 420 died children a year died and it was the leading cause of mental retardation in the united states. It's difficult to account for all of that but the hearing impairment because I don't really know how to come up with how many children under five there were (about 3.4 - 4 million kids born a year so 15-20 million, maybe?) but just looking at hearing impairment, 15- 30% of 1 in 200 is a .007%-.014% chance of hearing impairment from hib before the vaccine.
what is the leading bacterial infection now? from my understanding, and I am open to correction, is that there is always a "leading" bacterial infection. first it was HiB, then various strains of pneumococcal pneumonia, then another set of strains of pneumococal pneumonia...each time a new leading one comes out, a vaccine follows. Are that many children being impaired or killed by HiB, no, but are that many children being impaired or killed by some other, newer, possibly more deadly bacterial infection.
More info on serotype replacement is in the thread below.