The Hib vaccination worked too well. It prevented carriage of the bacteria which no one expected. This has changed how Hib circulates but no one is quite sure exactly how or what implications this has. The big risks now are that we don't get (or boost) natural immunity to it and that other types of Hi may become more prevalent and even more invasive. In general, the scientific consensus is that there is no serotype replacement going on but others have sounded warnings after observing a slow and steady increase in invasive infections caused by non-b Hi. Also, Hib is on the rise in a couple other countries even though the vaccine is still being used -- and it is speculated that one of the big reasons may be the change in carriage and circulation.
Overall, from what I have read, I've gathered that under the age of three, children have less of an ability to react to Hib if it invades. This isn't to say they can't mount any kind of an immune response. Our immune systems are complicated and pretty good at defending us but Hib is pretty virulent and good at invading. This is what puts kids under 5 at greater risk.
The question a few of us have been tossing around though is whether or not older (5+) non-vaccinated kids might remain or become more at risk given that they don't get a chance to develop or boost natural immunity.
It is funny because the articles I've read against serotype replacement have speculated that Hib wasn't very common pre-vaccine era so that is why we aren't seeing much of a difference once carriage was eliminated in kids. Personally, I think that is a poor guess. If the opposite was true and Hib was common in kids but not as commonly invasive as we tend to think it is, then we might only see a slow and steady increase in non-type b infections. Hah, lots of armchair research, here.

Also, I think breastfeeding is protective and that a kid's risk probably increases when weaned - no matter what age that is. In fact, in some European countries where the average age of weaning is older, the risk period for Hib is later than in the US. And in general, the longer you wait to wean, probably the better.
(Sorry, no references separating actual research from my opinions! I'm a bit disorganized as of late.

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