Originally Posted by Marnica
google serotype replacement.
It's discussed in the (well referenced) wikipedia article about PCV.
Basically a "serotype" is one of the versions of bacteria which cause pneumoccal disease. And serotype replacement refers to the most common types of bacteria causing these diseases shifting away from the strains in the vaccination.
From that (go to link to see references):
The original 7-valent formulation [of prevnar] contains serotypes 4,6B,9V,14,18C,19F, and 23F, and results in a 98% probability of protection against these strains, which caused 80% of the pneumococcal disease in infants in the US. In 2010, Pfizer introduced Prevnar 13 which contains six additional strains (i.e., 1, 3, 5, 6A, 19A and 7F), which protect against the majority of the remaining pneumococcal infections
While an overall decline in invasive pneumococcal disease is well documented, concerns have been raised regarding a potential increase in the rate of infections caused by serotypes not covered in the vaccine. Recent data suggest that serotype replacement is increasing (1.61- and 1.28-fold increase in children and adults) but remains minimal when compared to the significant reduction observed in the burden of this vaccine-preventable disease.
It seems to me this could be a natural result of a successfull vaccination program. If the serotypes in prevnar (and now prevnar 13) are becoming less common because of vaccination, then while overall rates of pneumoccocal go down you would still see a proportional increase in cases caused by serotypes not in the vaccine.
Is it something more than this?