The impact of specific and non-specific immunity on the ecology of Streptococcus pneumoniae and the implications for vaccination

More than 90 capsular serotypes of Streptococcus pneumoniae coexist despite
competing for nasopharyngeal carriage and a gradient in fitness. The underlying
mechanisms for this are poorly understood and make assessment of
the likely population impact of vaccination challenging. We use an individual-
based simulation model to generalize widely used deterministic models
for pneumococcal competition and show that in these models short-term
serotype-specific and serotype non-specific immunity could constitute the
mechanism governing between-host competition and coexistence. We find
that non-specific immunity induces between-host competition and that serotype-
specific immunity limits a type’s competitive advantage and allows
stable coexistence of multiple serotypes. Serotypes carried at low prevalence
show high variance in carriage levels, which would result in apparent outbreaks
if they were highly pathogenic. Vaccination against few serotypes can
lead to elimination of the vaccine types and induces replacement by others.
However, in simulations where the elimination of the targeted types is
achieved only by a combination of vaccine effects and the competitive pressure
of the non-vaccine types, a universal vaccine with similar-type-specific effectiveness can fail to eliminate pneumococcal carriage and offers limited herd
immunity. Hence, if vaccine effects are insufficient to control the majority of
serotypes at the same time, then exploiting the competitive pressure by
selective vaccination can help control the most pathogenic serotypes.