Firstly, vaccination may reduce the individual’s susceptibility to acquisition of colonisation. In general, susceptibility to acquisition is quantified by the rate of acquisition in those not colonised or otherwise considered susceptible to acquire the target (vaccine) serotypes (cf. [11] and [15]).

Lumacaftor cell line Secondly, vaccination may enhance the clearance of colonisation so that duration of future colonisation is shortened. Thirdly, vaccination may decrease the density of future colonisation, i.e. the quantitative load of pneumococcal carriage in the nasopharynx, as compared to a non-vaccinated carrier. All these three primary endpoints (acquisition, duration, density) can be considered either specific to the individual protective components of the vaccine or “overall” in an aggregate manner. For example, for PCVs, the serotypes included in a vaccine formulation can be considered mTOR inhibitor either individually or as a set of all vaccine serotypes. Although the main interest often lies in estimating the aggregate efficacy against all vaccine serotypes, vaccine effects on non-vaccine serotypes are also important if serotype replacement is considered (see Section 3). In addition to the primary endpoints,

various summary endpoints can be used to quantify vaccine effects on colonisation. In particular, a combined endpoint involving both acquisition and duration proves to have many desirable epidemiological properties. It is defined as T = (hazard rate of acquisition) × (mean duration of colonisation).

The risk of T is related to a susceptible individual’s expected (i.e. future) time spent colonised and thereby capable of spreading the organism. If transmissibility varies over the course of the colonisation episode, T is only an approximation of an individual’s capacity to spread pneumococci. Cell press However, even in this case T is likely to offer a feasible measure of transmission potential (cf. [16]). Moreover, if the density of colonisation is associated with both the transmissibility and the sensitivity of detection of colonisation, T reflects the transmission potential even without factoring density explicitly in this parameter. Finally, vaccine efficacy based on T can be estimated from cross-sectional data under weak assumptions about the colonisation processes in the study subjects, which makes it a particularly useful endpoint (see Section 4 for further discussion on this issue). There is evidence for current PCVs reducing a vaccinated individual’s susceptibility to acquisition of pneumococcal serotypes included in the vaccine formulation. This has been shown most clearly by studies addressing the effect of vaccination on early acquisition in infants. Lower levels of VT colonisation prevalence among the vaccinated infants as compared to the controls have been reported soon after immunisation, i.e.