[1]) and assuming that vaccination does not affect duration of colonisation. The main find more factor affecting how the bias in the estimated vaccine efficacy becomes negligible is the prevalence of colonisation at the time of vaccination. When the prevalence is close to 0 (left-hand panel), the mean of VEacq estimates from cross-sectional data closely approximate the true VEacq as long as the samples are collected

2–3 months after vaccination. When the prevalence of colonisation is higher (right-hand panel), the bias is initially clearly negative and becomes relatively small only after several months since vaccination. As a rule-of-thumb for both scenarios, the time from vaccination until nasopharyngeal SB431542 purchase sampling is determined by the rate of clearance rather than the rate of pneumococcal acquisition. This is shown by comparison between the “high” vs. “moderate” scenarios for overall acquisition in Fig. 1. Under both scenarios, colonisation should be sampled

only after at least twice the average duration of a carriage episode has passed since the immune-response. In the example, the mean duration was approximately 2 months and the sampling should thus occur 4 months after the immuno-response or somewhat later. The results for the combined vaccine efficacy against acquisition and duration (VET) were similar (data not shown). Apart from the requirement of approximate steady-state at the time

of sampling, Idoxuridine there are other factors that rather favour early measurement of colonisation (e.g. the possibility of waning immunity or changes in exposure with age and/or season). In addition to bias, the precision of estimation and sample size (cf. Section 5) need to be considered. In general, the precision was poor in the first 2 months, in particular with low individual prevalence and moderate rate of pneumococcal acquisition (data not shown). Also serotype-specific estimates can be obtained from a cross-sectional study (cf. Section 4 in [1]). In general, their estimation performs similarly to the aggregate (i.e., all vaccine-type) efficacy. For serotypes with very low prevalence, however, the negative bias in the efficacy estimates is obviously somewhat bigger unless the sample size is very large. The sensitivity of detecting pneumococcal colonisation depends on the technique of specimen sampling and handling, and the methodology to culture, identify and serotype pneumococci [2]. The current standard, which is based on using a single nasopharyngeal swab to measure the prevalence of pneumococcal carriage, is simple and rapid. The sensitivity of a single swab to detect and identify the dominant pneumococcal serotype is high, being in the range of 85–100% [2], [3] and [4]. A key challenge to nasopharyngeal sampling remains the identification of multiple serotypes simultaneously colonising the nasopharynx.