Transmission probabilities and durations of immunity for three pathogenic group B Streptococcus serotypes

Abstract

Group B Streptococcus (GBS) remains a major cause of neonatal sepsis and is an emerging cause of invasive bacterial infections. The 9 known serotypes vary in virulence, and there is little cross-immunity. Key parameters for planning an effective vaccination strategy, such as average length of immunity and transmission probabilities by serotype, are unknown. We simulated GBS spread in a population using a computational model with parameters derived from studies of GBS sexual transmission in a college dormitory. Here we provide estimates of the duration of immunity relative to the transmission probabilities for the 3 GBS serotypes most associated with invasive disease: Ia, III, and V. We also place upper limits on the durations of immunity for serotype Ia (570 days), III (1125 days) and V (260 days). Better transmission estimates are required to establish the epidemiological parameters of GBS infection and determine the best vaccination strategies to prevent GBS disease.

Figure 1

Distributions of relationship lengths, gap lengths, and rates of sexual contacts for male and female college freshmen (n = 299) in the study of Foxman et al. [20].

Figure 2

Probability per sex act that a susceptible individual will become infected, for females (β_f) and males (β_m), for various values of 1/δ, the average duration of immunity. The individual panels show (a) the general case (no serotype specificity), (b) serotype Ia, (c) serotype III, and (d) serotype V. Open circles represent the values of β_f and filled circles the values of β_m. Error bars are standard deviations on the sample mean for twenty repetitions of the calculation.