Comparability of Results from Pair and Classical Model Formulations for Different Sexually Transmitted Infections

The “classical model” for sexually transmitted infections treats partnerships as instantaneous events summarized by partner change rates, while individual-based and pair models explicitly account for time within partnerships and gaps between partnerships. We compared predictions from the classical and pair models over a range of partnership and gap combinations. While the former predicted similar or marginally higher prevalence at the shortest partnership lengths, the latter predicted self-sustaining transmission for gonorrhoea (GC) and Chlamydia (CT) over much broader partnership and gap combinations. Predictions on the critical level of condom use (Cc) required to prevent transmission also differed substantially when using the same parameters. When calibrated to give the same disease prevalence as the pair model by adjusting the infectious duration for GC and CT, and by adjusting transmission probabilities for HIV, the classical model then predicted much higher Cc values for GC and CT, while Cc predictions for HIV were fairly close. In conclusion, the two approaches give different predictions over potentially important combinations of partnership and gap lengths. Assuming that it is more correct to explicitly model partnerships and gaps, then pair or individual-based models may be needed for GC and CT since model calibration does not resolve the differences

Real-Time Epidemic Monitoring and Forecasting of H1N1-2009 Using Influenza-Like Illness from General Practice and Family Doctor Clinics in Singapore

10.1371/journal.pone.0010036PLoS ONE54

Critical level of condom use (<i>C_c</i>) predicted to prevent self-sustaining GC/CT and HIV transmission for the pair (A to D), classical uncalibrated (E to H), and classical model following calibration of <i>π</i> to the pair model output (I to L).

<p>The horizontal axes give partnership length in days while the vertical axes give gap length in days. <i>C_c</i> values are denoted by a gradient of colours as indicated; values of 0% demarcate the most extreme combination of partnership and gap lengths which supports self-sustaining transmission, while values above 100% (up to a theoretical maximum of 111% since condoms are assumed to be only 90% effective in preventing transmission) show partnership and gap combinations where consistent condom use is insufficient to prevent self-sustaining transmission.</p

Predictions from the classical and pair model formulations for the steady-state <i>π^s</i>of GC/CT (A and B), and the peak <i>π^p</i> of HIV with and without cofactor enhancement (C and D).

<p>The horizontal axes give partnership length in days while the vertical axes give <i>π</i>. The different lines denote predictions from using gap lengths () of 1 day, 7 days, 30 days and 90 days. The inset in each figure magnifies crossover point, if any, in the region where the classical and pair models diverge in <i>π</i> predictions. Models in (A) and (C) are unable to provide predictions at a gap length of 90 days.</p

Data from Lycke et al. [45] on case contact pairs for gonorrhoea and Chlamydia

1<p>Chlamydia.</p>2<p>Gonorrhoea.</p

Model parameters.

<p>Model parameters.</p

Absolute difference in predicted critical level of condom use (Abs(Δ<i>C_c</i>)) for GC/CT and HIV with and without cofactor enhancement (A to D), with its corresponding adjustment factor, (E to H).

<p>The horizontal axes give partnership length in days while the vertical axes give gap length in days. Abs(Δ<i>C_c</i>) is computed from the absolute difference in the corresponding values from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0039575#pone-0039575-g002" target="_blank">Figure 2</a>. Coloured bars in the left (A to D) and right (E to H) panels give the values of Abs(Δ<i>C_c</i>) and by the respective gradient of colours.</p