Internet-based surveillance of Influenza-like-illness in the UK during the 2009 H1N1 influenza pandemic.

BACKGROUND: Internet-based surveillance systems to monitor influenza-like illness (ILI) have advantages over traditional (physician-based) reporting systems, as they can potentially monitor a wider range of cases (i.e. including those that do not seek care). However, the requirement for participants to have internet access and to actively participate calls into question the representativeness of the data. Such systems have been in place in a number of European countries over the last few years, and in July 2009 this was extended to the UK. Here we present results of this survey with the aim of assessing the reliability of the data, and to evaluate methods to correct for possible biases. METHODS: Internet-based monitoring of ILI was launched near the peak of the first wave of the UK H1N1v influenza pandemic. We compared the recorded ILI incidence with physician-recorded incidence and an estimate of the true number of cases over the course of the epidemic. We also compared overall attack rates. The effect of using different ILI definitions and alternative denominator assumptions on incidence estimates was explored. RESULTS: The crude incidence measured by the internet-based system appears to be influenced by individuals who participated only once in the survey and who appeared more likely to be ill. This distorted the overall incidence trend. Concentrating on individuals who reported more than once results in a time series of ILI incidence that matches the trend of case estimates reasonably closely, with a correlation of 0.713 (P-value: 0.0001, 95% CI: 0.435, 0.867). Indeed, the internet-based system appears to give a better estimate of the relative height of the two waves of the UK pandemic than the physician-recorded incidence. The overall attack rate is, however, higher than other estimates, at about 16% when compared with a model-based estimate of 6%. CONCLUSION: Internet-based monitoring of ILI can capture the trends in case numbers if appropriate weighting is used to correct for differential response. The overall level of incidence is, however, difficult to measure. Internet-based systems may be a useful adjunct to existing ILI surveillance systems as they capture cases that do not necessarily contact health care. However, further research is required before they can be used to accurately assess the absolute level of incidence in the community.

Polygyny and symmetric concurrency: comparing long-duration sexually transmitted infection prevalence using simulated sexual networks.

OBJECTIVE: To compare the effects of polygyny (only men can form concurrent partnerships) and gender-symmetric concurrency (both genders can form concurrent partnerships) on prevalence of long-duration sexually transmitted infections (STIs) using a dynamic stochastic network model. METHODS: We modelled two pairs of scenarios: polygyny and gender symmetry at higher and lower levels of network concurrency (measured by the average number of concurrent partnerships per partnership). The same level of sexual activity was modelled in all scenarios (measured by mean per capita partnership incidence and per capita number of sex acts). Partnership duration and network concurrency were constant within each of the polygyny/symmetry pairs. Infections that mimicked characteristics of herpes simplex virus type 2 (HSV2) and HIV were introduced onto the networks separately. The mean prevalence 100 years after introduction for the HSV2-like infection and 30 years after introduction for the HIV-like infection were calculated over 1000 model iterations. RESULTS: Prevalence of both simulated STIs was significantly lower in the polygyny scenarios than in the symmetry scenarios. At lower concurrency, polygyny resulted in a relative reduction in HSV2-like infection prevalence of 19% (95% CI 15 to 23) compared to gender symmetry. At higher concurrency polygyny led to a relative reduction of 20% (16 to 23). The relative reduction in prevalence of the HIV-like infection after 30 years was 14% (10 to 17) at lower concurrency and 8% (5 to 11) at higher concurrency. CONCLUSIONS: Polygyny can result in lower STI prevalence compared to populations where both genders practise concurrency. Further work is required to explore whether this reduction is observed when modelling more realistic populations and infection characteristics.