The representativeness of a European multi-center network for influenza-like-illness participatory surveillance

Abstract

Background: The Internet is becoming more commonly used as a tool for disease surveillance. Similarly to other surveillance systems and to studies using online data collection, Internet-based surveillance will have biases in participation, affecting the generalizability of the results. Here we quantify the participation biases of Influenzanet, an ongoing European-wide network of Internet-based participatory surveillance systems for influenza-like-illness.

Methods: In 2011/2012 Influenzanet launched a standardized common framework for data collection applied to seven European countries. Influenzanet participants were compared to the general population of the participating countries to assess the representativeness of the sample in terms of a set of demographic, geographic, socio-economic and health indicators.

Results: More than 30,000 European residents registered to the system in the 2011/2012 season, and a subset of 25,481 participants were selected for this study. All age classes (10 years brackets) were represented in the cohort, including under 10 and over 70 years old. The Influenzanet population was not representative of the general population in terms of age distribution, underrepresenting the youngest and oldest age classes. The gender imbalance differed between countries. A counterbalance between gender-specific information-seeking behavior (more prominent in women) and Internet usage (with higher rates in male populations) may be at the origin of this difference. Once adjusted by demographic indicators, a similar propensity to commute was observed for each country, and the same top three transportation modes were used for six countries out of seven. Smokers were underrepresented in the majority of countries, as were individuals with diabetes; the representativeness of asthma prevalence and vaccination coverage for 65+ individuals in two successive seasons (2010/2011 and 2011/2012) varied between countries.

Conclusions: Existing demographic and national datasets allowed the quantification of the participation biases of a large cohort for influenza-like-illness surveillance in the general population. Significant differences were found between Influenzanet participants and the general population. The quantified biases need to be taken into account in the analysis of Influenzanet epidemiological studies and provide indications on populations groups that should be targeted in recruitment efforts.

Figure 1

Flow chart of Influenzanet data collection. The schematic diagram illustrates the processes of registration, account confirmation, and data collection through intake and weekly symptoms surveys.

Figure 2

Geographic distribution of Influenzanet participants at the level of NUTS2 regions. The color code indicates the relative difference between the geographic distribution of Influenzanet population and the corresponding general population data. The map was created with the collected data using ArcGIS Software and publicly available geographic datasets [25].

Figure 3

Gender and age profiles of Influenzanet population and comparison with the general population. Gender repartition is displayed for each country and aggregated for all countries (a); age profile in 10-years classes per gender is shown aggregated for all countries (country level statistics are reported in Additional file 1: Figure S4) (b).

Figure 4

Age profile of Influenzanet participants and comparison with the general population per country. Age distribution is shown in 10-years age classes. Country profiles by age and gender are reported in Additional file 1: Figure S4.

Figure 5

Quantitative analysis of the Influenzanet commuting network. (a) Linear correlation between the fraction of commuting links represented in Influenzanet and the fraction of active participants per country (R ² =0.96). (b) Statistical analysis of the traffic weights of the links represented in Influenzanet. For each country, the median rank of the commuting links represented in the Influenzanet population (red dot) is compared with a random sample (grey bar). Commuting links are ranked for decreasing probability of occurrence P _OD. Median ranks are smaller than the corresponding value for the random sample, and outside of the confidence interval for all countries except Sweden.

Figure 6

Comparison between the Influenzanet commuting network (left) and the backbone of the census commuting network (right). The color code associated to the links in the census commuting network is proportional to the adjusted weight (from yellow to dark-red). Both networks are directed, arrows are omitted for the sake of visualization. Maps were created with the collected data using ArcGIS Software and publicly available geographic datasets [25].

Figure 7

Distribution of the use of transportation modes for Influenzanet participants and comparison with national statistics.

Figure 8

Household size distribution for Influenzanet participants and comparison with national statistics.

Figure 9

Prevalence of different health indicators: smoking in the 15+ population, asthma, diabetes, and vaccination against influenza in the 65+ population. Influenzanet prevalence is compared to national statistics.