Evaluation of an Internet-Based Monitoring System for Influenza-Like Illness in Sweden

<div><p>To complement traditional influenza surveillance with data on disease occurrence not only among care-seeking individuals, the Swedish Institute for Communicable Disease Control (SMI) has tested an Internet-based monitoring system (IMS) with self-recruited volunteers submitting weekly on-line reports about their health in the preceding week, upon weekly reminders. We evaluated IMS acceptability and to which extent participants represented the Swedish population. We also studied the agreement of data on influenza-like illness (ILI) occurrence from IMS with data from a previously evaluated population-based system (PBS) with an actively recruited random sample of the population who spontaneously report disease onsets in real-time via telephone/Internet, and with traditional general practitioner based sentinel and virological influenza surveillance, in the 2011–2012 and 2012–2013 influenza seasons. We assessed acceptability by calculating the participation proportion in an invited IMS-sample and the weekly reporting proportion of enrolled self-recruited IMS participants. We compared distributions of socio-demographic indicators of self-recruited IMS participants to the general Swedish population using chi-square tests. Finally, we assessed the agreement of weekly incidence proportions (%) of ILI in IMS and PBS with cross-correlation analyses. Among 2,511 invited persons, 166 (6.6%) agreed to participate in the IMS. In each season, 2,552 and 2,486 self-recruited persons participated in the IMS respectively. The weekly reporting proportion among self-recruited participants decreased from 87% to 23% (2011–2012) and 82% to 45% (2012–2013). Women, highly educated, and middle-aged persons were overrepresented among self-recruited IMS participants (p<0.01). IMS (invited and self-recruited) and PBS weekly incidence proportions correlated strongest when no lags were applied (r = 0.71 and r = 0.69, p<0.05). This evaluation revealed socio-demographic misrepresentation and limited compliance among the self-recruited IMS participants. Yet, IMS offered a reasonable representation of the temporal ILI pattern in the community overall during the 2011–2012 and 2012–2013 influenza seasons and could be a simple tool for collecting community-based ILI data.</p></div

Distribution of socio-demographic characteristics among self-recruited and invited IMS participants during the 2011–2012 and 2012–2013 influenza seasons and the corresponding distribution of the general Swedish population 2011 and 2012.

<p>*Chi square goodness of fit test participants vs. Swedish population.</p><p>**Participants who contributed with at least one <i>active</i> report. For definition of active reports, see Methods section.</p><p>***Among participants 16–95+ year old.</p><p>****Including children in age group 0–15 yrs.</p

Bland-Altman plots 2011–2012 and 2012–2013.

<p>The upper graph shows a Bland-Altman plot of data from the 2011–2012 season and the lower graph shows a Bland-Altman plot of data from the 2012–2013 season. The black dots represents the differences of the weekly incidence proportions between the IMS and PBS (y) by the average of the IMS and PBS weekly incidence proportions (x). The thick blue line represents a simple linear regression model of the differences on the averages, while the thin blue lines represent the respective 95% limits of agreement. The limits of agreement for the difference between the IMS and PBS can be calculated from the equation in the bottom of the graphs, when their average is known. With the equations at the top of the graphs one system’s incidence proportions can be transformed to the other.</p

The weekly number of reports and reporting proportion among self-recruited and invited IMS participants during the 2011–2012 and 2012–2013 influenza seasons.

<p>*Active as defined in the Methods section.</p><p>**Number of reports per participant.</p

Summary of system components of the IMS and PBS during the influenza seasons 2011–2012 and 2012–2013.

<p>Summary of system components of the IMS and PBS during the influenza seasons 2011–2012 and 2012–2013.</p

Epidemic curves 2011–2012 and 2012–2013.

<p>The upper graph shows the smoothed weekly ILI incidence proportions generated by IMS and PBS (corrected for estimated demographic misrepresentation <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0096740#pone.0096740-Bexelius1" target="_blank">[4]</a> and underreporting <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0096740#pone.0096740-Merk1" target="_blank">[5]</a>) and number of laboratory confirmed influenza cases, Stockholm 2011–2012. The lower graph shows the smoothed weekly ILI incidence proportions generated by IMS (based on self-recruited and invited participants) and PBS (corrected for estimated demographic misrepresentation <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0096740#pone.0096740-Bexelius1" target="_blank">[4]</a> and underreporting <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0096740#pone.0096740-Merk1" target="_blank">[5]</a>), number of laboratory confirmed influenza cases, and ILI per 1,000,000 listed patients in GP-sentinel reports, Sweden 2012–2013.</p