Early detection of influenza outbreaks using the DC Department of Health's syndromic surveillance system

In a typical year, influenza results in 36,000 or more deaths and more than 200,000 hospitalizations in the United States alone. In addition to this human toll, influenza is annually responsible for a total cost of over $10 billion in the United States. A pandemic, or worldwide outbreak of a new influenza virus such as the AH1N1 virus that emerged in 2009, could dwarf this impact by overwhelming our health and medical capabilities, potentially resulting in hundreds of thousands of deaths, millions of hospitalizations, and hundreds of billions of dollars in direct and indirect costs [1].

Epidemiological characteristics of both seasonal and pandemic influenza suggest that syndromic surveillance systems are likely to make an important contribution beyond the capabilities of existing surveillance systems, and thus enable a more effective public health response to influenza outbreaks in a particular area. Indeed, a number of studies have demonstrated the potential that syndromic surveillance of influenza-like illness (ILI) offers, and such systems are now common at the national, state, and local levels [2]. Brownstein and colleagues [3] show that children and infants presenting to pediatric emergency departments (ED) with respiratory syndromes represent an early indicator of impending influenza morbidity and mortality, sometimes with as much as a three week lead on EDs serving adults. Lemay and colleagues [4] have found similar results in Ottawa, Canada. Using data from New York City, Lu and colleagues [5] have shown that monitoring both outpatient and ED data can enhance detection of ILI outbreaks. Lau and colleagues [6], Zheng and colleagues [7], and Cooper and colleagues [8] have found similar results in Hong Kong (China), New South Wales (Australia), and the United Kingdom respectively. Olson and colleagues [9] note that age-stratified analyses of ED visits for fever and respiratory complaints offer the potential earlier warning of the arrival of epidemic influenza (compared to non-stratified analyses) because they allow for detection of the characteristic age-shift of pandemic influenza. Olson and colleagues also identified the impact of respiratory syncytial virus (RSV) in children earlier than the flu season most years [9]. More recently, Polgreen and colleagues [10] and Ginsberg and colleagues [11] have suggested that data describing internet searches for flu symptoms could also be useful in detecting influenza outbreaks.

Statistical detection algorithms play a critical role in any syndromic surveillance system, yet at times they are applied without careful considerations of their statistical properties and performance. For example, many syndromic surveillance systems utilize "packaged" statistical detection algorithms such as the Centers for Disease Control and Prevention's (CDC) Early Aberrations Reporting System (EARS) [12] with little control over the parameters values used in those algorithms. However, selection of parameter values for an algorithm has a large impact on the statistical performance of the algorithm. In general, the performance of a statistical detection algorithm is characterized by a tradeoff among three factors: sensitivity, false positive rate, and timeliness. Sensitivity represents an algorithm's ability to flag when an outbreak is really happening. The false positive rate represents the likelihood it alarms incorrectly, when an outbreak is not occurring, and timeliness refers to an algorithm's ability to flag as early as possible when an outbreak is occurring.

The goals of this paper were two-fold. First, we studied the performance of three commonly used statistical detection algorithms against detecting simulated outbreaks and previously "known" outbreaks detected in data from the District of Columbia's (DC) Department of Health's Emergency Room Syndromic Surveillance System (ERSSS) [13]. The three algorithms included a standard CUSUM, a version of the CUSUM based on deviations from an exponentially weighted moving average, and the multivariate CUSUM [13]. Our primary aim in this analysis was to fine-tune the key parameters in each detection algorithm to ensure optimal performance in terms of reasonable trade-offs between sensitivity, timeliness, and the false positive rate of the algorithms. Additionally, we compared the three fine-tuned versions of the algorithms to determine which algorithm offered optimal performance for public health practice and used the improved detection algorithms to characterize the DC ERSSS's performance with regard to determining the onset of seasonal influenza outbreaks.

Second, we aimed to assess the performance of the DC ERSSS. To do so, we compared a number of different 'candidate' syndromic surveillance systems within the system to determine which offered the greatest benefit in detecting the beginning of influenza outbreaks. Specifically, we examine the use of data from DC's Children's National Medical Center (CNMC) as a sentinel syndromic surveillance system and the use of different symptom groups both individually and together to determine which conditions are strongly correlated with the beginning of influenza outbreaks. Some studies have suggested that children's symptoms are an especially sensitive indicator of the start of seasonal influenza outbreaks. Children, and the elderly, are likely to be the first victims of influenza due to their weaker immune systems. Such results have been seen in the analysis of ER surveillance data from both Boston [14] and New York [9]. Given such high sensitivity to influenza, it is important to study more comprehensively the possible benefits that could be gleamed from a syndromic surveillance system which focuses on data from hospitals whose primary patients are children as we do here.

In the proceeding sections, we regard a flu outbreak as a period characterized by a sudden increase in the number of people with flu-like symptoms in a given location. Since flu outbreaks in the United States usually occur from December through April, we defined the "flu season" to be December 1 through April 30 of the subsequent calendar year and the "non-flu season" to be the remainder of the year.

Fine-tuning approach

To fine-tune the three algorithms defined below, we used a two-pronged approach that aims to characterize the performance of the algorithms in rigorous, yet practical terms. First, we utilized a simulation study to estimate receiver operating characteristic (ROC) curves and determine which parameter values provided an optimal trade-off between false positive rates and sensitivity of an algorithm. Second, we used "known" outbreaks in the DC Department of Health data to assess the timeliness and sensitivity of an algorithm when faced with detecting actual (non-simulated) outbreaks. Data available for the initial fine-tuning analysis spanned the calendar dates from September 12, 2001 to May 17, 2004, i.e. 980 days

To perform the simulation study, we first created 970 datasets from the DC data with simulated outbreaks as follows. Linearly increasing outbreaks were inserted into each dataset such that x extra cases were inserted on day one of the outbreak, 2× on day two, and 3× on day three. The datasets each had a different start date for the simulated 3-day outbreak, that is, the first dataset's outbreak began on September 12, 2001, the second's on September 13, 2001, and the 970th data set's on May 8, 2004. Since we use standardized daily counts, x was set equal to values between zero and one where x = 0.50 would correspond to inserting multiples of half the average number of daily cases in the non-flu seasons into the data.

After creating the simulated datasets, we applied the algorithms for a given set of parameter values and false positive rate (between 0.001 and 0.05) to each one. We then computed the sensitivity of the algorithm to detect the simulated outbreak by day three of the outbreak in the non-flu seasons over all the simulated data sets. It was of primary interest to understand how well the algorithms did at detecting a simulated outbreak against a "normal" background level of disease activity. No known outbreaks occurred during the "normal" background periods. For this reason we did not use data from the flu seasons, December 1 to April 30, which likely contain actual outbreaks in addition to the simulated ones, yielding biased sensitivity rates. Thus, we computed the sensitivity of the algorithm using the following formula:

Finally, we plotted the sensitivity of the algorithm to flag by day three in the non-flu seasons by the false positive rate and computed the area under the sensitivity curve (see below). The set of parameters of the algorithm that gave the curve with the maximum area was considered to be the optimal set of parameter values in the simulation study since it gave the best balance between sensitivity and false positive rates [16].

After selecting a small number of candidate values that performed well under the ROC curve approach, we proceeded to the second part of our fine-tuning analysis. In the second part of our analysis, we assessed how well the selected candidate values did at detecting previously "known" outbreaks in the DC Department of Health data set. Specifically, we assessed how well the algorithms did at flagging the beginnings of the flu outbreaks in 2002 and 2004 and at flagging gastrointestinal outbreaks that occurred in three hospitals during the winter of 2003.

We selected data from three hospitals for our fine-tuning analysis based on the number of emergency department admissions. Throughout this paper, we refer to these as hospitals S (small), M (medium), and L (large). For each, we used data for two conditions: unspecified infection and gastrointestinal.

The results of our fine-tuning analysis are presented primarily in two different graphical formats. The results of the simulation studies are presented in terms of ROC curves, as in Figures 1 and 2. For this format, the horizontal axis represents the false positive rate and the vertical axis represents the sensitivity rate. The curves display the sensitivity rate on day three of the simulated outbreak in the non-flu season that is achieved for each pre-determined false positive rate. (The non-flu season is used under the assumption that there are no actual outbreaks during this period.) The different curves correspond to different outbreak sizes or parameter values.

The application of the detection algorithms to actual data in comparison with known outbreaks is displayed in a different format, as exemplified by Figures 3 and 4. In this form, smoothed values for one or more data streams (unspecified infection cases in hospital S in Figure 3 and hospital L in Figure 4) are presented in terms of a curve. The flagging of the detection algorithms is represented by symbols plotted according to the day they flagged on the horizontal axis and the value of the test statistic (e.g. CUSUM) on that date on the vertical axis. The flagging times are also represented along the time axis with vertical lines that are color coded to the symbol. Note that the vertical axis is on a square root scale because of its variance stabilizing properties. "Winter" is used in these graphs and throughout the remainder of the paper as a shorthand for November 1 of the previous year to April 1 of the year displayed or discussed. This definition, which differs from our definition of the "flu season," was chosen simply to ensure that the figures focus on the part of the data where flu outbreaks actually began during the study period.

Statistical detection algorithms

We utilize three detection algorithms in this study. The first, the CUSUM (for "cumulative summation") algorithm, monitors the daily statistic S _i which is defined by the recursive formula

where X _i denotes the observed daily count on day i, μ denotes the overall mean daily count estimated from the data, and k is an off-set parameter set by the user [17, 18]. This statistic cumulates positive deviations from the average in order to detect small but persistent increases in cases. The CUSUM algorithm alarms or flags at time = inf{i : S _i > h} where h is computed empirically to guarantee a fixed false positive rate (specified by the user) in the non-flu seasons. In our analysis, we focus on fine-tuning the user-specified value of k. The parameter k is a result of the theoretical derivation of the univariate CUSUM as a sequential likelihood ratio test for a shift δ in the mean parameter of a normal distribution. Under standard assumptions, k is usually set to δ/2, for a one-time shift in level that must be detected quickly. In syndromic surveillance however, we typically expect outbreaks that increase in size over time, so the standard theory does not determine an optimal k. Instead, k must be determined empirically using methods such as those described below. The theory does, however, suggest that choice of k matters a great deal. If k is too small, the algorithm will detect every fluctuation in the data while if it is too large, the algorithm will not detect anything at all.

The CUSUM based on deviations from an exponentially weighted moving average, which we refer to throughout as EXPO, adds one additional step to the CUSUM algorithm described above [13]. First, the EXPO algorithm predicts the daily counts, X _i , using an exponentially weighted moving average. Specifically, it defines

where 0 ≤ λ ≤ 1 is a user-specified parameter and represents the degree of smoothing that is to be done in the data (i.e. smaller values correspond to more smoothing). The algorithm then monitors the differences between the actual and predicted counts using the statistic S _i which is defined by the following recursive formula

As with CUSUM, the EXPO algorithm flags at time = inf{i : S _i > h} where h is computed empirically to guarantee a fixed false positive rate (user-specified) in the non-flu seasons. In contrast to CUSUM, EXPO should allow for more sensitive detection when outbreaks appear against a linearly increasing background pattern. With EXPO, two parameters must be fine-tuned: k and λ. Both parameters are supplied by users in this algorithm and their selected values will have a large impact on the statistical performance of the algorithm.

Lastly, we worked to fine-tune the multivariate CUSUM algorithm, which we refer to as MV CUSUM. The MV CUSUM was developed for monitoring multiple streams of data on a daily basis (e.g., streams of data from more than one hospital or streams of data from more than one condition within a hospital) [19] and can offer greater utility when an outbreak is likely to influence the daily counts of more than one symptom group. It follows the same logic as the standard CUSUM except that now daily counts are represented by a vector X _i whose dimension is p × 1 where p represents the number of streams being analyzed together. We define

and S _i = 0 if C _i ≤ k where

and Σ ^-1 is the estimated variance-covariance matrix for the p streams of data being analyzed using only daily counts from the non-flu seasons. The MV CUSUM algorithm flags at time = inf{i : C _i > h}. For the MV CUSUM algorithm, k is the parameter of interest for fine-tuning purposes. It is user-specified and its value will impact the statistical performance of the algorithm.

Our results suggest that careful analysis of unspecified infection cases, and especially those from CNMC, was useful in detecting the beginning of influenza outbreaks in the District of Columbia. Comparing the earliest dates at which various systems flag, we found that for unspecified infection, CNMC consistently flags earlier than any of the other six hospitals. It alone also regularly flags before a multivariate analysis of the other six hospitals in our analysis. In addition, we found that although the respiratory syndrome group also provides information about influenza, it does not appear to add anything to the information contained in unspecified infection. These results suggest that the CNMC unspecified infection data be monitored more closely than others, and a flag in this system be regarded as having higher predictive value.

When we compared unspecified infection cases in CNMC and in the other DC hospitals (using optimal detection algorithms as discussed above) to CDC's sentinel physician data for the South Atlantic states for four years in which there was a discernable influenza outbreak in DC (2002, 2004, 2005, and 2006), we found that in two of these four years, syndromic surveillance from CNMC outperformed the other two systems, and in one year it flagged only two days after the CDC system. It is important to note that the CDC sentinel physician data system has a built in delay of 1-2 weeks, for example, data for patients seen in Week 12 in 2007 (March 18 - March 24) was published on March 31. Given this delay, analysis of the DC ERSSS provides a substantial advantage for real-time surveillance, particularly when using data from CNMC. Similar results were seen in an analysis of ER surveillance data from Boston [14] and New York [9], supporting the idea that children's symptoms are an especially sensitive indicator of the state of seasonal influenza.

These results are subject to limitations. Our method for setting the false positive rate assumes that there were no true outbreaks during the non-flu seasons. The cut-off value of five cases for the mean standardized daily counts in the non-flu seasons for the univariate CUSUM was developed after observing that 0.5 worked well for hospital C (used in the sensitivity analysis) which had a mean standardized count of 5.3 cases for respiratory infection while for hospital A with a mean standardized count of 4.8 for unspecified infection, 1.5 was superior. While not exact, this rule of thumb worked very well in the DC data and should only be used as a starting point for other data sets. We also acknowledge that we cannot generalize our results for the univariate CUSUM beyond a mean standardized count of 28.1 cases (e.g. the largest stream of daily counts available to us in the DC data). Similar caution must be taken when trying to extrapolate our results for the EXPO and MVCUSUM algorithms. In addition, the detection algorithms we used do not account for day of the week effects. According to our previous analyses [13], these do exist in the data but are smaller than in many other data sources. Adding day of the week terms to a prediction model or pre-filtering the data to remove day of the week effect would presumably result in a greater signal to noise ratio in the data, but not fundamentally change its nature. Thus increasing the sophistication of the detection algorithms by accounting for day of the week effects might improve their performance, but it is unlikely to alter the conclusions of this paper about the need for tuning the parameters. Furthermore, our simulation studies use a relatively simple linear pattern to describe the simulated outbreak. While actual outbreaks may have a very different pattern, the outbreaks of most public health concern, including seasonal and pandemic influenza, will grow in size over time, the general pattern that our simulations represent. More complex patterns might vary the specific results but again will not the key finding concerning the need to fine-tune parameters in statistical detection algorithms.

The primary limitation of the influenza analysis is that it is based on only four influenza outbreaks in one city, one of which was atypical. It should be replicated in other areas to confirm the potential benefit that careful monitoring of data from CNMC can have in a local area's syndromic surveillance system. The use of CDC sentinel surveillance data for the South Atlantic area, rather than for DC, is also a limitation. Unfortunately no more specific data of this type, or viral surveillance data, are available for the District.

The results of this evaluation have implications for DC's ERSSS and for syndromic surveillance systems in general. First, the analysis shows that the choice of a detection algorithm's parameters matters in terms of statistical performance (here, assessed via sensitivity analysis and detection of known outbreaks). For the standard CUSUM algorithm, the optimal choice of k depended on the average number of cases per day outside the flu seasons. For the CUSUM based on deviations from an exponentially-weighted moving average (referred to as EXPO in this report), choosing k = 0.25 and λ = 0.20 was optimal, regardless of average number of cases. For the multivariate CUSUM, the choice of the k (defined differently than the k in the standard CUSUM) was much more sensitive to the data, and especially the number of data streams.

Although we explored only a limited set of detection algorithms and parameter values, our results suggest that simply using "canned" statistical detection algorithms with preset parameters may not give the optimal results for a given algorithm or data set. Rather, data need to be examined regularly using approaches similar to those above (perhaps with the consultation of a statistician) to fine-tune the parameters in the statistical detection algorithms to the data upon which they are being applied. Alternatively, Tokars and colleagues [21] explore how CDC's EARS algorithms can be improved by changing baseline periods and stratifying by day of the week.

The second conclusion is that, comparing the three detection algorithms applied to three or six data streams simultaneously in simulation studies, the multivariate CUSUM algorithm out-performed the two univariate methods in two of three settings examined in the simulation study, and in the detection of the beginning of the flu outbreak when applied to real data. The EXPO algorithm performed the best in one setting, namely detecting linearly increasing outbreaks in unspecified infection data from three hospitals. These results were also found in [19]. The results show that it is difficult to identify any particular characteristics of the data that would suggest that one algorithm is uniformly better than another. A system could be "tuned" to for different outbreak types, but that would require advance knowledge of what next season's flu would look like, which is clearly not available.

The optimal parameters and detection algorithms that we found in this analysis, of course, apply only to the particular data sets analyzed and the types of outbreaks studied (namely, linearly increasing outbreaks). The results cannot be generalized to other data streams and especially to other health departments. However, the conclusion that the parameters need to be tuned to the data does apply in general. Fine-tuning parameters on an annual or biannual basis would be prudent for any Emergency Room Syndromic Surveillance System.

In comparison with limitations of the ability of syndromic surveillance to detect bioterrorist events in a timely way unless they are very large and thus obvious [22], the analysis in this paper suggests that the most important contribution of syndromic surveillance to public health practice may be for natural disease outbreaks, such as seasonal and pandemic flu. The "situational awareness" that these systems provides can be quite valuable [23]. Indeed, in August 2009, the U.S. President's Council of Advisors on Science and Technology recommended that CDC aggregate real-time data from existing emergency department syndromic surveillance systems as a way to monitor the development and spread of the 2009-H1N1 influenza virus [24]. In addition, the World Health Organization now suggests monitoring outpatient or emergency department visits for acute respiratory illness, absenteeism rates from schools or work places, and other approaches that could be regarded as syndromic surveillance in countries in which 2009-H1N1 has already been established [25].