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ISDS 2012 Conference Abstracts

Early Detection of Influenza Activity Using Syndromic
Surveillance in Missouri
Fei Wu* and Amy Kelsey

Missouri Department of Health and Senior Services, Jefferson City, MO, USA

Objective
To assess how weekly percent of influenza-like illness (ILI) re-

ported via Early Notification of Community-based Epidemics
(ESSENCE) tracked weekly counts of laboratory confirmed influenza
cases in five influenza seasons in order to evaluate the early warning
potential of ILI in ESSENCE and improve ongoing influenza sur-
veillance efforts in Missouri.

Introduction
Syndromic surveillance is used routinely to detect outbreaks of

disease earlier than traditional methods due to its ability to automat-
ically acquire data in near real-time. Missouri has used emergency
department (ED) visits to monitor and track seasonal influenza ac-
tivity since 2006.

Methods
The Missouri ESSENCE system utilizes data from 84 hospitals,

which represents up to 90 percent of all ED visits occurring in Mis-
souri statewide each day. The influenza season is defined as starting
during Centers for Disease Control and Prevention (CDC) week num-
ber 40 (around the first of October) and ending on CDC week 20 of
the following year, which is usually at the end of May.

A confirmed influenza case is laboratory confirmed by viral cul-
ture, rapid diagnostic tests, or a four-fold rise in antibody titer be-
tween acute and convalescent serum samples. Laboratory results are
reported on a weekly basis. To assess the severity of influenza activ-
ity, all flu seasons were compared with the 2008-09 season, which
experienced the lowest influenza activity based on laboratory data.
Analysis of variance (ANOVA) was applied for this analysis using
Statistical Analysis Software (SAS) (version 9.2). 

The standard ESSENCE ILI subsyndrome includes ED chief com-
plaints that contain keywords such as “flu”, “flulike”, “influenza” or
“fever plus cough” or “fever plus sore throat”. The ESSENCE ILI
weekly percent is the number of ILI visits divided by total ED visits.

Time series of weekly percent of ILI in ESSENCE were compared
to weekly counts of laboratory confirmed influenza cases. Spearman
correlation coefficients were calculated using SAS. The baseline
refers to the mean of three flu seasons with low influenza activity
(2006-07, 2008-09 and 2010-11 seasons). The threshold was calcu-
lated as this baseline plus three standard deviations. 

The early warning potential of the ESSENCE weekly ILI percent
was evaluated for five consecutive influenza seasons, beginning in
2006. This was accomplished by calculating the time lag between the
first ESSENCE ILI warning versus the first lab confirmed influenza
warning. A warning was identified if either lab confirmed case counts
or weekly percent of ILI crossed over their respective baselines.

Results
For each influenza season evaluated, weekly ILI rates reported via

ESSENCE were significantly correlated with weekly counts of lab-
oratory-confirmed influenza cases (Table 1). The baseline of ILI ac-

tivity in ESSENCE was 1.8 ILI /100 ED visits/week and the thresh-
old was set at 4.1 ILI visits per 100 ED visits/week. The ESSENCE
ILI baseline provided, on average, two weeks of advanced warning
for seasonal influenza activity. Figure 1 shows that two influenza sea-
sons (2007-08 and 2009-10) were more severe than others examined
based on the ESSENCE percent ILI threshold analysis, this result is
consistent with the examination of severity of influenza activity based
on lab confirmed influenza data (p<0.05).

Conclusions
The significant correlation between ILI surveillance in ESSENCE

and laboratory confirmed influenza cases justifies the use of weekly
ILI percent in ESSENCE to describe seasonal influenza activity. The
ESSENCE ILI baseline and threshold provided advanced warning of
influenza and allowed for the classification of influenza severity in
the community.

Table 1. Correlation between laboratory confrmed influenza cases and
ESSENCE ILI weekly percent in five influenza seasons, 2006-2011

Keywords
ESSENCE; syndromic surveillance; influenza-like illness (ILI); base-
line; threshold

Acknowledgments

We thank Lesha Peterson, Rong He and Peggy Hartman at the Missouri
Department of Health and Senior Services for providing influenza case
data.

*Fei Wu
E-mail: Fei.Wu@health.mo.gov

Online Journal of Public Health Informatics * ISSN 1947-2579 * http://ojphi.org * 5(1):e37, 2013