Lynnette Brammer, MPHEpidemiology and Prevention Branch
Influenza DivisionNational Center for Immunization and Respiratory Diseases
Centers for Disease Control and Prevention
Influenza-like Illness Surveillance at the National Level
ILINet — Outpatient Influenza-Like Illness Surveillance Network
National outpatient ILI surveillance piloted in 1982 CDC run system Family practice only
Shift to a CDC – state collaborative system began with the 1997-98 season States are responsible for recruiting and maintaining
contact with participating outpatient sites CDC is responsible for developing and maintaining
reporting and data feedback systems Data are simultaneously available to both CDC and states
ILINet — Outpatient Influenza-Like Illness Surveillance Network
Report weekly the total number of patient visits for any reason and the number of patients with ILI by age group (0–4, 5–24, 25–49, 50–64, and >64 years)
ILI definition: fever ≥100oF AND cough and/or sore throat (without a known cause other than influenza)
Can submit respiratory specimens from a subset of ILI patients to their public health laboratory for virologic testing
ILINet — Outpatient Influenza-Like Illness Surveillance Network
~ 3,000 primary care sites enrolled for the 2013-14 influenza season
All 50 states, DC, Puerto Rico, and Virgin Islands participate
ILINet — Outpatient Influenza-Like Illness Surveillance Network
Mix of practice types enrolled Varies by state System allows enrollment of any practice type that may
provide primary care Family practice, pediatrics, internal medicine, OB/Gyn,
emergency medicine, student health, urgent care, and other
For the 2012-13 influenza season collected information on approximately 35 million patient encounters Estimated coverage: ~5% of primary care/ED visits
ILINet — Outpatient Influenza-Like Illness Surveillance Network
Mix of manually compiled data and electronic data
Electronic data are a mix of chief complaint and ICD coded data
Definition used to pull electronic data should match case definition as closely as possible It should match trend and magnitude of manually collected
data Because of the large volume of data from most
electronically reporting sites, we need historical data to include for baseline development
ILINet Composition by Practice Type — 2013-14 Season
Number of Sites Number of Visits
DATA ANALYSIS
ILINet Data Analysis - Traditional
0
1
2
3
4
5
6
7
8
40 42 44 46 48 50 52 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38
Week
% o
f V
isits
for
ILI
2013-14 season
2012-13 season
2011-12 season
2009-10 season
2007-08 season
2002-03 season
National Baseline
Percentage of visits for ILI weighted by state population
National and regional baselines: mean percentage of patient visits for ILI during non-influenza weeks for the previous three seasons plus two standard deviations
A non-influenza week is defined as periods of ≥ 2 consecutive weeks in which each week accounted for less than 2% of the season’s total number of influenza positive tests
ILINet – Data Analysis – State Activity Levels
An extension of national and regional baseline development
Each reporting site has its own baseline – site- or practice-type specific
Jurisdiction-level baselines are adjusted each week based on which sites provide data Weighted sum of the baseline ratios for each contributing
provider Activity level based on number of standard
deviations from mean 1 = less than mean, 10 = > 8 SD
ILINet – Data Analysis – State Activity Levels
Allows for direct state-to-state comparison of ILI activity levels
No numbers or percentages that might differ from state reported data
ILINet Data Analysis – CBSA Level – Internal use only
ILINet Data Sharing
All data are available to state influenza surveillance coordinators
National and regional aggregate visit and ILI numbers and weighted and unweighted percentages are available on CDC Web site using FluView and FluView Interactive
State ILI activity levels are also available Both data downloads and graphics are
available
ILINet — Weaknesses
A lot of work for a lot of people Correct data interpretation is not
straightforward Large volume sites may drown out signals
from smaller sites
ILINet — Strengths
Allows early detection of relatively mild illness Most helpful system in early stage of the 2009 pandemic
Broad geographic coverage Urban and rural areas
Collaborative system Shared workload Shared monitoring Linked to virologic data
Flexible reporting Network of primary care providers with interest in
respiratory illness and with contacts to public health Facilitates early event reporting by the astute clinician