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