A Decade of Indiana Sentinel Influenza Data Surveillance

Indiana Epidemiology Newsletter
June 2007

Shawn Richards, BS

The Indiana State Department of Health (ISDH) has participated in the Centers for Disease Control and Prevention (CDC) national Influenza Sentinel Provider Surveillance program for the past 10 years. This article describes the influenza baseline for the past 10 years as well as other information that has been observed over the past decade.

The Indiana Sentinel Provider Surveillance program includes 40 of the 1,000 sentinel sites included in the national program. These health care providers report the number of patients seen in their offices and, specifically, the number of patients with influenza-like-illness (ILI) on a year-round basis. For surveillance purposes, the CDC defines ILI as fever (>100 oF [37.8 oC] oral or equivalent) and cough or sore throat (in the absence of a known cause). Sentinel sites report weekly, submitting their data to the repository at the CDC via Internet or fax. Additionally, sentinel participants collect nasopharyngeal swabs from random patients with ILI whose onset of classic clinical symptoms started within 72 hours of the appointment. The swabs are sent to the ISDH Laboratories for viral isolation and identification by IFA, DFA, or PCR methods.

The ISDH uses these data to monitor influenza activity and establish baseline levels over time. A baseline level assists in monitoring trends of influenza activity and provides comparison to indicate influenza activity that may be significant. A baseline of 10 years provides a strong determination of ILI activity in Indiana. The baseline level of ILI for the past 10 years is shown in Figure 1.

Click here for data for individual years.

In a typical influenza season, the baseline level peaks in early-to-mid February in Indiana. Influenza activity was particularly significant during 1998, 1999, 2003, and 2004. Figure 2 compares the levels in these 4 years to the 10-year baseline.

In addition to providing baseline levels of activity and patterns of transmission, influenza sentinel surveillance data also help determine the viral components for influenza vaccine. The ISDH sends laboratory results of viral strains identified in specimens collected at sentinel sites to the CDC, as do all other states. Each year, usually in February, the CDC compiles these laboratory results to determine which influenza strains are most likely to circulate during the following influenza season. Most of the time, the vaccine strains closely match the circulating strains. Occasionally, due to antigenic drift, the circulating viruses are different enough from the vaccine strains that they do not match well. When this occurs, the vaccine does not provide as much protection, and influenza epidemics usually result. The viral components of the influenza vaccine and the predominating strains of the corresponding year are found in Table 1.

It is well known that the seasonal impact of influenza is very significant. During 1990-1999, the CDC estimates that 36,000 deaths each year in the United States were related to influenza (1). Calculating the exact number of deaths from influenza is difficult, because influenza is not reportable in Indiana and is not always diagnosed at time of death, although it may be a contributing factor. According to CDC data, Indiana should record approximately 600 deaths per year from influenza. However, Indiana death certificates reflect only a percentage of those deaths. Due to the delay in filing death certificates and the current method of tabulating these data, ISDH death data are usually unavailable for up to 1½ years following the event. In October 2006, the ISDH Executive Board approved mandating the reporting of influenza deaths in Indiana. An emergency rule mandates that all influenza deaths are reportable within 72 hours of the knowledge of death. The rule is expected to be processed through formal promulgation by the end of 2007. Figure 3 shows the total number of Indiana deaths due to influenza reported on an official death certificate for the years 1996-2005.


1. Thompson WW, Shay DK, Weintraub E, et al. Mortality associated with influenza and respiratory syncytial virus in the United States. JAMA 2003;289:179--86.