Self-Reported Influenza-Like Illness During the 2009 H1N1 Influenza Pandemic --- United States, September 2009--March 2010

CDC identified the first case of 2009 H1N1 pandemic influenza on April 15, 2009. During the first 3 months of the outbreak, approximately 43,000 cases were reported to CDC (1). In June 2009, the World Health Organization declared the outbreak an influenza pandemic. Because no existing influenza surveillance system in the United States monitored influenza-like illness (ILI) among persons with ILI who did not seek health care, CDC initiated community-based surveillance of self-reported ILI (defined as the presence of fever with cough or sore throat) and health-care--seeking behavior through a supplementary module of the Behavioral Risk Factor Surveillance System (BRFSS). This report summarizes results from BRFSS surveys conducted during September 2009--March 2010. Among 216,431 adults and 43,511 children (aged <18 years), the average monthly percentage of respondents reporting ILI in the 30 days preceding the interview was 8.1% among adults (range: 5.5% for September interviews to 9.5% for November) and 28.4% among children (range: 20.4% for September interviews to 35.9% for November). Health care was sought by 40% of adults and 56% of children with self-reported ILI. The results indicate that reported symptoms of ILI were widespread during the 2009--10 influenza season, with a substantial percentage of those reporting ILI seeking health care.

BRFSS conducts state-based, random-digit--dialed telephone surveys of the noninstitutionalized U.S. population aged ≥18 years to determine the prevalence of health conditions and health risk behaviors (2). From September 1, 2009, to March 31, 2010, BRFSS respondents in 49 states (excluding Vermont), the District of Columbia (DC), and Puerto Rico were interviewed using a new module for ILI. One of each respondent's children also was eligible to be the subject of the interview in 39 of the 49 states, DC, and Puerto Rico. Reported fever with cough or sore throat during the 30 days preceding the interview (for themselves or their child) was defined as having ILI in the past month.* Those respondents who reported ILI also were asked if they (or their child) visited a health-care professional for the illness.†

Average monthly percentages of self-reported ILI were calculated. ILI was analyzed by age group, month of interview, sex, race/ethnicity,§ and U.S. Census region.¶ Percentage estimates for race/ethnicity were age-adjusted by the direct method to the 2000 U.S. Census population. Respondents with missing ILI status or who resided in states where interviews were conducted for <6 months were excluded from analysis. Results were weighted to reflect selected demographic and geographic population estimates, in accordance with BRFSS weighting methodology (2). Response rates for BRFSS were calculated using Council of American Survey and Research Organizations (CASRO) guidelines. Median survey response rates were 55.2%, calculated as the percentage of persons who completed interviews among all eligible persons, including those who were not contacted. Median cooperation rates were 75.3%, calculated as the percentage of persons who completed interviews among all eligible persons who were contacted. Statistical significance was determined by t tests where appropriate; comparisons were considered statistically significant at p<0.05.

In interviews conducted during September 2009--March 2010, the average monthly percentage of adults reporting ILI was 8.1%, and the percentage of children with reported ILI was 28.4% (Table). Reported ILI among adults (9.5%) and children (35.9%) peaked in November interviews, which corresponds with illness in October or November. This pattern was observed in all census regions, except among adults in the South, where ILI peaked in December interviews (Figure 1). Reported ILI among adults (5.5%) and children (20.4%) was lowest for September interviews.

The monthly percentage of respondents reporting ILI decreased with age; the percentage was highest among children aged 0--4 years (32.7%) and lowest among adults aged ≥65 years (3.2%). Among all adults, a significantly higher percentage of women (9.0%) reported ILI than men (7.1%), but no statistically significant difference by sex was observed among children (p=0.11). Compared with white adults (8.4%) and children (29.9%), those identified as American Indian/Alaska Native adults (16.3%) and children (40.8%) reported significantly higher ILI prevalence, and those identified as black adults (7.2%) and children (23.3%) reported significantly lower ILI prevalence.

Among adults, no statistically significant differences were observed by census region. However, among children, those in the Midwest (29.4%) and South (29.5%) were significantly more likely to have reported ILI than those in the Northeast (26.2%). By state, adults in Arkansas were the most likely to report ILI (11.5%), whereas those in Delaware were the least likely (5.3%) (Figure 2). Among children, those in Oklahoma were the most likely to report ILI (33.4%), whereas those in DC were the least likely (21.5%). Health care was sought by 40% of adults and 56% of children with ILI.

Reported by

M Biggerstaff, MPH, L Kamimoto, MD, L Finelli, DrPH, Influenza Div, National Center for Immunization and Respiratory Diseases; L Balluz, PhD, ScD, Div of Behavioral Surveillance, Office of Surveillance, Epidemiology, and Laboratory Svcs, CDC.

Editorial Note

With the emergence of 2009 H1N1 influenza, CDC initiated community-based monitoring of self-reported ILI as a method for assessing the impact of 2009 H1N1 influenza on persons who did not seek health care. Results indicate that self-reported ILI was more common among younger respondents and less common among older respondents, a finding consistent with reports from other influenza surveillance systems (1).

After adjusting for age, the percentage of self-reported ILI in adults and children was similar among whites and Hispanics, moderately lower among black adults and children, and significantly higher among American Indian/Alaska Native adults and children. Reasons for these differences are not clear and are being investigated, but severe outcomes from influenza and other respiratory infections have been reported more commonly among children in certain racial/ethnic groups than in others (3).

The findings in this report are subject to at least five limitations. First, BRFSS ILI data are self-reported or reported by parents for their children; thus, symptoms, including fever, were not confirmed. However, self-reported symptoms of infectious illness, including respiratory illnesses, have shown close congruence with physician documentation (4). Second, the reported cases of ILI were not laboratory-confirmed as influenza infection. Influenza infection can cause illnesses that do not meet the ILI case definition (e.g., respiratory illness without fever); conversely, illnesses meeting the ILI criteria can be caused by multiple pathogens other than influenza (e.g., respiratory syncytial virus). Studies have indicated that 10%--25% of all respiratory illnesses occurring during periods when influenza viruses are known to be circulating are actually laboratory-confirmed influenza (5--7). The sensitivity and specificity of the ILI definition for influenza illness can vary substantially, but requiring fever as part of the ILI criteria has been shown to increase the specificity of ILI for influenza illness (8,9). Third, BRFSS data are collected only from households with a landline telephone. Selection bias related to exclusion of households with only cellular phones or no telephone service is possible; however, BRFSS weighting methodology partially compensates for the exclusion of households without telephones. Fourth, respondents from nine states were excluded from the analysis of ILI in children, and six of the excluded states were in the South census region. Therefore, results from the ILI analysis for children might not be representative of the United States overall and particularly of the South census region. Finally, the median state response rate for this survey period was only 55.2%; low response rates can increase the potential for bias.

Community-based ILI surveillance through BRFSS provided important information to help describe the 2009 H1N1 influenza pandemic and the epidemiology of ILI in the United States. These data were a useful adjunct to routine influenza surveillance and provided the only source of information for persons with ILI who did not seek health care. CDC continues to use the supplementary ILI module during the 2010--11 influenza season; the results will enable analysis of year-to-year trends and factors associated with self-reported ILI.

Acknowledgments

This report is based, in part, on contributions by BRFSS state coordinators; M Jhung, MD, Influenza Div, National Center for Immunization and Respiratory Diseases; and W Garvin and M Qayad, Div of Behavioral Surveillance, Office of Surveillance, Epidemiology, and Laboratory Svcs, CDC.

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