2009 Pandemic Influenza A (H1N1) Virus Infections --- Chicago, Illinois, April--July 2009

On April 21, 2009, CDC reported the first cases of 2009 pandemic influenza A (H1N1) virus* infection in the United States (1). On April 24, in response to those reports, the Chicago Department of Public Health (CDPH) established enhanced surveillance for 2009 pandemic influenza A (H1N1) virus infections. The first cases were identified on April 28. This report summarizes laboratory-confirmed cases identified during April 24--July 25 and provides clinical and epidemiologic data for a subset of those cases. By July 25, a total of 1,557 laboratory-confirmed cases had been reported to CDPH. The overall attack rate was highest among children aged 5--14 years (147 per 100,000 population), which was 14 times higher than for adults aged ≥60 years. A total of 205 (13%) patients were hospitalized, with the highest rate observed among children aged 0--4 years (25 per 100,000), followed by children aged 5--14 years (11 per 100,000). These findings affirm prevention strategies that target children and young adults, who are at a disproportionate risk for infection and hospitalization. The Advisory Committee on Immunization Practices (ACIP) recommends that these populations should be among the first groups targeted for vaccination with influenza A (H1N1) 2009 monovalent vaccine (2).

On April 24, CDPH issued a citywide health alert to physicians and infection control professionals recommending influenza testing for persons with influenza-like illness (ILI) who had traveled to Mexico or affected counties in California and Texas, or had been in contact with ill persons from these areas in the 7 days before their illness onset. Infection with the 2009 pandemic influenza A (H1N1) virus is a reportable disease in Illinois, and health-care providers and hospitals were instructed to report suspected cases to CDPH. A probable case was defined as ILI in a person with a positive result by real-time reverse transcription--polymerase chain reaction (rRT-PCR) for influenza A and a negative result for seasonal H1 and H3 influenza (i.e., unsubtypeable for seasonal influenza A). A confirmed case was defined as a probable case that additionally had a positive result for the 2009 H1N1 virus by rRT-PCR. The Illinois Department of Public Health (IDPH) Division of Laboratories served as the reference laboratory for novel influenza testing for the entire state, including Chicago. In addition, four Chicago-area laboratories serving local hospitals were equipped, as a result of prior pandemic influenza preparedness efforts, to perform rRT-PCR to detect influenza A viruses.

On April 26, CDPH and other city departments held their first press conference regarding the outbreak and recommended home isolation for persons with ILI. On April 28, CDPH received the first four reports of probable 2009 pandemic influenza A (H1N1) virus infection among Chicago residents, which included two health-care workers from the same medical facility, an office worker, and an elementary school student; all four specimens were later confirmed to be the 2009 H1N1 virus at CDC and IDPH laboratories.†

On April 30, CDPH issued an alert advising health-care providers to limit testing for 2009 pandemic influenza A (H1N1) virus to hospitalized ILI patients because large numbers of respiratory specimens had been sent to IDPH for confirmatory testing. This excess had been created when emergency departments and outpatient clinics in Chicago and the surrounding suburbs evaluated large volumes of patients with mild illness who sought care after local and national media coverage.

During April 24--May 15, CDPH conducted telephone interviews with all persons reported as having confirmed or probable cases. A standardized case report form was used to record demographic, clinical, and exposure information. After May 15, because of widespread community transmission, interviews were discontinued. Subsequently, the principal source of information about nonhospitalized cases was demographic data included on IDPH laboratory reports.

From April 24 to July 25, a total of 1,557 laboratory-confirmed 2009 pandemic influenza A (H1N1) virus infection cases among Chicago residents were reported to CDPH with specimen collection dates of April 23 to July 16 (Figure 1). Although an initial cluster was identified in one northeastern community area during the first week of the outbreak, cases soon were reported among residents of multiple community areas throughout the city (Figure 2). By May 23, the fifth week of the outbreak, cases had been reported in 68 of Chicago's 77 community areas.

During April 24--July 25, the median age of reported confirmed cases was 12 years (range: 24 days--91 years). The attack rate was highest among children aged 5--14 years (147 per 100,000 population), followed by children aged 0--4 years (113 per 100,000). The attack rate for children aged 5--14 years was 14 times higher than for adults aged ≥60 years (Table). Attack rates for males and females were similar. Among 433 patients for whom the data were available, the most common symptoms were fever (315 patients; 73%) and cough (295; 68%), followed by sore throat (124; 29%) and shortness of breath (64; 15%); no information about vomiting or diarrhea was collected.

Of the 205 laboratory-confirmed patients who were admitted to the hospital; the median age was 16 years (range: 24 days--91 years). By age group, the hospitalization rate was highest among children aged 0--4 years (25 per 100,000), followed by children aged 5--14 years (11 per 100,000). Race/ethnicity data were more complete for hospitalized patients (90%) than nonhospitalized patients (40%). Hospitalization rates were higher for non-Hispanic blacks (nine per 100,000), Asian/Pacific Islanders (eight per 100,000), and Hispanics (eight per 100,000) versus non-Hispanic whites (two per 100,000), a pattern that persisted even when cases were limited to only those patients ≤14 years. Within each of these four racial/ethnic populations, hospitalization rates were higher among children aged 0--14 years than among patients aged ≥15 years.

Among the 205 hospitalized patients, 40 (20%) patients were admitted to an intensive-care unit, and nine were reported to have required mechanical ventilation. The duration of hospitalization ranged from 1 day to 11 days (median: 2 days) for the 97 surviving patients who had both admission and discharge dates reported. Among hospitalized patients, 14 (7%) were pregnant women, including a woman aged 20 years who died of respiratory failure a day after giving birth by emergency cesarean section (3). Among 177 hospitalized patients with information on underlying illness, 37 (21%) had a previous diagnosis of asthma noted and 13 (7%) had a previous diagnosis of diabetes noted.

As of August 24, seven deaths attributed to 2009 pandemic influenza A (H1N1) virus infection among Chicago residents had been reported, including the pregnant woman, a woman aged 54 years with acute myeloid leukemia, a man aged 22 years with renal disease requiring chronic hemodialysis, a man aged 32 years with asthma and obesity, a man aged 52 years with lymphoma, and two women with no reported chronic health conditions, one aged 26 years and one aged 47 years. Investigation of these deaths is ongoing; however, respiratory compromise was a factor in all of the deaths.

Reported by: KA Ritger, MD, RC Jones, MPH, KN Weaver, MPH, E Ramirez, S Smith, MPH, JY Morita, MD, CJ Lohff, MD, SB Black, MD, JD Jones, MD, W Wong, MD, U Samala, MPH, SI Gerber, MD, Chicago Dept of Public Health; G Dizikes, PhD, J Nawrocki, PhD, J Price, MS, Illinois Dept of Public Health. LA Hicks, DO, National Center for Immunization and Respiratory Diseases; JE Staples, MD, M Fischer, MD, D Swerdlow MD, National Center for Zoonotic, Vector-Borne, and Enteric Diseases; S Lyss, MD, Office of Workforce and Career Development; F Serdarevic, MD, K Janusz, DVM, J Cortes, MD, EIS officers, CDC.

Editorial Note:

During the 14-week period covered by this report, 1,557 confirmed cases of 2009 pandemic influenza A (H1N1) virus infection were reported to CDPH. The highest attack rates, both overall and among hospitalized patients, were among children aged 0--4 years and aged 5--14 years, with substantially lower rates in persons ≥15 years. Previous reports have indicated that age-specific attack rates for 2009 pandemic influenza A (H1N1) virus infection cases are higher in younger persons and lower in older persons, compared with seasonal influenza infections (4,5). Older persons, as a group, might have preexisting immunity to the 2009 H1N1 virus (6). One small study indicated that approximately one third of adults aged >60 years had cross-reactive antibody to 2009 pandemic influenza A (H1N1) virus detected, compared with none detected among children (7). Another factor might be higher contact rates among teenagers (8).

In Chicago, Hispanics, non-Hispanic blacks, and Asian/Pacific Islanders had higher reported rates of hospitalization for 2009 pandemic influenza A (H1N1) virus infection than did non-Hispanic whites. The cause for these higher rates is unknown and could not be explained entirely by differences in the age distribution of these populations. These differences are likely the result of variations in exposure rather than differences in susceptibility. However, underlying conditions, such as asthma and diabetes, are more prevalent among blacks and Hispanics in Chicago, which might explain some of the difference in rates among hospitalized cases (9,10).

The number of cases reported in Chicago likely represents in part the high public and health-care provider awareness of the outbreak, and also the establishment of enhanced hospital laboratory surveillance. To raise awareness about the outbreak, CDPH sent 21 health alert messages via its health alert network (a secure, web-based communication portal) to physicians, infection control professionals, laboratorians, and emergency department personnel and hosted four press conferences. These messages might have played a role in increasing case findings early in the outbreak, before more limited testing recommendations were issued. The additional molecular laboratory capacity provided by the four area laboratories serving Chicago hospitals might have allowed more patients to be tested, resulting in increased numbers of cases confirmed in Chicago.

Despite the CDPH recommendation to limit testing, large numbers of outpatients continued to be tested. This might have occurred because the CDPH health alert network does not have extensive reach to community health-care providers, and these providers might not have known about the limited testing recommendation right away. CDPH also does not have a means by which to limit specimen submission from hospitals and community health-care providers to the IDPH laboratory. By mid-June, case reports declined substantially; the exact reason for this is unknown but could be related to the end of the school year.

The findings in this report are subject to at least one limitation. The number of cases in Chicago was likely underestimated because many infected persons might not have sought medical attention and because testing was discouraged for outpatients after April 30. Accordingly, the proportion of confirmed cases that resulted in hospitalization might be overestimated because the true denominator of 2009 pandemic influenza A (H1N1) virus infection cases in the city might be higher than the number of confirmed cases contained in this report. However, the rate of hospitalization was likely less affected because surveillance for hospitalized cases was uniform throughout the period.

Because of the high attacks rate of 2009 pandemic influenza A (H1N1) virus infection among children and young adults, CDPH intends to focus on infection prevention, vaccination, surveillance, and diagnostic and education strategies in this population. CDPH is strengthening absenteeism monitoring among schools and promoting home isolation for students with ILI. Health-care providers who care for children will be a priority group for CDPH communications, in addition to other groups recommended by ACIP (2).

Enhanced molecular laboratory capacity will be critical for distinguishing 2009 pandemic influenza A (H1N1) virus from other circulating influenza viruses. Additional hospital laboratories in Chicago are initiating rRT-PCR testing to characterize influenza strains in the fall. Each week, CDPH will collect reports of influenza-positive results by strain type from PCR-equipped laboratories and aggregate ILI data from hospital emergency departments to track the onset and extent of ILI in the city. In addition, CDPH will continue surveillance of hospitalized influenza A cases to monitor influenza morbidity.

References

1. CDC. Swine influenza A (H1N1) infection in two children---southern California, March--April 2009. MMWR 2009;58:400--2.
2. CDC. Use of influenza A (H1N1) 2009 monovalent vaccine. MMWR 2009;58(No. RR-10).
3. Jamieson DJ, Honein MA, Rasmussen SA, et al. H1N1 2009 influenza virus infection during pregnancy in the USA. Lancet 2009;374:451--8.
4. CDC. Update: novel influenza A (H1N1) virus infection---Mexico, March--May, 2009. MMWR 2009;58:585--9.
5. CDC. Neurologic complications associated with novel influenza A (H1N1) virus infection in children---Dallas, Texas, May 2009. MMWR 2009;58:773--8.
6. Novel Swine-Origin Influenza A (H1N1) Virus Investigation Team, CDC. Emergence of a novel swine-origin influenza A (H1N1) virus in humans. N Engl J Med 2009;360:2605--15.
7. CDC. Serum cross-reactive antibody response to a novel influenza A (H1N1) virus after vaccination with seasonal influenza vaccine. MMWR 2009;58:521--4.
8. Nishiura H, Castillo-Chavez C, Safan M, Chowell G. Transmission potential of the new influenza A (H1N1) virus and its age-specificity in Japan. Eurosurveillance 2009;14:1--4.
9. Bocskay KA, Harper-Jemison DM, Gibbs KP, Weaver K, Thomas SD. Community area, health inventory part one: demographic and health profiles. Health Status Index Series Vol. XVI, No. V. Chicago, Illinois: Chicago Department of Public Health, Office of Epidemiology; 2007.
10. Woldemichael G, Bocskay KA, Thomas SD. Age, race and place: two decades of health disparities in Chicago. Chicago, Illinois: Chicago Department of Public Health, Office of Epidemiology; 2006.

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Date last reviewed: 8/27/2009