Outbreak of Novel H1N1 Influenza A Virus — Chicago, April–June 2009

Background: On April 28, 2009, Chicago Department of Public Health (CDPH) received the first reports of probable novel influenza A (H1N1) virus infections among Chicago residents. CDPH initiated enhanced surveillance for novel influenza A to understand the epidemiology of this emergent virus.

Methods: We defined a confirmed case as influenza-like illness and a positive H1N1 polymerase chain reaction on a respiratory specimen. Between April 28 and May 10, CDPH interviewed persons reported with confirmed infection. We used a standardized case report form to collect demographic and clinical information. After May 10, we collected demographic information through laboratory reports and medical providers.

Results: During April 24¨CJuly 2, 2009, 1495 confirmed novel influenza A (H1N1) cases were reported to CDPH with onset from April 20¨CJune 19, 2009; 967 (65%) were aged <20 years, 315 (21%) 20¨C39 years, 154 (10%) 40-59 years, and 36 (2%) ≥60 years. Of 552 cases with reported location of health-care visit, 187 (34%) were hospitalized; 104 (56%) were <20 years of age, 33 (18%) 20-39 years, 36 (19%) 40-59 years, and 13 (7%) ≥60 years. The six decedents were aged 20, 22, 26, 47, 52 and 54 years old.

Conclusions: The proportion of cases and hospitalizations due to novel influenza A among children and young adults was high. Children and young adults should be targeted along with traditional groups at risk for complications of influenza when developing prevention and treatment strategies to lower morbidity and mortality should H1N1 reemerge in the fall.