Comparing the clinical severity of the first versus second wave of 2009 Influenza A (H1N1) in a New York City pediatric healthcare facility.

Objective: We previously reported the epidemiology of 2009 Influenza A (H1N1) in our pediatric healthcare facility in New York City during the first wave of illness (May-July 2009). We hypothesized that compared with the first wave, the second wave would be characterized by increased severity of illness and mortality.

Design: : Case series conducted from May 2009 to April 2010.

Setting: Pediatric emergency departments and inpatient facilities of New York-Presbyterian Hospital.

Patients: All hospitalized patients ÷ 18 yrs of age with positive laboratory tests for influenza A.

Measurements And Main Results: We compared severity of illness during the first and second wave assessed by the number of hospitalized children, including those in the pediatric intensive care unit, bacterial superinfections, and mortality rate. Compared to the first wave, fewer children were hospitalized during the second wave (n = 115 vs. 76), but a comparable portion were admitted to the pediatric intensive care unit (30.4% vs. 19.7%; p = .10). Pediatric Risk of Mortality III scores, length of hospitalization in the pediatric intensive care unit, incidence of respiratory failure and pneumonia, and peak oxygenation indices were similar during both waves. Bacterial superinfections were comparable in the first vs. second wave (3.5% vs. 1.3%). During the first wave, no child received extracorporeal membrane oxygenation and one died, while during the second wave, one child received extracorporeal membrane oxygenation and there were no deaths.

Conclusions: At our pediatric healthcare facility in New York City, fewer children were hospitalized with 2009 Influenza A (H1N1) during the second wave, but both waves had a similar spectrum of illness severity and low mortality rate.