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  • The H1N1 virus that caused that pandemic is now a regular human flu virus and continues to circulate seasonally worldwide.
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Questions and Answers: NEJM article "Hospitalized Patients with 2009 H1N1 Influenza in the United States- April-June 2009"

October 23, 2009, 12:00 PM ET

What was the purpose of this study?

The purpose of this study published by the New England Journal of Medicine (NEJM) was to analyze the clinical characteristics of patients hospitalized with 2009 H1N1 flu virus infections in the United States during April through June 2009. This article is available online at NEJM.org.

How was this study conducted?

CDC researchers, in collaboration with state public health departments, analyzed the clinical characteristics of 272 patients hospitalized with 2009 H1N1 flu virus infections in the United States from April to mid-June 2009.
Cases were defined as any person hospitalized for 24 hours or more with influenza-like illness (ILI)* and who tested positive for 2009 H1N1 by real-time reverse transcriptase polymerase chain reaction (rRT-PCR)**.

What were the different age groups of hospitalized patients?

Unlike seasonal flu, few cases of severe illness associated with 2009 H1N1 have been reported among people 65 years of age and older. In fact only 5% of patients hospitalized in this study were 65 years of age or older. In comparison, this group makes up more than 60% of the seasonal flu hospitalizations in the U.S. each year. Forty-five percent of the patients hospitalized in this study were children younger than 18 years old, and 50% were adults, ages18 to 64.

How many of the hospitalized patients had underlying medical conditions?

A large proportion (73%) of patients hospitalized with 2009 H1N1 had one or more underlying conditions, including asthma, diabetes, heart, lung, or neurologic disease, or pregnancy.  Asthma was the most common underlying medical condition, and was found in 29% of children and 27% of adults. Although underlying medical conditions were common, severe illness among young healthy persons was also identified.

Is obesity considered an underlying medical condition that puts people at higher risk for 2009 H1N1-related complications?

Although at this time obesity has not been linked with increased risk for 2009 H1N1 flu-related complications, this study shows that further investigation is warranted.  Although data regarding height and weight were available for only 70% of patients in the study, 45% of these patients were either obese or morbidly obese, based on chart abstractions. However, the majority (81%) of these patients had an underlying condition known to increase their risk for flu-related complications.  For adults included in the study, the prevalence of obesity (29%) was comparable to the estimated obesity prevalence in the adult U.S. population (27%). However, the prevalence of morbid obesity (26%) was higher than the estimated (5%) in the adult U.S. population.

What symptoms did hospitalized patients have?

Symptoms on presentation to a health care provider included fever (95%) and cough (88%).  Diarrhea or vomiting was reported in 39% of patients, including 42% of children and 37% of adults.  The average time from the start of illness to hospital admission was 3 days.

What type of treatments did hospitalized patients receive?

About 75% of hospitalized patients received flu antiviral medicines. Of these patients, 188 received oseltamivir (Tamiflu), and 19 received zanamivir (Relenza).  Antiviral therapy began an average of three days from the start of illness.  Thirty-nine percent of patients received antiviral drugs within 48 hours of illness onset.  Delays in therapy were associated with severe outcomes, such as death and admittance into the ICU.

Two-hundred and six patients received antibiotic treatment. Commonly used antibiotics included ceftriaxone, azithromycin, vancomycin, and levofloxacin. Seventy-three percent of patients with radiographs consistent with pneumonia were treated with antiviral drugs, and 97% were treated with antibiotics.

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How many hospitalized patients were admitted to an Intensive Care Unit (ICU)? 

Sixty-seven hospitalized patients were admitted to an ICU. The average age of those admitted was 29 years.  Forty-five (67%) of these patients had an underlying medical condition, including pregnancy.  Among these patients, the average time from the beginning of the illness to the start of antiviral therapy was 6 days. Only 23% of patients admitted into the ICU or who died received antiviral drugs within 48 hours of illness onset.

How many hospitalized patients died?

Seven percent (19) of the hospitalized patients died, all of whom had been admitted to an ICU and required mechanical ventilation. The average age of the patients who died was 26 years. Thirteen (68%) of the patients had an underlying medical condition, including neurologic disease, asthma or chronic obstructive pulmonary disease (COPD), and pregnancy.  The average time from the start of illness to death was 15 days. 

ICU patients and patients who died were more likely than non-ICU patients who survived to have shortness of breath, a neurologic disorder, radiographically confirmed pneumonia, acute respiratory distress syndrome  (ARDS), or sepsis, and were more likely to have received antivirals, antibiotics, or steroids.  Also, ICU patients and those who died were older and had a longer time between start of illness and the beginning of antiviral treatment.

How many of the patients who died received antiviral medications?

Of the 19 patients who died, 90% received antiviral drugs, and all received antibiotics. The average time from the start of illness to the beginning of antiviral therapy was 8 days.  None of the patients who died received antiviral therapy within 48 hours after the beginning of symptoms.

*ILI is defined as fever (temperature of 100°F [37.8°C] or greater) and a cough and/or a sore throat in the absence of a KNOWN cause other than influenza

**More information on rRT-PCR and testing for flu is available.

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