In contrast with some common perceptions regarding 2009
influenza A(H1N1) infections, an examination of cases in California indicates
that hospitalization and death can occur at all ages, and about 30 percent of
hospitalized cases have been severe enough to require treatment in an intensive
care unit, according to a study in the Nov. 4 issue of JAMA.
“Since April 17, 2009, when the first tywo cases of
pandemic influenza A(H1N1) virus infection were reported in California, the
virus has rapidly spread throughout the world,” the authors write. They add that
preliminary comparisons with seasonal influenza suggest that this influenza
infection disproportionately affects younger ages and causes generally mild
disease.
Janice K. Louie, MD, MPH, of the California Department
of Public Health, and colleagues examined the clinical and epidemiologic
features of the first 1,088 hospitalized and fatal cases due to pandemic 2009
influenza A(H1N1) infection reported in California, between April 23 and Aug.
11, 2009. On April 20 of this year the California Department of Public Health
and 61 local health departments initiated enhanced surveillance for hospitalized
and fatal cases of this infection.
The researchers found that of the 1,088
A(H1N1) cases, 344 (32 percent) were children younger than 18 years, with
infants having the highest rate of hospitalization and persons age 50 years or
older having the highest rate of death once hospitalized. The median (midpoint)
age of all cases was 27 years. Fever, cough, and shortness of breath were the
most common symptoms. Underlying conditions previously associated with severe
influenza were reported in 68 percent of cases. Other underlying medical
illnesses recorded included obesity, hypertension, hyperlipidemia and
gastrointestinal disease. The median length of hospitalization among all cases
was four days.
Three hundred forty cases (31 percent) were admitted to intensive care units,
and of the 297 intensive care cases with available information, 65 percent
required mechanical ventilation. Of the 884 cases with available information, 79
percent received antiviral treatment, including 496 patients (71 percent) with
established risk factors for severe influenza. Of the 833 patients who had chest
radiographs, 66 percent had infiltrates (evidence of infection involving the
lungs), suggestive of pneumonia or acute respiratory distress syndrome. Rapid
antigen tests were falsely negative in 34 percent of cases evaluated.
“Overall fatality was 11 percent
(118/1,088) and was highest (18 percent - 20 percent) in persons aged 50 years
or older,” the researchers write. “Of the deaths, 8 (7 percent) were children
younger than 18 years. Among fatal cases, the median time from onset of symptoms
to death was 12 days.” The most common causes of death were viral pneumonia and
acute respiratory distress syndrome.
“In the first 16 weeks of the current
pandemic, 2009 influenza A(H1N1) appears to be notably different from seasonal
influenza, with fewer hospitalizations and fatalities occurring in elderly
persons. In contrast with the common perception that pandemic 2009 influenza
A(H1N1) infection causes only mild disease, hospitalization and death occurred
at all ages, and up to 30 percent of hospitalized cases were severely ill. Most
hospitalized cases had identifiable established risk factors; obesity may be a
newly identified risk factor for fatal pandemic 2009 influenza A(H1N1) infection
and merits further study.”
“Clinicians should maintain a high level
of suspicion for pandemic 2009 influenza A(H1N1) infection in patients
presenting currently with influenza-like illness who are older than 50 years or
have known risk factors for influenza complications, regardless of rapid test
results. Hospitalized infected cases should be carefully monitored and treated
promptly with antiviral agents,” the authors conclude.
Reference: JAMA. 2009;302[17]:1896-1902.