CHICAGO -- November 3, 2009 -- In contrast with some common perceptions regarding influenza A(H1N1) infections, an examination of cases in California indicates that hospitalisation and death can occur at all ages, and about 30% of hospitalised cases have been severe enough to require treatment in an intensive care unit (ICU), according to a study in the November 4 issue of JAMA.
Preliminary comparisons with seasonal influenza suggest that H1N1 disproportionately affects younger ages and causes generally mild disease.
Janice K. Louie, MD, California Department of Public Health, Richmond, California, and colleagues examined the clinical and epidemiologic features of the first 1,088 hospitalised and fatal cases due to H1N1 reported in California between April 23 and August 11, 2009. On April 20, the California Department of Public Health and 61 local health departments initiated enhanced surveillance for hospitalised and fatal cases of H1N1.
The researchers found that of the 1,088 H1N1 cases, 344 (32%) were children aged younger than 18 years, with infants having the highest rate of hospitalisation and persons aged 50 years or older having the highest rate of death once hospitalised. The median 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% of cases. Other underlying medical illnesses recorded included obesity, hypertension, hyperlipidemia, and gastrointestinal disease. The median length of hospitalisation among all cases was 4 days.
A total of 340 cases (31%) were admitted to ICUs, and of the 297 intensive care cases with available information, 65% required mechanical ventilation. Of the 884 cases with available information, 79% received antiviral treatment, including 496 patients (71%) with established risk factors for severe influenza. Of the 833 patients who had chest radiographs, 66% had infiltrates, suggestive of pneumonia or acute respiratory distress syndrome. Rapid antigen tests were falsely negative in 34% of cases evaluated.
"Overall fatality was 11% (118/1,088) and was highest (18-20%) in persons aged 50 years or older," the researchers wrote. "Of the deaths, 8 (7%) 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, H1N1 appears to be notably different from seasonal influenza, with fewer hospitalisations and fatalities occurring in elderly persons," they continued. "In contrast with the common perception that H1N1 causes only mild disease, hospitalisation and death occurred at all ages, and up to 30% of hospitalised cases were severely ill. Most hospitalised cases had identifiable established risk factors; obesity may be a newly identified risk factor for fatal H1N1 and merits further study."
"Clinicians should maintain a high level of suspicion for 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," they concluded. "Hospitalised infected cases should be carefully monitored and treated promptly with antiviral agents."
SOURCE: JAMA
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