"Pandemic influenza A(H1N1) emerged rapidly in California in April 2009," write Janice K. Louie, MD, MPH, from the California Department of Public Health, Richmond, and colleagues. "Preliminary comparisons with seasonal influenza suggest that pandemic 2009 influenza A(H1N1) disproportionately affects younger ages and causes generally mild disease."
However, data on the clinical features and populations at risk for complications from H1N1 influenza infection are still emerging, the authors add.
The aim of the study was to describe the clinical and epidemiologic features of H1N1 influenza cases that led to hospitalization or death.
The investigators studied all cases of California residents who were hospitalized or died with laboratory evidence of H1N1 infection that were reported to the California Department of Public Health between April 23 and August 11, 2009.
During that time, 1088 cases that led to hospitalization or death were reported in 41 of 61 local health departments, with most occurring in June and July. Of these, 344 (32%) were children younger than 18 years.
The median age of all cases was 27 years (range, <1>. The overall rate of hospitalization or fatality per 100,000 for all age groups was 2.8 and ranged from 11.9 in infants younger than 1 year to 1.5 in those aged 70 years or older.
The highest hospitalization rates per 100,000 were in infants 1 month old (35.8) and 2 months old (21.1). These rates were lower in infants aged between 3 and 12 months, ranging from 4.2 to 12.6 per 100,000.
The median length of stay in hospital was 4 days.
Although infants were hospitalized at greater rates than adults, individuals aged 50 years or older had the highest rate of death once hospitalized, the authors report.
The overall fatality rate was 11%. In children younger than 18 years, the death rate was 7%, and in persons older than 50 years, it was 18% to 20%. The median time from onset of symptoms to death was 12 days, and the most common causes of death were viral pneumonia and acute respiratory distress syndrome.
Sixty-eight percent (741/1088) of patients had risk factors for seasonal influenza complications.
Obesity a Risk Factor for Hospitalization
The study also found that a high number of hospitalized adult patients were obese. Of the 268 adults aged 20 years or older with a known body mass index (BMI), 156 (58%) were obese, as defined by a BMI 30 kg/m2 or higher. Of these, 67 patients (43%) were morbidly obese (BMI ≥ 40 kg/m2).
Sixty-three percent of obese patients had comorbidities associated with influenza complications such as diabetes, asthma, and renal disease.
"We found a surprising number of obese persons in this study," Dr. Louie commented to Medscape Infectious Diseases. "Our proportion of morbidly obese persons was much higher than in the general population. Others are reporting similar findings. We need to study further to find out if obese persons were also more likely to have other risk factors not yet diagnosed, like asthma or diabetes."
The study findings emphasize the importance of vaccinating children, as well as caregivers of infants younger than 6 months of age and pregnant women, she added.
"H1N1 has a reputation as a mild illness, but we found that almost one third of our hospitalized cases became severely ill and required mechanical ventilation. Over 10% died," she said. "Also, there is a perception that the elderly have some immunity. However, we found that if elderly persons were admitted to a hospital, they tended to be more likely to die from their H1N1 infection."
She emphasized that clinicians should be thinking about H1N1 infection, as well as seasonal influenza, when a person presents with fever and respiratory symptoms.
Rapid Influenza Tests Not Reliable
She added, "Clinicians should not rely on the rapid tests for influenza done in the clinic, because these can be unreliable, especially in adults. If a patient is not looking well or has risk factors like pregnancy, antiviral treatment should be started right away while awaiting test results with [polymerase chain reaction]. Many studies have shown that early treatment can make a big difference as far as hospitalization and death for influenza."
Commenting on the study for Medscape Infectious Diseases, John Bartlett, MD, professor of medicine at Johns Hopkins University School of Medicine in Baltimore, Maryland, said the authors did everybody a big favor by doing the study.
The big surprise here, he said, was the severity of H1N1 infection in older individuals.
"We've all emphasized the fact that this is a disease of young people and elderly people are relatively spared. A lot of people, including myself, have told older people not to worry about H1N1, that it's not their problem. I think we have to think twice about that now," he said.
This does not mean that all older people need to be vaccinated, he added. Rather, it means that clinicians need to be more cautious in advising them of their risk.
Dr. Bartlett also noted the finding that obesity appears to be a risk factor for hospitalization. "Everybody has been talking about this, but the CDC has never really recognized that as a risk per se. But it just keeps coming up. It came up in Canada, it came up in Mexico, and it came up in California. I think we are going to have to try to figure out why people who are morbidly obese tend to handle this flu less well. A lot of them in this series had comorbidities, but a lot of them did not."
Dr. Louie and Dr. Bartlett have disclosed no relevant financial relationships.