In a prospective analysis of data from children who had laboratory-confirmed influenza, researchers found that most influenza cases were not recognized clinically, and the resulting symptoms were treated on an outpatient basis far more often than through inpatient care.
Dr. Katherine A. Poehling of Vanderbilt University, Nashville, Tenn., and her colleagues reported data from the New Vaccine Surveillance Network, organized by the Centers for Disease Control and Prevention, in which they observed hospitalizations and pediatric clinic visits resulting from acute respiratory illness or fever in children younger than 5 years who were living in one of three U.S. counties during 2000–2004 (N. Engl. J. Med. 2006;355:31–40).
They found that among the outpatient population of 1,668 children, 16% (267 children) tested positive for influenza, compared with less than 6% (160 children) of the 2,797 in the inpatient group. However, discharge diagnoses identified influenza in only 17% of outpatients and 28% of hospitalized patients with laboratory-confirmed influenza.
Outpatient care was overwhelmingly predominant in the study cohort. During the 4-year study period, the average hospitalization rate for all included age groups was 1 per 1,000 children, and outpatient treatment rates for each group (younger than 6 months, 6 months to 1 year old, and 1–2 years old) were 10, 100, and 250 times higher than the rates of hospitalizations, respectively.
Moreover, the investigators found that laboratory-confirmed flu infections accounted for more than 10% of weekly clinic visits for acute respiratory tract infection or fever during the 2002–2003 flu season and more than 19% of such visits during the 2003–2004 season. In the emergency department, laboratory-confirmed flu infections accounted for nearly 6% of weekly visits for acute respiratory tract infection or fever in 2002–2003 and 29% of such visits in 2003–2004.
“Much of this influenza disease burden may be prevented through vaccination,” the researchers wrote. Influenza vaccination for all children aged 6–23 months was recommended beginning in 2004.
Notably, younger patients had greater influenza incidence. In the hospitalized patients, 80% were younger than 2 years and 49% were younger than 6 months. In the outpatient group, 47% were younger than 2 years, and 7% were younger than 6 months.
In an accompanying editorial, Dr. W. Paul Glezen of Baylor College of Medicine, Houston, observed that uninsured families, who compose almost one-fifth of the U.S. population, were underrepresented in the study, which might have contributed to artificially low hospitalization rates in the study (N. Engl. J. Med. 2006;355:79–81).