RESUMO
BACKGROUND: Pandemic influenza A/H1N1 carries a relatively high morbidity, particularly in young people. Early identification would enable prompt initiation of therapy, thereby improving outcomes. OBJECTIVE: To describe the epidemiological, clinical and laboratory characteristics of children admitted to hospital with the clinical diagnosis of influenza with reference to pandemic influenza A/H1N1. METHODS: We conducted a prospective study of all children aged 16 years or less admitted to the pediatric department with the clinical diagnosis of influenza-like illness from July to October 2009. The presence of A/H1N1 virus was confirmed using real-time reverse transcriptase polymerase chain reaction (RT-PCR) analysis of nasopharyngeal secretions. Positive cases were compared with negative cases concerning epidemiological data, risk factors, clinical presentation and laboratory parameters, with emphasis on changes in the differential blood count. RESULTS: Of the 106 study patients, 53 were positive to influenza A/H1N1 and 53 were negative. In both groups nearly all patients had fever at presentation and approximately two-thirds had both fever and cough. All patients had a mild clinical course, no patient needed to be admitted to the intensive care unit and no mortalities were recorded. Hyperactive airway disease was more common in the A/H1N1-positive group. Pneumonia occurred in 30% of children in both groups. Laboratory findings included early lymphopenia and later neutropenia in theA/H1N1-infected patients. CONCLUSIONS: Leukopenia consisting of lymphopenia and later neutropenia was common in patients with A/H1N1 infection but was not correlated with disease severity or clinical course, which were similar in both groups. However, reduced leukocyte count can be used as an additional criterion for diagnosing A/H1N1 infection until RT-PCR results are available.