Clinical features of influenza A and B in children and association with myositis

Influenza virus is among the most common causes of respiratory illness, which may manifest as a range of conditions, from mild upper respiratory tract infection to bronchiolitis and pneumonia. Acute childhood myositis associated with influenza occurs mostly in influenza B infection. In this retrospective study, we analyzed the characteristics of 197 children with influenza virus treated from January 2000 to December 2001. Among them, 73 children had influenza A infection and 124 had influenza B infection. Influenza A virus outbreaks occurred in January 2000, July 2001, and December 2001, while influenza B virus outbreaks occurred from March 2000 to May 2000 and from December 2000 to February 2001. The most common clinical manifestations of influenza A and influenza B virus infection included fever, cough, and rhinorrhea. These infections also frequently manifested as laryngotracheobronchitis, pneumonia, and unexplained fever, which led to hospitalization. The most common clinical diagnosis was upper respiratory tract infection. The rates of benign acute childhood myositis in influenza A and influenza B were 5.5% and 33.9%, respectively. Creatine kinase levels were elevated in most myositis cases and boys were more commonly affected. Acute childhood myositis was more commonly seen in influenza B infection.