History of infections and hemagglutination inhibition titers (HIT) in non-vaccinated children.

*“Infection” was defined as any infection, since the clinical picture of flu in children may not be confined to typical symptoms of upper or lower respiratory tract infection.**without suspected or confirmed diagnosis of A (H1N1) 2009.***Cumulative missing values in “history of infections” or/and “diagnosis of A (H1N1) 2009”.****There were 161 children with missing values for the variable “diagnosis of A (H1N1)” (no such diagnosis; suspected by physician; lab-confirmed) and an additional 19 children with missing values in “history of infections” (no infection; one or more infection) (see table 1). Four children had missing values regarding both variables, therefore the total of missing values was 176 (161+19−4 = 176).