Humoral immune responses to influenza. Hemagglutination inhibition (HI) antibody titers to influenza A/H1N1/7/California/2009 (H1N1/pdm09), A/Perth/16/2009(H3N2) (H3N2/Per2009), A/H3N2/361/Victoria/2011-like (H3N2/Vic2011), B/Brisbane/60/2008 (B/Bris2009), and B/Wisconsin/1/2010 (B/Wis2010) viruses, in plasma from 103 preschool (2–5 years old) children living in Addis Ababa, Ethiopia. Hemagglutination inhibition values <10 were considered negative and assigned a value of 5 for calculation purposes. Horizontal lines represent geometric mean titers.