Each winter, viruses belonging to two kinds of influenza A ("A/H1N1" & "A/H3N2") and two kinds of influenza B ("B/Yamagata" & "B/Victoria") can cause illness. The yearly influenza vaccine is designed to protect against both kinds of influenza A but only one or the other kind of influenza B. Current recommendations in Canada are that if an eligible child less than nine years of age has received two doses of influenza vaccine before, then that child only requires a single dose of influenza vaccine in subsequent years of immunization. In a previous study conducted in early 2010 we measured the antibody response to influenza B in children who had previously received two doses of a B/Yamagata kind of virus contained in the 2008-09 influenza vaccine and just one dose of the B/Victoria kind of virus contained in the 2009-10 recommended vaccine. The purpose of this follow-up study is to see if the protection (antibodies in the blood) provided against the influenza B/Victoria kind of virus that was in the 2009-10 vaccine can be improved with another (second) dose of the same B/Victoria kind of virus included in the 2010-11 vaccine. Since influenza B is an illness especially of children, understanding how to best protect children against both kinds of influenza B is important.

Inclusion Criteria:

• Child previously participated in the TITRE II study;
• Child is healthy (stable chronic conditions acceptable) as established by health assessment interview and verbal history-directed health examination;
• Child is available and can complete all relevant procedures during the study period;
• Parent or legal guardian is available and can be reached by phone during the study period;
• Parent/guardian provides written informed consent;
• And, parent/guardian is fluent in English/French.

Exclusion Criteria:

• Child has already received the 2010-11 seasonal (TIV) influenza vaccine
• Child has received immune globulin or other blood products within the prior six weeks;
• Child has received injected or oral steroids within prior six weeks;
• Child is or will be enrolled in any other clinical trial of a drug, vaccine or medical device during the study period;
• Or, child has a recently acquired health condition which, in the opinion of the investigator, would interfere with the evaluation or pose a health risk to the child.

Participants of an earlier clinical trial (TITRE I); enrolled in winter 2008/09 as previously unimmunized infants and toddlers (6-23 months of age) who later participated in 2009-10 study (TITRE II) to receive 2009-10 influenza vaccine