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. Historically, the yearly influenza vaccine that was recommended in children was designed to protect against both kinds of influenza A but only one kind of influenza B. In a series of trials conducted between 2008-09 and 2010-11 (TITRE I, II, and IIB), the TITRE investigators measured antibody response to influenza B in children who were primed with two doses of trivalent inactivated influenza vaccine (TIV) containing B/Yamagata. Overall, the investigators found that 2 doses of vaccine containing B/Yamagata did not adequately prime children for response to the alternate B/Victoria antigen and that subsequent vaccine doses containing B/Victoria-lineage antigen strongly boosted antibodies to the B/Yamagata antigen that was introduced during first immunization priming, but with lower responses to B/Victoria.

For the first time since 2009-10, the recommended B/Victoria component of the seasonal influenza vaccine has been changed, from B/Brisbane/60/2008 to B/Colorado/60/2007 for the coming 2018-19 season. The investigators thus have a unique opportunity to clarify lineage-specific influenza B responses in a well-characterized cohort of children originally primed to Yamagata. The investigators' main interest is to assess whether TITRE I children primed with two doses of B/Yamagata in 2008-09 have since or are now capable of achieving a sufficient antibody response to B/Victoria following a single dose of 2018-19 QIV, ten years after their initial TIV B/Yamagata priming exposure.

Inclusion Criteria:

• Child previously completed the TITRE I study in British Columbia or Quebec;
• 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;
• Parent/guardian is fluent in English/French

Exclusion Criteria:

• Child has already received the 2018-19 seasonal (TIV or QIV) influenza vaccine;
• Child has a bleeding condition that would prevent vaccine injection or blood collection;
• Child has known or suspected immunodeficiency;
• Child has a suspected or known anaphylactic reaction to any of the vaccine components used in this study;
• Child has a health condition which, in the opinion of the investigator, would interfere with the evaluation or pose a health risk to the child;
• Child has received immune globulin or other blood products within the prior six weeks;
• Child has received injected or oral steroids within the prior six weeks defined by more than 1 week of immunosuppressants or immune modifying drugs (e.g. oral prednisolone >0.5mL/kg/day or intravenous glucocorticoid steroid). Nasal, topical or inhaled steroids are allowed;
• Child has received any live vaccine within 28 days of the study vaccine or is scheduled to receive live vaccine during the study period;
• Child has received any inactivated vaccine within 14 days of the study vaccine;
• Child is or will be enrolled in any other clinical trial of a drug, vaccine or medical device during the study period.