Mucosal and Systemic Immunity Correlates of Protection Against Influenza in Young Children, Aged 2-5 Years (MUSIC-Flu)

Inclusion Criteria: * Children aged 2-5 years of age (i.e. from the second birthday until 5 years and 11 months of age) at the time of enrolment. * Healthy children as established by medical history and clinical examination before entering the study. * Parent availability and willingness to provide written informed consent to all study procedures * Child planning to reside in the selected community for the duration of the study. Exclusion Criteria: * Presence of malnutrition (defined as z score for age \<-2 based on WHO child grown standard at time of screening for eligibility). * Presence of any serious systemic disorder determined by medical history and/or physical examination that would compromise the participant's health or is likely to result in non-conformance to the protocol or a child who is on treatment for any chronic medical condition. * Prior history of receiving influenza vaccination. * Not providing consent for storage and shipment of samples for immunological testing. * Parent of participating child not able, available or willing to accept active weekly follow-up by the study staff, performing a twice-weekly caregiver swab and completing symptom diary on an electronic device or any medical condition in the parents that, in the judgment of the investigator, would interfere with or serves as a contraindication to protocol adherence or a parent's ability to give informed consent. History of chronic administration (defined as more than 14 days) of immunosuppressant medications, including corticosteroids, in the last 6 months (those on inhaled or topical steroids may be permitted to participate in the study. * Child is HIV-infected as identified by self-report or if screening of study eligibility identifies that the child is HIV-infected.