Subject recruitment for the TCRI Study occurred over 4 years, and was completed in May 2008. Overall, 9329 visits were screened, representing 7632 unique infants, and 2986 of these infants met study eligibility requirements. From the 2986 eligible infants, 674 infants and their biological mothers were enrolled (Fig. 1) . Among the 2312 subjects who were available during the recruitment periods, the major reasons for non-response were refusal (22%), insufficient time/ unwilling to stay for the visit (outpatients) (39%), conflict with or already enrolled in another study (20%), and other (18%) (includes language barrier, mother/ guardian not present, previously enrolled and other miscellaneous). In 99.9% of the cohort, the nasal/ throat swab was obtained, and in 79% of the hospitalized infants one spot urine sample was obtained at hospital admission. Weekly enrolment into the cohort is depicted in Figure 2 .