BACKGROUND: It has been suggested that the transiently increased infection risk following childcare enrolment is compensated by decreased infection risk later in childhood and adolescence. We investigated how childcare enrolment affected rates of antimicrobial-treated infections during childhood and adolescence.

METHODS: In a register-based cohort study of all children born in Denmark 1997-2014 with available exposure information (n = 1 007 448), we assessed the association between childcare enrolment before age 6 years and infection risks up to age 20 years, using antimicrobial exposure as proxy for infections. Nationwide childcare and prescription data were used. We estimated infection rates and the cumulative number of infections using adjusted Poisson regression models.

RESULTS: We observed 4 599 993 independent episodes of infection (antimicrobial exposure) during follow-up. Childcare enrolment transiently increased infection rates; the younger the child, the greater the increase. The resulting increased cumulative number of infections associated with earlier age at childcare enrolment was not compensated by lower infection risk later in childhood or adolescence. Accordingly, children enrolled in childcare before age 12 months had experienced 0.5-0.7 more infections at age 6 years (in total 4.5-5.1 infections) than peers enrolled at age 3 years, differences that persisted throughout adolescence. The type of childcare had little impact on infection risks.

CONCLUSIONS: Early age at childcare enrolment is associated with a modest increase in the cumulative number of antimicrobial-treated infections at all ages through adolescence. Emphasis should be given to disrupting infectious disease transmission in childcare facilities through prevention strategies with particular focus on the youngest children.