Background: The season in which a child is born may affect the immune system development and thereby influence the risk of infections. In this study, we examined the associations between birth season and the risk of hospital admission or symptoms associated with a wide range of infections.
Methods: This study is a prospective cohort study of 2434 children with an average follow-up of 3.5 years. Admission data were obtained from the Danish National Patient Registry. Via short message service (SMS) questionnaires, 1279 families reported symptoms of infections in a 1-year period.
Results: Of the 2434 children, 639 (26.3%) were admitted to the hospital, and the children experienced on average 64.4 days with symptoms of infection within 1 year. There was no association between birth season and hospital admissions due to all infectious causes [incidence rate ratio (IRR) = 0.89; 95% confidence interval (CI), 0.65-1.22; P = 0.471]. However, children born in the fall had a higher IRR for admission due to all infectious causes when excluding admissions within the first year of life. Winter- and spring-born children had lower IRRs for admission due to gastrointestinal infections than summer-born children, but this association was alone present when admissions within the first year of life were included. The short message service-survey showed significantly lower IRRs for any symptom of infection among winter-born (IRR = 0.85; 95% CI, 0.75-0.96; P = 0.009) and fall-born children (IRR = 0.88; 95% CI, 0.78-0.99; P = 0.033) in comparison with summer-born children.
Conclusions: Birth season was not associated with hospital admission due to all infectious causes within the first 5 years of age; however, fall-birth was associated with a higher IRR for admissions due to all infectious causes after the first year of life. The association between birth season and admissions due to gastrointestinal infections was only seen when including children admitted under the age of one. Being born in fall or winter was associated with a decreased IRR for number of days with any symptom of infection registered at home.