Relationship Between Number of Older Siblings and Common Cold Among Children Aged 5 Years: The Kyushu Okinawa Maternal and Child Health Study

In view of the lack of epidemiologic information on the relationship between sibship size and the common cold, this prebirth cohort study investigated the association between the number of older siblings and the high frequency of common colds in Japanese children aged 5 years.

The study included 1197 mother–child pairs in the Kyushu Okinawa Maternal and Child Health Study; data were collected through questionnaires. From April 2007 through March 2008, the baseline survey, conducted during pregnancy, collected information on the number of children (older siblings). The questionnaire in the eighth survey (at approximately 60 months postpartum) asked participants about the frequency of common colds in their children since their fourth birthday. We defined high frequency of common colds as experiencing ≥6 common colds since turning age 4 years.

Among the 1197 children aged 59 to 71 months, 140 (11.7%) experienced a high frequency of common colds. Compared with having no older siblings, having 1 older sibling and having ≥2 older siblings were independently associated with a lower likelihood of high frequency of common colds at age 5 years. The adjusted odds ratios (95% CI) for high frequency of common colds were 0.67 (0.45-0.99) for 1 older sibling and 0.42 (0.23-0.73) for ≥2 older siblings (P for trend = .001), using no older siblings as the reference group.

This study identified an independent inverse exposure–response relationship between the number of older siblings and high frequency of common colds at age 5 years. Given that the sibling effect is beneficial for colds, it may be necessary to focus on preventing colds in the first child.