Following the “hygiene hypothesis”, a plethora of studies have
investigated the role of sibship composition in atopic diseases, but
findings are largely conflicting. We undertook the first systematic
review to synthesize the global literature linking birth order and
sibship size to the risk of allergic rhinitis. Fifteen databases were
searched from inception until 20 th October 2021.
Screening and data extraction were performed in pairs. Comparable
numerical data were analyzed using meta-analysis with robust variance
estimation (RVE). We included 76 reports based on 66 studies that met
our inclusion criteria, encompassing >2 million subjects.
Being second- or later-born child was associated with protection against
both current (pooled risk ratio [RR] 0.79, 95%CI 0.73-0.86) and
ever (RR 0.77, 95%CI 0.68-0.88) allergic rhinitis. Having siblings,
regardless of birth order, was associated with a decreased risk of
current allergic rhinitis (RR 0.89, 95% CI 0.83-0.95) and allergic
rhinoconjunctivitis (RR 0.92, 95%CI 0.86-0.98). These effects were
unchanged across age, time period, and geographical regions. Our
findings indicate that primarily, a higher birth order, and to a lesser
extent the number of siblings, is associated with a lower risk of
developing allergic rhinitis.