Number of siblings, birth order influence risk for allergic rhinitis in children

Children with siblings had a reduced risk for developing allergic rhinitis, particularly if they were younger and had multiple older siblings, according to study results.

“Since David Strachan, MD, PhD, in the late 1990s reported an inverse relationship between birth order and allergic rhinitis, this association has been extensively studied,” Daniil Lisik, a PhD student at the Krefting Research Centre at University of Gothenburg in Sweden, said during the presentation at the European Academy of Allergy and Clinical Immunology Hybrid Congress.

“This association basically is largely based on the suggestion that cross infection between siblings at an early age causes a modulation of the immune system in a way that reduces the risk of allergy development, but the results have been conflicting,” Lisik said.

A search of 15 databases with no restriction on publication date or language yielded 77 studies, with 59 published in the last 20 years. These studies included 36 examining sibship size, or the number of siblings regardless of birth order, and 59 examining birth order.

Also, 37 studies recruited participants from Europe, 24 from Asia, seven from North America, six from Oceania, three from South America and one from Africa, with 31 studies considered high quality, 33 considered moderate and 13 considered weak.

Children with any number of siblings had a significantly decreased risk for allergic rhinitis (OR = 0.88; 95% CI, 0.82-0.94). When the researchers divided sibship sizes into different categories, however, significantly decreased risks only persisted for children with three siblings (OR = 0.93; 95% CI, 0.87-0.99) or five or more siblings (OR = 0.59; 95% CI, 0.33-0.84).

Risks varied as well in these studies based on age at assessment, with a significant risk reduction observed for those aged 0 to 7 years (OR = 0.63; 95% CI, 0.41-0.84) and aged 8 to 15 years (OR = 0.95; 95% CI, 0.9-0.99), and a trend toward reduced risk among those aged 15 years and older (OR = 0.89; 95% CI, 0.77-1.01).

When birth order was examined, the researchers found that any number of older siblings had a significant association with a decreased risk for allergic rhinitis in a dose-dependent fashion.

Children who were born second had an odds ratio for allergic rhinitis of 0.82 (95% CI, 0.75-0.9). Risks then further reduced for those born third (OR = 0.63; 95% CI, 0.5-0.78), fourth (OR = 0.5; 95% CI, 0.35-0.88) or fifth or later (OR = 0.42; 95% CI, 0.27-0.56).

“It seems that there is a significant association between having siblings or being second born or later and having a smaller risk for developing allergic rhinitis,” Lisik said.

However, Lisik cautioned that it is difficult to precisely identify causality and different paths of effect in the epidemiological and observational studies that were the basis of this systematic review.

“Furthermore, what limits our results is that the studies were highly heterogenous, both in terms of methodology and participants,” Lisik continued, adding that studies ranged in publication date from the 1970s to 2022.

“We have a lot of studies that look at this in different ways,” he said.