Secondhand smoking (SHS) is an independent risk factor in pediatric bronchial asthma hospitalizations in young children, especially among those who had respiratory syncytial virus (RSV) infection during infancy, according to study findings published in the Annals of Allergy, Asthma & Immunology.

Researchers assessed the effect of secondhand smoking on bronchial asthma (BA) in children who previously had RSV and human metapneumovirus (hMPV) infection, using data from the Japan Health Insurance Association database.

Participants were beneficiaries’ dependents, born from April 1, 2004, to March 31, 2017, who had a diagnosis of RSV/hMPV infection. SHS was based on beneficiary smoking status during an annual examination. The primary outcomes were the diagnosis of BA and hospitalization to control its symptoms.

The RSV-alone group had 2248 participants, the hMPV-alone group had 444 participants, and the RSV + hMPV group had 56 participants. SHS exposure was 38%, 40%, and 39%, respectively, in the 3 groups, with no significant difference (P =.56 with Kruskal-Wallis test). The proportion of patients hospitalized for control of BA was greater in the SHS group vs the no-SHS group (4.5% vs 2.7%; P =.047). In the 509 patients who were infected at 12 months of age or older, the median age at BA diagnosis in the SHS group was younger compared with the no-SHS group (22 months vs 25 months; P =.011).

The close link of SHS to the earlier onset and increased severity of BA in children with a history of RSV or hMPV infection was shown in this study, possibly supporting a synergistic effect of viral infection and tobacco on BA.

For patients with hMPV infection alone, 47 (11%) had their first infection before age 12 months, and no cases of hospitalization for the control of BA occurred in the SHS group. Of the 397 patients with a first infection at age 12 months or older, the median age at BA diagnosis was younger in those with SHS exposure compared with those without (33 months vs 47 months; P =.0022). No significant differences in outcomes occurred in patients with both infections.

According to multivariable logistic regression analysis, among the RSV infection-alone patients, a BA diagnosis before the infection was a significant risk factor for hospitalization. In those infected before the age of 12 months, SHS (odds ratio, 1.69; 95% CI, 1.00-2.85) had an independent association with hospitalization for the control of BA. No significant relationship was observed for the hMPV infection-alone group.

“The close link of SHS to the earlier onset and increased severity of BA in children with a history of RSV or hMPV infection was shown in this study, possibly supporting a synergistic effect of viral infection and tobacco on BA,” the study authors stated. “Protection of these high-risk infants from SHS may reduce the prevalence of pediatric BA and hospitalization rates.”

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