Secondhand Smoke Raises Risk for Hospitalization in Young Children With Influenza

Daniel M. Keller, PhD

October 27, 2010

October 27, 2010 (Vancouver, British Columbia) — Young children are put at risk for hospitalization because of influenza if the mother is young, the household income is below the poverty level, or if the child is not up to date on vaccinations or has an underlying medical condition. In addition, a child is at risk if most household members are smokers. However, vaccination of any household member protects a child from such hospitalizations.

Nila Dharan, MD, fellow in the Division of Infectious Diseases at New York University School of Medicine in New York City, presented these findings here at the Infectious Diseases Society of America (IDSA) 48th Annual Meeting. The work was conducted when she was in the Influenza Division of the Centers for Disease Control and Prevention in Atlanta, Georgia.

Other child risk factors for complications and hospitalization from influenza are being male, being younger than 12 months, having been premature, and having had a low gestational birth weight.

Dr. Dharan and coworkers investigated influenza-associated hospitalizations in children between 6 and 59 months of age as part of a population-based surveillance of vaccine effectiveness in the United States. The study was conducted over 3 influenza seasons, from 2005 to 2008, by the Emerging Infections Program, and represented hospitals providing care to 9% of American children. The researchers looked at this age group because, compared with older children, these children are at increased risk for severe illness and hospitalization from influenza.

Cases (n = 290) were defined as children hospitalized within 14 days of diagnosis with laboratory-confirmed influenza. Cases were matched, in a 1:4 ratio, with ZIP-code-matched controls 4 years or older (n = 1089) who were not hospitalized with influenza in the same season. The investigators conducted interviews with caregivers and physicians within 120 days of admission to determine child, caregiver, and household characteristics.

More than half of the cases and controls came from the 2007/08 season. The median age was 20 months (range, 5 to 59 months), and 60% of subjects were in the 6- to 23-month age range.

In a multivariate analysis, several independent factors affected the risk for hospitalization related to influenza. "Some of these factors had already been recognized as increasing risk, including certain medical conditions (like asthma), blood disorders (like sickle cell disease), cancer, and neurologic conditions (like seizure disorders)," Dr. Dharan reported.

Pulmonary conditions more than doubled the risk (odds ratio [OR], 2.7; 95% confidence interval [CI], 1.8 - 4.2), hematologic and oncologic conditions raised it 12-fold (OR, 12.1; 95% CI, 4.6 - 31.6), and neurologic conditions quadrupled the risk (OR, 4.1; 95% CI, 1.7 - 9.9).

Having a mother younger than 26 years doubled the risk (OR, 2.1; 95% CI, 1.3 - 3.4), as did a household income below the poverty level (OR, 2.1; 95% CI, 1.2 - 3.4). There was a greater risk for hospitalization if the child was not up to date on all vaccinations (OR, 1.7; 95% CI, 1.1 - 2.7). If a child was not fully vaccinated against influenza, vaccination of any household member dropped the child's risk for hospitalization in half (OR, 0.5; 95% CI, 0.3 - 0.8) (P < .05 for all). If more than half the household members were smokers, the risk for hospitalization more than doubled (OR, 2.3; 95% CI, 1.0 - 5.3).

"Secondhand exposure to cigarette smoke has been suggested as a risk factor in previous studies, but this study is the largest to date examining the risk of childhood exposure to cigarette smoke and hospitalization specifically for influenza," Dr. Dharan said.

She noted that some limitations of the study related to laboratory testing for influenza, in that physicians might have ordered tests not based on standardized criteria, and the tests could have varied in their specificity and sensitivity. Another limitation is that the date of the household member's vaccination was not collected, and recall bias might have affected some responses.

"Our study highlights the importance of reducing children's environmental exposure to smoke and the need for vaccination of both the child and their household contacts to reduce children's exposure to influenza," she said. "It also highlights the benefits of routine childhood vaccination, which may reduce the risk of secondary bacterial infections for children who are infected with influenza."

Andrew Pavia, MD, chair of the Pandemic Influenza Task Force for IDSA and chief of the Division of Pediatric Infectious Diseases at the University of Utah in Salt Lake City, said that the risk factors that Dr. Dharan and colleagues described are important because they have not previously been demonstrated. Secondhand smoke has been shown to be a major risk factor for hospitalization for infection with respiratory syncytial virus and for asthma but, to his knowledge, never before for influenza.

"Given that flu hospitalization affects a much wider variety of pediatric ages and can be a lot more severe, it's pretty important to identify that as something that is a preventable, modifiable risk factor," he told Medscape Medical News. "In a home with young children, I think there's a major issue there."

He said it is helpful to know that vaccinating other family members helps to protect a child in the home. Previous studies have shown that such vaccination reduces the risk of a child acquiring flu, but "the fact that it actually prevents hospitalization, compared with people who had flu and weren't hospitalized, adds an extra layer to it. It suggests that vaccine in the household members may attenuate severity, not just whether or not you get infected," he said.

Dr. Pavia speculated that one explanation for the attenuation of the severity of influenza symptoms with the vaccination of household contacts is that the severity of the disease might be related to the quantity of viral exposure and the form of exposure, such as transmission by hand contact as opposed to inhaled aerosols from coughs and sneezes.

Dr. Dharan and Dr. Pavia have disclosed no relevant financial relationships.

Infectious Diseases Society of America (IDSA) 48th Annual Meeting: Abstract 1430. Presented October 24, 2010.


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