Preschoolers signal flu season, study finds

Oct 18, 2005 (CIDRAP News) – A study that tracked influenza cases by age-groups found that preschoolers led the annual parade of flu-related doctor visits, with sick adults following about 29 days later.

The findings, which appear in the October issue of the American Journal of Epidemiology, support the idea of vaccinating all preschool children, write the authors, led by John S. Brownstein at Boston's Children's Hospital. Current recommendations emphasize flu shots for 6- to 23-month-old children.

The authors sought to identify the best populations for influenza vaccination by using real-time health-monitoring systems to pull age-group data on respiratory illness and look for relationships with overall deaths due to flu and pneumonia. The data were drawn from two real-time population health-monitoring systems,the Automated Epidemiologic Geotemporal Integrated SurveillanceSystem and the National Bioterrorism Syndromic SurveillanceDemonstration Project.

Five healthcare populations were used. These included patients reporting to four different emergency departments that served different age-groups, including children, adults, or all ages. The fifth population consisted of patients who were members of a large health-maintenance organization (HMO) in Massachusetts and sought care for respiratory illness at an ambulatory care clinic.

Several years' worth of data were sorted by age-groups: 0-2; 3-4; 5-11; 12-17; 18-39; 40-64; and older than 64. The researchers compared these data with influenza surveillance findings from the Centers for Disease Control and Prevention (CDC) and with CDC data on overall deaths due to flu and pneumonia in New England.

A pediatric emergency department had the earliest peak in cases of respiratory illness, usually 38 days before the peak in flu-related mortality. Three- to 4-year-olds were seen before other patients, with a mean lead time of 34 days (95% confidence interval, 14.5 to 53.5). Adult emergency department, general emergency department, and community populations offered the shortest warning of flu deaths, with a mean lead time of 2 weeks, the report says. Each healthcare population was found to be a statistically significant predictor of mortality (p<0.0001).

"What we think is most likely is that 3- and 4-year-olds are early spreaders of influenza because of the preschool setting," Brownstein told the Associated Press for a story earlier this month.

Overall, children's illnesses were the best predictors of pneumonia and flu deaths across all the healthcare sites studied, the authors wrote. They concluded that patient age is a key determinant of timing of visits for respiratory illness and that respiratory illness in patients younger than 5 is significantly associated with flu and pneumonia deaths.

"Pediatric populations are sentinels of infection, andthey signal the consequent burden of illness," the report states. "Although this finding does not necessarily prove that preschool-age children are driving the yearly influenza epidemics, they intriguingly suggest that preschool-age children are the initial group infected and may be important in the subsequent spread." The vulnerability of this age group, coupled with the healthcare-seeking behavior of their parents, may help make them such effective sentinels, the researchers add.

The study strengthens arguments for recommending universal vaccination of preschoolers, the authors conclude. The CDC's Advisory Committee on Immunization Practices (ACIP) is currently considering such a recommendation. However, one limitation of the study was that few children had laboratory testing to confirm that their illness was flu.

Brownstein JS, Kleinman KP, Mandl, KD. Identifying pediatric age groups for influenza vaccination using a real-time regional surveillance system. Am J Epidemiol 2005:162(7):686-693 [Abstract]

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