Healthy 3 and 4-year-olds drive flu epidemics

Current immunization policies recommend universal flu vaccination for children aged 6-23 months, but shots are advised for older children only if they have high-risk medical conditions.

New data compiled by researchers at Children's Hospital Boston and Harvard Medical School, reported in October 1 issue of the American Journal of Epidemiology, suggest that otherwise healthy 3- and 4-year-olds drive flu epidemics, a pattern that may warrant consideration when formulating immunization policy.

The researchers leveraged a real-time computerized biosurveillance system linking five diverse health-care settings in Greater Boston, and examined medical visits from 2000 to 2004. Children aged 3 to 4 clearly led influenza epidemics, presenting with flu-like respiratory illness as early as late September. Children aged 0-2 began arriving a week or two later, while older children first arrived in October and adults began arriving only in November.

Moreover, flu-like illness in children under age 5, compared with all other age groups, was the most predictive of pneumonia and influenza deaths in the general population as determined from a Centers for Disease Control and Prevention database. Visits by children aged 0-2 provided the best prediction of mortality, but those of 3- and 4-year-olds followed close behind, suggesting that preschoolers, not just infants and toddlers, are important spreaders of flu to vulnerable groups.

"The data make sense because preschools and daycares, with their close quarters, are hotbeds of infection," says Dr. John Brownstein, the paper's lead author and a faculty member of the Children's Hospital Informatics Program at the Harvard-MIT Health Sciences and Technology program. "The data suggest that when kids are sneezing, the elderly begin to die. Three- and 4-year-olds are sentinels that allow us to focus our surveillance systems."

Influenza kills tens of thousands of Americans each year. Previous studies have shown decreases in household flu transmission and in adult flu mortality when children are immunized. Additional studies have also suggested that preschoolers drive flu epidemics, but they are based on simulations.

"Our study was not a simulation," says senior investigator Dr. Kenneth Mandl, an attending physician in Children's Department of Emergency Medicine and an informatics program faculty member. "This was real life."

Brownstein and Mandl believes that the surveillance data support a different approach to immunization, one based not on who's at risk for influenza, but on who's spreading the disease.

"General influenza immunization policies target high-risk individuals – kids under 24 months, the elderly, and people with underlying disease," Mandl says. "But if avian flu is coming in and you want to stop it from spreading, you might want to vaccinate the people who are transmitting it to everyone else. In a pandemic, where people are getting sick and dying, you might want to reallocate who gets the vaccine."

The study drew its data from two real-time population health monitoring systems: the Automated Epidemiological Geotemporal Integrated Surveillance system, or AEGIS, developed by Children's emergency department, and the National Bioterrorism Syndromic Surveillance Demonstration Project. The health care settings analyzed were pediatric, adult, general and community emergency departments and a large HMO network.

"Millions of federal dollars are invested in real-time surveillance to detect bioterrorism," Mandl points out. "But these systems have a dual use, allowing us to look very quickly at the transmission dynamics of diseases like influenza during peacetime."