WASHINGTON - Preschoolers may signal the arrival of flu season: Thirty days after hacking 3- and 4-year-olds start showing up in doctors' offices and emergency rooms, flu-ridden adults follow.
It's a provocative discovery sure to bolster growing calls to vaccinate more healthy children against influenza - to help keep the misery from spreading.
Moreover, researchers using a system that almost instantly tracks Boston-area health care found that a spike in respiratory illness among youngsters under 5 predicts that about five weeks later, influenza-related deaths among the elderly will peak.
The study doesn't prove preschoolers actually drive each winter's flu epidemic, just that they're harbingers for waves of illness.
"What we think is most likely is that 3- and 4-year-olds are early spreaders of influenza because of the preschool setting," said John Brownstein, an epidemiologist at Children's Hospital of Boston and co-author of the new study.
Brownstein calls the close quarters of preschool and day care, full of kids who don't yet cover their sneezes and are apt to pick their noses, "hotbeds of infection."
Regular schools are too, caution other experts who cite evidence that school-age children may be the biggest influenza spreaders. Older children may have better hygiene, but the 5- to 18-year-old population is much larger and more active in the community, giving them greater contact with the elderly who are most vulnerable to influenza, explains flu specialist Ira Longini, a biostatistician at Emory University.
-Some 85 percent of school-age children in Tecumseh, Mich., were vaccinated just before the 1968 influenza pandemic, resulting in 67 percent less flu-like illness in that community than in a neighboring one.
-Researchers now are giving the nasal flu vaccine FluMist to school-age children in two Texas towns, Temple and Bolton, to see if it lowers influenza's toll compared with similar communities. Once a quarter of the schoolchildren were immunized, researchers recorded an 8 percent to 18 percent decline in adults seeking care for respiratory illnesses.
Healthy children over age 2 aren't on the government's list of flu-shot targets each year because they're typically strong enough to fight off the virus without complications. Parents can choose to get them vaccinated anyway, as supplies allow.
More vaccine is expected this year than last. Still, the Centers for Disease Control and Prevention is urging doctors to reserve flu shots for people at highest risk from influenza until Oct. 24; then anyone else who wants a shot can seek one.
High-risk people include those 65 or older; babies and toddlers ages 6-23 months; anyone with a chronic illness such as heart or lung disease; pregnant women; and caregivers of high-risk patients.
As the government works to increase annual flu vaccine supplies, flu specialists question whether it's time to begin targeting not just those at risk of influenza's complications, but also those most likely to spread the virus. Already federal health officials are considering if one day everyone should get a yearly flu shot, the topic of a meeting hosted by Emory and CDC later this month that's sure to focus on the new child research.
Brownstein and Dr. Kenneth Mandl, a pediatric emergency physician at Children's, used health monitoring systems developed to detect bioterrorism to track Boston-area emergency rooms and a large HMO network during four flu seasons.
Preschoolers showed up with flu-like illnesses first, sometimes as early as the end of September, the researchers report in this month's American Journal of Epidemiology. Babies and toddlers followed a week or two later, followed by older children and finally adults.
Checking CDC's week-by-week estimates of flu-related deaths, the researchers concluded that respiratory-related visits to pediatric emergency rooms by children under 5 peaked, on average, five weeks before mortality did.
A caveat: Few of the children underwent laboratory testing to prove they had influenza and not a similar virus.
Spotting the leading wave can help communities gear up for flu outbreaks. This fall, Brownstein and Mandl are collecting daily data on doctor visits to see if they can predict how the flu season evolves in real-time.
While a flu diagnosis means staying home, it's hard to tell if a youngster has influenza, a cold or some other virus. "Some kids will sniffle for weeks," says Mandl, and not appear sick enough to remain cooped up. He advises nagging them to wash hands often and sneeze into their elbows to stem the germs' spread.