Lots of youngsters on your street? Watch out: flu may strike your community sooner and harder than it hits the hip singles neighbourhood down the road. Flu-shot season begins this month, and for the first time vaccination is being pushed for virtually all children in the U.S. — not just those under five.

It's a huge change, and one bolstered by provocative new evidence that children are key flu spreaders. Over four winters, Harvard researchers matched adult visits to Boston-area emergency rooms with census data for 55 zip codes. Flu-like symptoms struck first and worst in the zip codes that were home to the most kids.

'It doesn't mean the areas without kids are protected from flu. It just means they experience flu later and at lower rates.'— Dr. John Brownstein

Every one per cent increase in the child population brought a four per cent increase in adult ER visits, researchers reported this summer in Annals of Emergency Medicine.

"The impact of kids and the flu is clear," says study co-author John Brownstein, an epidemiologist at Children's Hospital Boston. "It doesn't mean the areas without kids are protected from flu. It just means they experience flu later and at lower rates."

Any parent can attest that youngsters are germ factories. It takes years of nagging before they cover coughs and sneezes. Little ones tend to pick their noses. Even teenagers aren't great hand-washers. Crowded schools, preschools and day-care centres act as incubators.

Previously, flu vaccine was recommended for youngsters under five, who can become dangerously ill from influenza. This year, the U.S. government is recommending that children from age six months to 18 years be vaccinated — expanding inoculations to 30 million more school-age children. While they seldom get as sick as the younger tots, it's a bigger population that catches flu at higher rates, so the change should at least cut missed school and work.

And it could have the added benefit of protecting entire communities if less influenza virus gets passed around.

"We're all very enthusiastic and anticipate seeing an indirect benefit, but that's something we need to study and carefully watch," says Dr. Jeanne Santoli of the Centers for Disease Control and Prevention.

Protecting seniors

That cocooning effect could be especially important for people over age 65, who account for most of the 36,000 flu-caused deaths each winter — possibly because the flu vaccine simply doesn't work as well inside aged bodies as it does in the young. While flu vaccine protects 75 per cent to 90 per cent of young healthy people, some research suggests that protection may plummet to 30 per cent among their grandparents. And while more seniors are getting vaccinated, sadly there hasn't been a corresponding drop in their death rates.

Still, some protection is better than none. So scientists urge older Americans to keep getting vaccinated even as they hunt improvements. Research is under way to see if higher doses or adding immune-boosting compounds helps.

But the kid connection is getting increasing attention. Helping to drive the new inoculate-all-children advice is "a belief that if you could do that, you would limit transmission" to the elderly, says University of Rochester flu vaccine specialist Dr. John Treanor.

There's no proof yet, he cautions. But there's mounting suggestive evidence:

• New York City's health department tracked emergency-room visits for five flu seasons, and found school-age children are among the first sickened.
• Earlier work by Boston's Brownstein and Dr. Kenneth Mandl had implicated preschoolers, finding that a spike in respiratory illness in the under-five crowd predicts that about five weeks later, flu-related deaths among the elderly will peak. In their newest study, flu struck the earliest in zip codes with the most preschoolers.
• Carroll County, Md., gave free doses of the nasal vaccine FluMist to 44 per cent of its elementary school students in 2005, and saw less absenteeism that winter than a neighbouring county. More intriguing, Carroll County's unvaccinated high school students missed less school that winter, too, suggesting some community-wide protection.
• Tecumseh, Mich., vaccinated 85 per cent of school-age children just before the 1968 influenza pandemic, resulting in 67 per cent less flu-like illness in that community than in a neighbouring one.

Brownstein's next study, paid for by the government, will track the community impact of vaccinating more school-age children.