TORONTO (Canadian Press) -- A landmark study looking at how to limit the spread of influenza has shown what experts have long believed but hadn't until now proved: Giving flu shots to kids helps protect everyone in a community from the virus.

The study, led by Dr. Mark Loeb of McMaster University in Hamilton, Ont., showed the risk of catching the flu was lowered by nearly 60 per cent in communities where a substantial portion of kids aged three to 15 got flu shots.

That level of indirect protection is nearly as good as what healthy adults might expect from getting a flu shot themselves and is perhaps better than what a senior with a waning immune system might expect from a flu vaccination.

"Children are heavily involved in (flu) transmission. If you prevent disease in children, then you're going to prevent infection in older individuals," Dr. Arnold Monto, a flu expert at the University of Michigan in Ann Arbor, said when asked to comment on the study.

The work, published Wednesday in the Journal of the American Medical Association, was funded by the Canadian Institutes of Health Research and the U.S. National Institute of Allergy and Infectious Diseases.

Monto was not involved in this research. But he did lead an earlier study on which Loeb's work builds, called the Tecumseh study. In that seminal work, published in 1970, Monto and colleagues showed that flu rates were reduced by about one-third in a community where school children were vaccinated compared to a community where they were not.

But Monto's study was an observational one, meaning the communities were not randomly assigned to receive vaccine. And evidence generated by observational studies isn't viewed as being as strong as evidence from randomized controlled trials, which is the type of study Loeb led.

Protecting the whole of a community or population by vaccinating a portion is a phenomenon known as herd immunity.

Loeb wanted to see whether one could trigger herd immunity against influenza by vaccinating children, whose susceptibility to flu viruses and patterns of close contact with each other and with adults makes them perfect little vectors for spread.

He figured the answer to the question could be found in Hutterite communities in Alberta, Saskatchewan and Manitoba.

Hutterites, descendents of the 16th-century Anabaptist movement of Switzerland, practise communal farming in small colonies that are relatively isolated from towns and cities. Their colonies are generally made up of between 80 and 120 people, a high proportion of them children.

The combination of lots of children and limited contact with outsiders make these communities perfect places in which to see if there is a herd effect when children are vaccinated against flu. "If it didn't work in this community, there would be no point" studying it elsewhere, said Loeb, an infectious diseases researcher.

He and his team randomly assigned 46 colonies to one of two sides of the study. In half, all children aged three to 15 got flu shots in the fall of 2008. In the other half, the children got shots against hepatitis A. Neither the communities nor the researchers knew which was which.

The communities were closely followed over the 2008-2009 flu season, with nurses visiting twice a week looking for evidence of flu-like illness. People who were sick were tested for flu; positives were later confirmed with a second test, based on blood samples.

A vaccination rate of about 80 per cent in children was seen to confer about a 60 per cent reduced risk of catching flu within a community. Loeb said the effect might have been higher still if they'd vaccinated all children over six months, the youngest age at which flu shots are given.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said the study is a proof of the concept that herd immunity against influenza can be achieved.

"It tells me ... that the more people you vaccinate, the better you are," Fauci said from Bethesda, Md.

"You get better bang for the buck ... when you vaccinate young people. You protect them and you immunize a very efficient source of spread."

But Dr. Allison McGeer, an influenza expert at Toronto's Mount Sinai Hospital, questioned whether the effect was due to the fact that Loeb's team vaccinated kids, or just to the fact that they vaccinated a sizable portion of the population of the colonies randomized to get flu shots.

She and others insisted these findings don't mean children should be vaccinated instead of seniors, but rather in addition to at-risk adults.

"Vaccinate the people that you want to protect and particularly vaccinate the people that are the best spreaders. But you can get away from all of that hair splitting by vaccinating everybody," said Fauci, noting the committee that advises the U.S. government on vaccination policy recently endorsed universal flu vaccination for the United States.

Loeb agreed the findings don't suggest children should be vaccinated instead of seniors. But he did argue they support moving kids higher up the vaccine priority list for future flu pandemics. And he said if flu vaccine shortages occur, one could argue that prioritizing children makes sense.

But McGeer said vaccinating kids against flu wouldn't have as large an impact in large urban settings as it does in small, insular communities. And there is still an argument to be made that while indirect protection is good for seniors -- who are generally at highest risk of dying from influenza -- direct protection is still important.

"The trade-off for a 70-year-old is: Is it better to have your risk of influenza reduced by 60 per cent but have no protection if you get infected, or your risk of influenza getting reduced by 30 per cent but your chances of dying from influenza reduced by 70 per cent if you get it?" she noted.

Copyright The Canadian Press, 2010