Vaccinating children against flu reduces influenza transmission in the wider community, a study carried out in Hutterite colonies in Alberta, Saskatchewan and Manitoba found. (Chuck Stoody/Canadian Press)

Vaccinating children against seasonal flu helps protect others in their community, a new Canadian study suggests.

Public health researchers had hoped that immunizing enough young people would pay off in protecting others who are not immunized, a concept known as herd immunity, but it hasn't been demonstrated in a randomized trial until now.

The trial was carried out in 49 Hutterite colonies in Alberta, Saskatchewan and Manitoba by Dr. Mark Loeb of McMaster University in Hamilton and his colleagues. Children in the communities between the age of three and 15 were randomly assigned to receive standard influenza vaccine or hepatitis A vaccine, which served as a control. Neither the subjects nor the researchers knew who received which vaccine.

In communities where four out of five children got flu shots, 39 unvaccinated people, or 3.1 per cent of the population, fell sick with lab-confirmed flu during the 2008-2009 flu season. In communities where children didn't get the flu shot, 80 residents, or 7.6 per cent of the population, fell ill.

The results of the trail appear in Wednesday's issue of the Journal of the American Medical Association.

The researchers chose Hutterite colonies for the study because they are composed of 60 to 120 residents who live in single-family homes in tightly knit religious communities that interact like an extended family with limited contact with outsiders. Randomizing entire communities to test the indirect benefit of vaccinating young people wouldn't be feasible in most other settings, the researchers said.

Vaccine priority groups

"Our findings offer experimental proof to support selective influenza immunization of school-age children with inactivated influenza vaccine to interrupt influenza transmission," the study's authors concluded.

Children and teens should be targeted for flu shots to reduce community transmission — even when vaccines are in short supply and delivery is constrained, the researchers said.

Children are contagious for longer and usually fall sick first, though flu is usually less dangerous for them compared with seniors.

The findings don't mean children should be vaccinated instead of seniors but that they should be considered a higher priority in future pandemic planning, Loeb said.

No serious adverse reactions to the vaccine were observed.

Dr. Allison McGeer, an influenza expert at Toronto's Mount Sinai Hospital, questioned whether the protective effect came from the vaccinated children or the mere fact that such a large proportion of the population was immunized.

"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 Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases in Bethesda, Md.

Last month, advisers to the U.S. Centers of Disease Control and Prevention recommended that everyone over the age of six months receive seasonal flu vaccines every year.

With files from The Canadian Press