McMaster University

McMaster University

Faculty of
Health Sciences

Giving the flu shot to children protects community

By Laura Thompson
Published: March 9, 2010
Mark Loeb
Dr. Mark Loeb, a professor in the Department of Pathology and Molecular Medicine and the Department of Clinical Epidemiology and Biostatistics

McMaster-led study finds herd immunity effect

Vaccinating children and adolescents against influenza can provide the same level of protection for the wider community.

This groundbreaking finding is the result of a study led by McMaster University infectious disease expert and physician Mark Loeb. The professor in the Michael G. DeGroote School of Medicine led a team of investigators from Canada and the United States to study whether selective immunization of healthy youngsters could prevent the spread of the flu in the wider community.

The researchers found that giving the flu shot to children and adolescents reduced the incidence of influenza by about 60 per cent in individuals who did not receive the vaccine. They also found that when they looked at the whole community, including the children who were immunized, the overall benefit was about the same.

"These results demonstrate conclusively that immunizing children provides a large indirect benefit to people who do not receive the vaccine, to the extent that the protection provided is comparable to directly vaccinating people who are older or at high-risk for the flu," Loeb said.

"Our study provides rigorous evidence showing that immunizing children and adolescents can stop the spread of influenza. In our opinion, this has important public health implications."

The results of the study will be published in the March 10 issue of the Journal of the American Medical Association (JAMA). Funding for the randomized control trial was provided by the Canadian Institutes of Health Research and the U.S. National Institute for Allergy and Infectious Diseases.

"Dr. Loeb’s research will help us better understand how immunization can inhibit the spread of influenza and other infectious diseases," said Dr. Marc Ouellette, scientific director of the Institute of Infection and Immunity at the Canadian Institutes of Health Research (CIHR). "The more we understand about the best methods and procedures for immunizing populations, the greater the chance we have at preventing the spread of infectious disease in the future, reducing illness and fatalities amongst vulnerable segments of the population."

To conduct the study, Loeb and his team recruited participants from Hutterite colonies in western Canada. Although relatively isolated from cities and towns, these tight-knit Anabaptist communities still experience influenza infections and outbreaks.

Participants in the study included 947 healthy children and adolescents aged three to 15 in 49 Hutterite colonies in Alberta, Saskatchewan and Manitoba. The children were randomly assigned according to community to receive a standard dosing of seasonal flu vaccine or hepatitis A vaccine, which was used as the control.

The study also recruited 2,326 community members who did not receive the vaccine, in order to assess the indirect effect of immunizing children and adolescents.

All study participants were monitored for signs and symptoms of the flu from December 2008 to June 2009 and suspected cases of influenza were confirmed through laboratory tests. The researchers found that vaccinating children and adolescents provided indirect protection of 61 per cent to people who did not receive the vaccine. The overall protection provided to both vaccine recipients and non-recipients was 59 percent.

The researchers concluded that giving healthy children and adolescents the influenza vaccine significantly protected unimmunized residents of the communities against the flu. They suggest that this "herd immunity effect" can be achieved when approximately 80 per cent of healthy children and adolescents are immunized.

Current vaccine policy focuses on immunizing people at high-risk of complications of the flu, such as those with chronic medical conditions, people 65 years and older, children under age two and pregnant women.

According to the Public Health Agency of Canada, influenza affects up to a quarter of Canadians each year. While most recover, the virus also results in about 20,000 hospitalizations and 2,000 to 8,000 deaths annually.

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