Scientists have long believed that immunizing some members of a community against a disease would protect the entire community and that this so-called 'herd immunity' could be used to control influenza outbreaks. But, until recently, researchers had never tested the theory in a controlled, randomized study.
So, Dr. Mark Loeb and his colleagues at McMaster University in Hamilton, Ontario, conducted a vaccine trial in 49 isolated rural communities in western Canada. They vaccinated children from three to 15 years old in half of the communities. In the others, the youngsters did not get the influenza vaccine.
"This type of study was to show that by immunizing children - who may not be at the highest for complications but can mount a very good response to the vaccine - we could stop the spread of the virus, and in doing so, protect people who, if they got infected, would be at high risk for those complications," says Loeb.
A dramatic effect
For six months, nurses checked in weekly with the participants to determine if anyone had come down with the flu. Many people had, but in the communities where children had been vaccinated, there were less than half as many cases of influenza.
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"We found that immunizing kids with influenza vaccine had about a 60 percent effectiveness at protection," Loeb reports. "In other words, the herd immunity that protected the other residents of those communities was almost as great as if they themselves had been vaccinated."
Loeb says the study's findings suggest that immunizing healthy children and adolescents could be an efficient way to help prevent the spread of influenza.
Those results have important implications for disease control, especially if there is only a limited supply of vaccine. "If you had one group to immunize, you should at least consider these groups because it could just stop influenza in its tracks," Loeb says.
The study appears in the March 10 issue of the "Journal of the American Medical Association."