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Vaccinating Children Against Influenza May Trigger 'Herd Immunity' BETHESDA, Md and CHICAGO -- March 9, 2010 -- Immunising children and adolescents with inactivated seasonal influenza vaccine can significantly protect unvaccinated community members against influenza as well. The study, published in the March 10 issue of JAMA, was conducted to determine if immunised children could act as a barrier to limit the spread of influenza to the wider, unvaccinated community, a concept known as herd immunity. “Current vaccine policy focuses on immunizing those at high risk of complications of influenza. As a component of a broader policy to prevent the spread of influenza and reduce its complications, using immunization to interrupt community-wide transmission of influenza may be effective for protecting the entire population, including those at high risk,” the study authors write. They add that children and adolescents appear to play an important role in the transmission of influenza, and that selective vaccination against influenza among this group may interrupt virus transmission and protect those not vaccinated. Mark Loeb, MD, MSc, McMaster University, Hamilton, Ontario, Canada, and colleagues recruited volunteers from 46 Canadian Hutterite religious colonies that have limited contact with surrounding, non-Hutterite populations. “These tightly knit communities resemble extended families but are composed of single families each residing in their own house, where children and adolescents between the ages of 3 years and 15 years attend school. Approximately 60 to 120 people reside on each colony,” the authors write. A total of 947 children aged 36 months to 15 years participated in the trial; 502 children in 22 colonies received 2008-09 seasonal influenza vaccine, while 445 youth in the other colonies received hepatitis A vaccine. The hepatitis A vaccine served as a control vaccine for comparison. The average vaccine coverage among healthy children of clusters assigned to the influenza vaccine was 83%, which was similar to the average vaccine coverage among colonies assigned to hepatitis A vaccine (79%). In the 6 months after the children were vaccinated, 119 of 2,326 unvaccinated community members (who were of all ages) developed laboratory-confirmed cases of influenza. Of these, 80 (7.6%) of 1,055 were from colonies where children received hepatitis vaccine, while 39 (3.1%) of 1,271 were from colonies where children received the influenza vaccine. The researchers found that influenza vaccination was 61% effective at indirectly preventing illness -- that is, protecting via herd immunity -- in unvaccinated individuals if they lived in a colony where approximately 80% of the children had received the influenza vaccine. Among all study participants (those who were and those who were not vaccinated), 80 (4.5%) of 1,773 in the influenza vaccine colonies and 159 (10.6%) of 1,500 in the hepatitis A vaccine colonies had confirmed influenza illness for an overall protective effectiveness of 59%. No serious vaccine adverse events were observed. “Considering, for instance, the rapid spread of influenza A(H1N1) in the 2009 pandemic, understanding whether influenza transmission can be prevented or reduced by immunizing children is of high priority so that groups such as pregnant women and aboriginal populations who are at high risk of complications may potentially be indirectly protected,” the authors write. The findings, they write, “...offer experimental proof to support selective influenza immunization of school aged children ... to interrupt influenza transmission. Particularly, if there are constraints in quantity and delivery of vaccine, it may be advantageous to selectively immunize children in order to reduce community transmission of influenza.” The research was funded in part by the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health, and by the Canadian Institutes for Health Research. SOURCE: The National Institutes of Health and JAMA E-mail this page to a friend or colleague! To print, use this version