In the March 10 issue of the medical journal JAMA, a new study has just been released on the concept of herd immunity.  The study was created to determine if immunized children could act as a barrier to limit the spread of influenza to the wider, unvaccinated community, a concept known as herd immunity.  Interesting idea?!?!

But first what is herd immunity??  The Virology Blog has a nice column on the topic. “Infection stops when the viral load drops below the threshold required to sustain the infection in the population. Not every individual in a population need be immunized to stop viral spread, but a sufficient number must become immune to impede virus transmission. It varies between 80-95% depending on the virus and the population.”

Cows Pasture 1
The point is that herd immunity is conferred when immunity of the population reaches a certain level. Whether that level is achieved by immunization programs, or by the spread of vaccine virus from a vaccinated to a non-vaccinated individual does not matter. I must confess, every time I hear the term - herd immunity - I think of cows in a field! Moooooo!

OK, back to the study…The researchers 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.”

This diagram from the National Institute of Allergy and Infectious Diseases demonstrates the concept.