You Herd It Here

Vaccines are so great at protecting people from disease that sometimes they protect even those people who haven't yet received them. This strange concept is called herd immunity, and I've blogged a bit about it before in relationship to the HPV vaccine. Basically, the way herd immunity works is that when enough people have been vaccinated against a disease, it becomes less likely for a non-vaccinated person to come into contact with someone who's sick. Herd immunity only works if a large enough proportion of the population has been vaccinated, but it can have a powerful effect. It can also help protect those who, for one reason or another, can't get vaccinated themselves.

I bring all this up because a paper recently published in the journal Pediatrics found that the HPV vaccine is causing some herd immunity effects. In a study of 13 to 26 year old girls attending two primary care clinics, the researchers found that the prevalence of vaccine-type HPV dropped substantially in the four years after the vaccine became available. Furthermore, not only did the prevalence of the virus go down significantly in vaccinated girls, it also dropped by almost half in girls who didn't receive the vaccine. This suggests that herd immunity was having its fascinating effect.

One important note - the study did find the prevalence that non-vaccine types of HPV seemed to increase in vaccinated girls. A shift in common types of HPV, rather than an elimination of HPV infection, is something that many scientists have been worried about with this vaccine. It will be interesting to see if the effect is confirmed by future studies. If it is, it could substantially change the debate.

• HPV Vaccine Basics
• HPV 101

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Source: http://std.about.com/b/2012/07/18/you-herd-it-here.htm

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