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"Herd immunity" causes dramatic drop in infant chicken pox

In the early 1990s, Stanford scientist Ann Arvin, MD, led research that helped explain immune responses to varicella zoster, the virus that causes chicken pox. Her work contributed important scientific background for introducing the chicken pox vaccine.

Now a new study from the Centers for Disease Control examines how widespread use of this vaccine has protected infants too young for the chicken pox shot. The new paper is a stunning example of the power of "herd immunity" – the ability of broadly-used vaccination to protect those who are too fragile to be vaccinated themselves. The vaccine consists of a live but weakened version of the varicella virus, and is not safe to give until 12 months of age.

The study's key finding is that chicken pox cases in infants dropped a whopping 90 percent between the vaccine's 1995 introduction and 2008, the most recent year for which data were available. This is a big deal because in babies chicken pox can be quite severe. In the pre-vaccine era, death rates among infected infants were four times higher than among children who contracted chicken pox between ages 1 and 14.

Continued efforts to improve and expand varicella vaccination programs are well worth it, the study concludes. Some babies are still being exposed to chicken pox via unvaccinated or under-vaccinated older children, supporting the need to broaden "catch-up" programs that deliver a second dose of the vaccine to kids who have gotten only one of the two recommended doses. Babies can also catch chicken pox from adults who suffer shingles, a re-activation of the virus in the body of someone who had chicken pox years before, so shingles vaccination for adults should be expanded, too.

Previously: Vaccination could eliminate chicken pox-related deaths in the U.S.

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