Since becoming part of routine vaccinations for kids in 1995, the chickenpox (varicella) vaccine has greatly reduced the number of cases of this very contagious illness. But a new study suggests that a single dose of the vaccine doesn't provide long-lasting protection.

Researchers from the Centers for Disease Control and Prevention (CDC) and the Los Angeles Department of Health Services looked at data on 350,000 Californians between 1995 and 2004. Of the more than 11,000 people who developed chickenpox, almost 1,100 had been vaccinated.

That means kids who were vaccinated early on with just one dose may still develop chickenpox at an older age, when the illness may be more severe. However, the study says that a second (booster) shot provides added protection.

Kids are usually vaccinated between 12 to 15 months of age. In 2006, the CDC recommended a booster shot at 4 to 6 years old for further protection. A single dose of the vaccine confers about 85% protection — since its introduction, chickenpox-related hospitalizations and deaths from complications have dropped dramatically.

The study also found that 8- to 12-year-olds who contracted chickenpox after being vaccinated at least 5 years earlier were twice as likely to have "moderate or severe" cases than those who had gotten the vaccine less than 5 years before. In other words, the longer it had been since getting the single dose of the vaccine, the more likely they were to be more sick when they came down with chickenpox at a later age.

What This Means to You

It's recommended that kids get both the vaccine (at 12 to 15 months old) and the booster shot (at 4 to 6 years old). It's also a good idea for older kids, teens, and adults who have never had chickenpox to get a "catch-up" vaccination if they've only gotten the single dose previously.

If you have any concerns about the chickenpox vaccine, or any other vaccine, be sure to talk to your doctor.

Reviewed by: Steven Dowshen, MD
Date reviewed: March 2007

Source: Sandra S. Chaves, MD, MSc; Paul Gargiullo, PhD; John X. Zhang, PhD; Rachel Civen, MD; Dalya Guris, MD, MPH; Laurene Mascola, MD, MPH; and Jane F. Seward, MB, BS, MPH. New England Journal of Medicine, March 15, 2007.