Varicella Vaccine Reduces Incidence; No Shift to Older Kids

Lara C. Pullen, PhD

June 09, 2014

( UPDATED June 12, 2014 ) The varicella vaccine was introduced 15 years ago. Since then, there has been a reduction in the incidence of varicella as well as subsequent hospitalizations from varicella. There is no evidence of a shift in disease burden from young children to older age groups, according to results from a series of cross-sectional surveys

In 1995, 76% of children aged 5 to 9 years had had chickenpox. In 2009 only 4.9% of children in this age group had experienced the disease. Pointing to these data, Edgar Marcuse, MD, MPH, pediatrician at Seattle Children's Hospital in Washington, told Medscape Medical News, "[This is] quite an impressive study."

The Advisory Committee on Immunization Practices licensed and recommended the varicella vaccine for routine use in 1995. "Many physicians were concerned that varicella immunization would simply push the disease into the adolescent years, where it can actually be more severe. This led to some hesitancy to strongly recommend the vaccine," explained L.J. Tan, PhD, chief strategy officer of the Immunization Action Coalition in St. Paul, Minnesota, to Medscape Medical News. Dr. Tan was not involved in the current study.

Roger Baxter, MD, from the Kaiser Permanente Vaccine Study Center in Oakland, California, and colleagues addressed these concerns by administering 5 surveys over the course of 15 years to patients of Kaiser Permanent Northern California (KPNC).

The researchers obtained information on varicella vaccination from the KPNC immunization database. They obtained information on varicella history and occurrence by telephone survey and used primary hospital discharge diagnoses to estimate the rate of varicella hospitalizations.

Varicella hospitalization rates declined by 90% from 1994 to 2009. The decline was evident in all age categories, suggesting the varicella vaccination program was having a positive effect in adolescents and adults, as well as in children.

As healthcare providers implemented vaccine recommendations over time, the investigators found the expected decrease in the proportion of varicella-unprotected children and adolescents in all age groups, including 15- to 19-year-olds (declining from 15.6% unprotected in 1995 to 7.6% unprotected in 2009).

From 1995 to 2009, the overall incidence of varicella in patients aged 5 to 19 years declined from 25.8 to 1.3 per 1000 person-years. When the researchers stratified data by age, they found a 90% to 95% reduction in varicella incidence in all age categories: 5 to 9, 10 to 14, and 15 to 19 years. Age-adjusted varicella hospitalization rates also declined in the general member population from 2.13 per 100,000 in 1995 to 0.25 per 100,000 in 2009 (a 90% decline).

In June 2006, the Advisory Committee on Immunization Practices recommended a second dose of varicella vaccine, and KPNC implemented the recommendation. The 2009 survey results reflected the 2-dose protocol and revealed that the age-specific incidence rates of varicella were at an all-time low. The investigators suggest the results are consistent with an added benefit from the second dose.

Dr. Tan agrees: "Two doses of varicella vaccine are a highly effective intervention to prevent chickenpox, and even for those who became ill, it's nice to see that vaccination reduced the severity of disease as indicated by hospitalization rates."

The study was funded by Merck Sharp & Dohme, Corp. One author is an employee of Merck and holds Merck stocks. The other authors have disclosed no relevant financial relationships. The Immunization Action Coalition receives funding from a variety of sources, including Merck Sharpe & Dohme, Corp. Dr. Marcuse has disclosed no relevant financial relationships.

Pediatrics. Published online June 9, 2014. Full text

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