The 14-year study followed 7,585 children who were vaccinated in 1995, when they were 12 to 23 months old, to assess the long-term effectiveness of the vaccine and the impact on the epidemiology of varicella (chicken pox) and herpes zoster (shingles). Researchers also observed the impact of the second dose of varicella vaccine, introduced in 2006.
The varicella vaccine was licensed in the United States in 1995, and recommended soon after by the Advisory Committee on Immunization Practices for routine administration to children. Prior to that, chicken pox was ubiquitous, with more than 90 percent of young people experiencing infection by the age of 20.
Over the entire follow-up period, the incidence rate of chicken pox in this cohort was 9 to 10 times lower than corresponding rates in unvaccinated children of the same age in the pre-vaccine era. This resulted in an overall vaccine effectiveness rate of approximately 90 percent.
"Clearly, the vaccine is a very effective tool in preventing or limiting the severity of chicken pox in young people," said Randy Bergen, MD, chief of outpatient pediatrics at Kaiser Permanente's Walnut Creek Medical Center and a pediatric infectious disease consultant. "As with any vaccine, though, the rate of vaccination has a huge impact on effectiveness. The more children vaccinated, the more effective the vaccine is for the entire community. At Kaiser Permanente, our use of a comprehensive electronic health record, KP HealthConnect, enables us to quickly identify children in the targeted age ranges who have not been vaccinated, and to reach out to their parents to ensure they get the shots. Keeping vaccination rates high confers benefit on the community as a whole because there are fewer children who can contract and spread the virus."
A total of 1,505 breakthrough cases of chicken pox were reported within the study cohort of 7,585 children in the 14 years following varicella vaccination. "Breakthough cases" are so named because they occur despite the child having received the varicella vaccine; the virus "breaks through" the defenses afforded by the vaccine. Cases were classified as "mild" (less than 50 lesions), "moderate" (51 to 300 lesions) and "severe" (more than 300 lesions). Very few cases were severe (only 28 of 7,585 children over 14 years), whereas in the pre-vaccine era most children experienced severe symptoms. Prevention of moderate to severe disease was achieved with one dose of varicella vaccine; no cases were reported after the second dose.
The incidence rate of breakthrough varicella steadily decreased over time and no increase was observed during the 14 years of follow-up. The apparent increase in the vaccine's effectiveness over time, according to lead author Roger Baxter, MD, co-director of the Kaiser Permanente Vaccine Study Center, "is likely the result of vaccine failure occurring early, while breakthroughs became rare due to high vaccine effectiveness both directly and through herd immunity."
The continuing decline in breakthrough rates observed in 2008 and 2009 may have been the result of the implementation of the second dose in 2006, researchers said. The second dose of varicella is typically given at ages 4 to 6 years. However, it could potentially be of more benefit if given early after the first dose — if varicella is circulating — by increasing protection for infants too young to receive the vaccine and immune-compromised children who cannot receive a live vaccine.
The risk of herpes zoster, commonly known as shingles, was not increased in vaccinated children, and appeared to be lower in vaccinated children than in the pre-vaccine era. There were 46 confirmed cases of shingles among the cohort, suggesting an approximately 40 percent decreased incidence of herpes zoster in vaccinated children.
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Additional authors on the study include Paula Ray, MPH, Edwin Lewis, MPH, and Bruce Fireman, MA, with the Kaiser Permanente Vaccine Study Center; Patricia Saddier, MD, PhD, and Trung N. Tran, MD, PhD, of the epidemiology department, Merck Sharp & Dohme, Corp., Whitehouse Station, N.J.; Steve Black, MD, of the Center for Global Health, Cincinnati Children's Hospital, Cincinnati; Henry R. Shinefield, MD, of the University of California San Francisco Medical Center; and Paul M. Coplan ScD, MBA, of Purdue Pharma, Stamford, Conn.
Trung Nam Tran, MD, and Paul Coplan were employees of Merck Sharp & Dohme, Corp. at the time of the study. Patricia Saddier, MD, is currently still an employee of Merck Sharp & Dohme, Corp. Roger Baxter, MD, has received research grants from Merck, Sanofi Pasteur, GSK and Novartis. Steve Black, MD, is a consultant for Novartis, and is on data safety monitoring boards for Novartis, GSK and the World Health Organization. All other authors report no conflicts of interest.
About the Kaiser Permanente Vaccine Study Center
Founded in 1985, the Kaiser Permanente Vaccine Study Center began as a way of responding to numerous requests to use Kaiser Permanente's large population for vaccine efficacy studies. Key studies have focused on Haemophilus influenza, type B (Hib), chickenpox, pneumococcus, rotavirus and flu vaccines. The center operates 31 sites in Northern California and collaborates with Kaiser Permanente's Northwest, Hawaii and Colorado regions, as well as participates in several Centers for Disease Control and Prevention and National Institutes of Health studies. For more information, visit http://www.dor.kaiser.org/external/DORExternal/vsc/index.aspx.
About the Kaiser Permanente Division of Research
The Kaiser Permanente Division of Research conducts, publishes and disseminates epidemiologic and health services research to improve the health and medical care of Kaiser Permanente members and the society at large. It seeks to understand the determinants of illness and well-being and to improve the quality and cost-effectiveness of health care. Currently, DOR's 600-plus staff is working on more than 250 epidemiological and health services research projects. For more information, visit http://www.dor.kaiser.org.
About Kaiser Permanente
Kaiser Permanente is committed to helping shape the future of health care. We are recognized as one of America's leading health care providers and not-for-profit health plans. Founded in 1945, our mission is to provide high-quality, affordable health care services and to improve the health of our members and the communities we serve. We currently serve more than 9 million members in nine states and the District of Columbia. Care for members and patients is focused on their total health and guided by their personal physicians, specialists and team of caregivers. Our expert and caring medical teams are empowered and supported by industry-leading technology advances and tools for health promotion, disease prevention, state-of-the-art care delivery and world-class chronic disease management. Kaiser Permanente is dedicated to care innovations, clinical research, health education and the support of community health. For more information, go to: kp.org/newscenter.
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