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Varicella incidence rates decrease while vaccination rates increase
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Varicella incidence and hospitalization rates have decreased among children since the introduction of varicella vaccine 15 years ago, according to study findings in Pediatrics.
Roger Baxter, MD, of Kaiser Permanente Vaccine Study Center in Oakland, California, and colleagues surveyed children aged 5 to 19 years to determine varicella incidence and varicella history. Study participants were members of Kaiser Permanente of Northern California. Varicella vaccination status was determined via telephone surveys or hospital records for 8,400 to 8,900 children. Researchers reviewed hospital records to assess changes in varicella-associated hospitalization rates. Surveys were conducted in 1995, prior to the introduction of varicella vaccine, and again in 2000, 2003, 2006, and 2009.
Roger Baxter
From 2000 to 2009, varicella vaccination coverage increased from 51% to 98.8% among children aged 5 to 9 years; from 11% to 94.7% among children aged 10 to 14 years; and from 3% to 53.6% among children aged 15 to 19 years.
Overall, the proportion of children who were not vaccinated against varicella decreased from 19% in 1995 to 5.8% in 2009. About 90% of study participants who were not previously vaccinated were recorded as being vaccinated by 2009.
Similarly, the proportion of children who reported ever having varicella decreased from 84.6% in 1995 to 79.5% in 2000; 70.7% in 2003; 59.4% in 2006; and 37.9% in 2009. The proportion of children aged 5 to 9 years who reported ever having varicella significantly decreased from 76% 4.9% between 1995 and 2009. These rates also decreased among other age groups; specifically from 87.5% in 1995 to 13.6% in 2009 among children aged 10 to 14 years; and 85.7% in 1995 to 48.1% among children aged 15 to 19 years.
Varicella incidence rates significantly decreased among all ages since the introduction of varicella vaccination in 1995.
Varicella-associated hospitalization rates decreased among all ages, including adults, according to the researchers. The rate of hospitalization with a primary diagnosis of varicella decreased 13% each year between 1994 and 2009 (95% CI, 0.84-0.9).
During the 15 years since the introduction of varicella vaccine, varicella incidence data from the baseline (1995), 2000, 2003, 2006 and 2009 surveys did not indicate a shift in the burden of varicella to older age groups among children and adolescents. In addition, the incidence of varicella incidence decreased among those vaccinated and as well as those unvaccinated.
“In parallel, varicella hospitalization rates decreased, emphasizing the impact of the varicella vaccination program in all age groups, including adolescents and adults. This study confirms that varicella vaccination has the ability to dramatically decrease varicella burden, both directly and possibly through herd immunity,” the researchers concluded.
Disclosure: One researcher reports financial ties with Merck. The other researchers report no relevant financial disclosures.
Perspective
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Anne A. Gershon, MD
When varicella vaccine was being considered for universal use in the United States, fears were expressed by many that vaccination might lead to an increase of chicken pox in teenagers and young adults, who are at higher risk to develop severe infections with varicella zoster virus than young children.
The data from this paper, collected between 1995, when live attenuated varicella vaccine was licensed in the US, through 2009, showed that the incidence of varicella decreased from 25.8 to 1.3 per 1000 person-years in children and teens aged 5 to 19 years old. This represents a decline of 90-95% in incidence of disease; the vaccine is thus highly effective in clinical use. Importantly, that there was no increase in the illness itself or severity in adolescents during this 15 year period, strongly suggests that waning immunity to varicella after vaccination of young children is not a significant problem.
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