Chickenpox Decline Continues, Even in Unvaccinated Groups

Laurie Barclay, MD

August 14, 2015

Varicella (chickenpox) has continued to decline in the United States as vaccination coverage with the two-dose program has increased, according to a retrospective cohort study published online August 12 in the Journal of the Pediatric Infectious Diseases Society. The findings suggest herd immunity, protecting those who were not vaccinated.

"We found that, in our study, rates for varicella in the U.S. continued to decline as the varicella vaccine program has become fully implemented," coauthor Jessica Leung, MPH, from the Centers for Disease Control and Prevention in Atlanta, Georgia, said in a news release. "We saw significant declines in rates of varicella after the one-dose vaccine was recommended in 1995 in the U.S., and we're continuing to see additional declines in varicella after two doses were recommended in 2006."

According to the Centers for Disease Control and Prevention, US incidence of varicella was 4 million in 1995, with 11,000 varicella-related hospitalizations and 100 to 150 deaths. Since the one-dose varicella vaccination program began in 1996, varicella prevalence has fallen, with an 88% decline in varicella-related hospitalizations and a 59% decline in outpatient visits during 1994 to 2002, as shown in previous studies. However, continued sporadic outbreaks prompted implementation of a two-dose vaccination program in 2007.

Using claims data from 1994 to 2012 in the Truven Health MarketScan databases, the investigators examined trends in rates of varicella-related outpatient visits and hospitalizations for enrollees aged 0 to 49 years. Analyzed data included outpatient laboratory testing and antiviral use and pediatric strokes, with 1994 to 1995 considered to be the prevaccination period and 2006 to 2012 the two-dose varicella vaccination period.

Outpatient visits for varicella decreased by 84% from the prevaccination period to 2012, and hospitalizations for varicella decreased by 93%. During the two-dose period, from 2006 to 2012, outpatient visits for varicella decreased by 60%, and hospitalizations for varicella decreased by 38%.

Children and adolescents aged 1 to 19 years, the population targeted for vaccination, had the greatest reductions in varicella outpatient visits and hospitalizations. However, largely unvaccinated populations, including infants younger than 12 months and adults, also had dramatic reductions in outpatient visits and hospitalizations, suggesting the possibility of herd immunity.

"The surrounding population that can be vaccinated are not getting sick, and therefore the data suggest that these infants are also being protected," Dr Leung said. "We're seeing that for adults as well."

Among patients with an outpatient visit for varicella, the proportion of those having laboratory testing for varicella increased from 6% in 2003 to 17% in 2012. Claims for antiviral treatment did not change significantly over time, with the total being 21,445 (17%). From 1994 to 2012, there was no decline in pediatric strokes.

The authors note several study limitations, including coding and diagnostic errors inherent in administrative data, changes in secular patterns of healthcare use possibly affecting the results, inability to control for changes in some demographic variables across time, and limited generalizability.

"[A]fter 16 years, the varicella vaccination program continues to mature and to provide increasing beneficial national impact," the study authors conclude. "We provide evidence from our large study population that outpatient visit and hospitalization rates continue to decline, as does case severity."

The authors have disclosed no relevant financial relationships.

J Pediatric Infect Dis Soc. Published online August 12, 2015. Abstract


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