Reduction in the Incidence of Chickenpox Among Children Since the Introduction of the Two-Dose Vaccine Regimen—Southeast Texas, 2005–2012

BACKGROUND:   Varicella-zoster virus causes chickenpox, formerly a widespread childhood disease within the United States. In 1995, a vaccine was introduced to prevent chickenpox, and in 2007 a two-dose regimen was adopted. Since then, incidence within the United States, and Texas, has decreased. The Houston-Galveston health region in southeast Texas serves a population of almost 7 million, accounting for over a quarter of Texas’s population. We aim to describe the epidemiology of chickenpox within the Houston-Galveston region from 2005 through 2012.

METHODS:   Using Texas’s electronic disease surveillance system, we analyzed records of confirmed and probable chickenpox cases with an event date from January 1, 2005 through December 31, 2012 and residence in one of the 16 counties within the region. Event dates were based on one of the following: onset, diagnosis, date reported to state or county, or published report date. We calculated incidence rates as the number of reported cases per 100,000 population using US Census estimates. We calculated rates by year, age group, and sex. We calculated relative change by comparing 2012 rates to a 2005–2007 baseline, and used a p-value <0.05 to determine statistical significance. We compared annual rates to those reported statewide.

RESULTS:   From 2005–2012, 8,748 cases were reported. Annual rates ranged from 6.1 in 2011 to 34.5 in 2006. Annual rates were consistently lower than those for Texas, and incidence within the region decreased by 90.1% compared to 78.8% for Texas. Overall incidence was highest among 7–9 year olds (137.5) and lowest among persons ≥15 years (3.4). Incidence decreased among all age groups <15 years, ranging from a decrease of 73.4% among children ≤3 years to 90.8% among 7–9 year olds. Incidence increased among persons ≥15 years by 73.3%, although this was primarily due to a peak in 2012 rather than a consistent pattern. Rates were similar for males (17.1) and females (17.4), yet decreased more among males. All changes were statistically significant.

CONCLUSIONS:   There has been a drastic reduction in chickenpox incidence within the Houston-Galveston region overall and relative to Texas. However, this reduction is only observed among the younger population. Furthermore, high rates of chickenpox continued to be observed prior to the introduction of a two-dose vaccine regimen, which provides support that the one-dose regimen was not as effective. Examining the correlation between vaccine adherence and chickenpox incidence within the region could help to provide further support, and to identify gaps and target interventions.