We report a significant decrease in rates of varicella-related ambulatory and hospital discharges since introduction of the varicella immunization program in the United States. Childhood varicella vaccination rates increased from 12.2% in 1996 to 87.5% in 2004.13,14
Our study identified a 66% decrease in varicella-related ambulatory discharges and a 53% decrease in VRHD between the pre- and post-licensure periods. In the ambulatory setting, the decreases were most pronounced among patients <4 years of age, where the rate of varicella-related ambulatory discharges decreased by 98%. The post-licensure decrease in VRHD noted from 1995–2001 by previous authors1, 4, 7, 15–20
continued between 2001 and 2004. These decreases parallel national increases in varicella vaccination coverage.5
In addition, primary varicella-related complications occurred less commonly overall in the post-licensure period with significant age-specific decreases among patients 5–14 years of age, the group with the highest overall complication rate. These results further emphasize the benefits of the national childhood varicella vaccination program in reducing rates of primary varicella infection.
Our study is the first to examine national trends in varicella-related ambulatory discharges in insured and uninsured patients. A major goal of the varicella vaccination program in the United States was to reduce the rates of morbidity and death associated with varicella infections. While previous studies have utilized hospitalization and mortality data to assess vaccine effectiveness, most cases of varicella are managed in the ambulatory setting. Ambulatory practices may also be the sentinel setting for tracking the post-vaccine epidemiology of varicella. Following licensure of the vaccine, we found that decreases in varicella-related ambulatory discharges occurred earlier and were more dramatic than decreases in VRHD. While examining epidemiologic trends in varicella in the outpatient setting is essential to describing the impact of the vaccine, few studies have addressed the burden of varicella infection in the ambulatory setting.7, 17, 21
Ambulatory visits also contribute significantly to the economic burden of varicella infection. Zhou et al7
demonstrated that ambulatory visits comprised 52.1% of varicella-related healthcare expenditures in the pre-licensure period, and 78% in the post-licensure period. These data suggest that as VRHD declines, tracking varicella in the ambulatory setting is the most accurate method of examining vaccine cost-effectiveness and efficacy. Tracking rates of varicella in the ambulatory setting is particularly important because outbreaks of varicella continue to occur, even in highly vaccinated populations.22, 23
Breakthrough varicella in vaccinated persons is often mild, making it unlikely that affected persons will require hospitalization.23
Widespread use of the heptavalent pneumococcal conjugate vaccine was associated with elimination of racial differences in the incidence of invasive pneumococcal disease.24
In contrast, despite significant decreases in varicella-related ambulatory discharges in both whites and non-whites in the post-licensure period, racial differences in varicella rates persisted in the post-vaccine period, with rates of ambulatory discharges remaining higher for non-whites than whites. VRHD also remained higher for non-whites in the post-licensure period when varicella was listed as the primary diagnosis. Staat et al17
found that varicella-related emergency department visit rates were higher for black children than white children in the post-vaccine period. While racial disparities in vaccine coverage existed in the early post-licensure period, there have been no significant differences in national coverage rates between whites and blacks after 1998.5
Lack of access to primary care or delayed diagnosis among non-whites may also lead to these racial differences. However our data, which are based in the outpatient setting, suggest that non-white patients frequently utilize primary care for diagnoses of varicella. The persistent difference in rates of disease may be due to baseline racial differences in varicella susceptibility, which have been previously reported,25
or to differences in the threshold for seeking medical attention for this condition.
The use of national health statistics is both a strength and limitation of this study. While active surveillance may be the most precise method for tracking varicella epidemiology, it is both costly and impractical for demonstrating the true nationwide impact of the vaccination program. Use of databases such as the NAMCS, NHAMCS, and NHDS is an accurate and low-cost method of monitoring national trends in disease. The results of our study closely parallel those of the Varicella Active Surveillance Project (VASP), a varicella active surveillance project implemented in Antelope Valley, California, and Philadelphia, Pennsylvania, which describe an approximately 90% decrease in the rate of primary varicella from 1995–2005.15
Both VASP and our study identified large decreases in cases of varicella, with the sharpest decline occurring from 1998–1999 and the greatest reduction in the rate among children under 4 years of age.4
One limitation of using national health statistics is the reliability of estimates based on smaller number of discharges. As noted previously, we were unable to compare post-licensure hospitalization rates after 2001 due to the relatively small number of VRHDs among children 0–14 years of age. Similarly, although there may have been a trend toward an increase in ambulatory discharges among children 5–14 years of age after 2002, the small number of cases precludes meaningful interpretation of this data. However, this increase may represent a true increase in varicella cases in older children. This may represent normal year-to-year variation of disease, or reflect the increased rates in breakthrough disease that prompted the Advisory Committee on Immunization Practices to recommend a booster varicella vaccine at 4–6 years of age in 2006.26
Additionally, we presumed that decreases in varicella cases and varicella-related complications were a consequence of vaccination. Although our findings are likely to be causal, we cannot make such inferences about causality with administrative data. The decline in varicella could also be due to underdiagnosis by physicians because in immunized populations the rash may be modified or atypical, and thus more difficult recognize. It is also possible that patients are less likely to present for care, especially in cases where vaccination has attenuated the severity of the disease. Finally, it is possible that varicella rates incrementally increased between 2001 and 2004 as a consequence of waning immunity in vaccinated children or increased numbers of vulnerable immunocompromised patients. However, because relatively few patients in the databases had varicella in this time period, our study was not powered to detect small increases in varicella rates.