Reported varicella annual incidence remained relatively stable during 2001–2006 at an average rate of 50.0 cases/100,000 persons (range, 45.5–54.5 cases/100,000 persons). Subsequently, in 2007 the rate declined to 41.0 cases/100,000 persons. In 2008, the incidence further decreased to 24.5 cases/100,000 persons, a ~50% decline compared with 2005 (P <.001).
Incidence by age group is presented in . In 2008 compared with 2005, the varicella rate decreased markedly among children aged 1–14 years, particularly among children aged 5–9 years. The decline among children aged 1–4 years was seen among both those aged 1–3 years and those aged 4 years. Rates among infants, older adolescents, and young adults did not decrease, whereas adults aged ≥30 years had a slight but statistically significant increase in incidence.
Changes in Varicella Incidence by Demographic Group, Connecticut, 2005 and 2008
Clinical and epidemiologic characteristics of varicella are presented in . Vaccination status was the only statistically significant difference among cases in 2008 and 2005. Although the total percentage of patients having received a vaccination was similar between 2005 and 2008 (78% vs 76%, respectively), the percentage of patients who had received 2 doses of varicella vaccine was higher in 2008 than in 2005 (13% vs <1%, respectively; P < .001).
Characteristics of Varicella, Connecticut, 2005 and 2008
The number and size of school outbreaks of varicella decreased dramatically during the study period, with 42 outbreaks during the 2005–2006 school year (mean size, 14; range, 5–62) and only 2 outbreaks during the 2008–2009 school year (mean size, 5; range, 3–6).
Analysis of the proportion of potentially preventable varicella cases by age group revealed opportunities for disease prevention among all vaccine-eligible age groups: A total of 23% of children aged 1–4 years were unvaccinated; 73% of those aged 5–9 years had received only 1 dose (68%) or were unvaccinated (5%); 78% of those aged 10–14 years had received only 1 dose (72%) or were unvaccinated (6%); 53% of those aged 15–19 years had received only 1 dose (39%) or were unvaccinated (14%); and 86% of those aged ≥20 years had received only 1 dose (8%) or were unvaccinated (78%). Overall, 59% of cases were potentially preventable, and of these, 81% were among persons aged 5–14 years.