Epidemiology of Varicella Disease Among Vaccinated and Unvaccinated Individuals—Georgia, 2012-2014

BACKGROUND:  Prior to the introduction of the vaccine in 1995, varicella was a common childhood disease in the United States with an estimated 11,000 hospitalizations and 100 deaths annually. Since vaccine licensure, the incidence of varicella has decreased significantly. Studies have shown that a single dose of varicella vaccine has an effectiveness range from 44% to 100%, with substantial protection against moderate and severe disease; however, breakthrough illness has been documented in up to 56% of individuals vaccinated with at least one dose. Despite high coverage rates, most of Georgia’s varicella cases are reported in vaccinated persons. The objective of this analysis is to compare severity of disease among individuals vaccinated with 1 or 2 doses with those who had no history of varicella vaccine.

METHODS:  Passive varicella surveillance is conducted in Georgia and information concerning age, signs and symptoms, and vaccination history is entered into the State Electronic Notifiable Disease Surveillance System (SendSS). SendSS data from January 1, 2012 to November 31, 2014 was analyzed.  A varicella case was defined as probable or confirmed using the Council for State and Territorial Epidemiologists (CSTE) case definition. Similar to previous studies, severity of disease was defined by the number of skin lesions. Chi-square analysis was used to determine association and to calculate odds ratios by vaccination status using SAS 9.4. 

RESULTS:  From January 1, 2012 to November 31, 2014, 466 varicella cases were reported in Georgia to SendSS. Among all cases, 183 (39.3%) received 2 doses of varicella vaccine, 119 (25.5%) received 1 dose, and 163 (35.0%) had no history of varicella vaccination. Cases with a 2-dose history were more likely to have mild disease (<50 lesions) (OR 4.47, p <0.0001, [95% CI, 2.72-7.35]), absence of vesicular lesions (0.352, 0.0004, [0.183-0.658]), and no fever (0.193, <0.0001, [0.11-0.317]) compared to those with no history of vaccination. Cases with a 1-dose history were more likely to have mild disease (OR 4.54, <0.0001, [0.11-0.317]), absence of vesicular lesions (0.456, 0.0183, [0.22-0.93]), and no fever (0.4832, 0.0029, [0.29-0.804]) when compared to those with no history of vaccination. There was no significant difference in disease severity among persons with one dose and two doses of varicella vaccine. 

CONCLUSIONS:  Breakthrough varicella continues to occur among individuals vaccinated with one or two doses of varicella vaccine in Georgia; however, vaccinated individuals present with milder symptoms than unvaccinated individuals, which is consistent with previous studies.