VARICELLA SUSCEPTIBILITY AMONG MEXICAN AND AMERICAN ADULTS

*M.C. Holliday (1), E. Gordon (2), C. Woernle (3), G. Higginbotham (3), R. Judy (3), S. Schmidt (4), A. Jumaan (1), J. Seward (1) (1) National Immunization Program, CDC, Atlanta, GA; (2) Brundidge, Alabama; (3) Alabama Department of Public Health, Montgomery, AL; (4) National Center for Infectious Diseases, CDC, Atlanta, GA

Background/Objectives:
Before licensure of varicella vaccine, 95% of varicella cases in the United States occurred by 15 years of age. However, in tropical countries varicella is less common in childhood resulting in higher levels of susceptibility in adulthood. We examined the serosusceptibility to varicella and predictive values of varicella history among American and Mexican adults in the context of a varicella outbreak.

Methods:
A varicella outbreak with 18 cases, resulting in severe complications, occurred among Mexican adults in Alabama from April to June 2000. We conducted a serosurvey in the residence (apartment complex) and workplace (poultry processing plant) where most of the cases occurred. All samples were tested at CDC using a VZV IgG EIA test; individuals testing IgG negative and equivocal were considered susceptible.

Results:
Apartment Complex. We obtained blood samples from 70 Mexican persons. Their median age was 25 years (range: 17 - 45) and 61 (87%) were male. Their median length of stay in the United States was 8 months (range: < 1 month-8 years). Excluding cases, 23 (33%) persons were susceptible to varicella. The positive predictive value of varicella disease history was 100%, and the negative predictive value was 48%. Of the 27 persons who reported a positive history of varicella, only 66% had the disease before age 15.
Poultry Processing Plant. Of the 461 workers tested for varicella IgG, 49 (11%) were susceptible. Varicella susceptibility was 20% among Mexican workers and 3% among American workers (Prevalence rate: 6.47, 95% CI: 2.81-14.91). Susceptibility was similar for males and females in each birth country category. The positive predictive value of varicella history was 100% for Americans and 97% for Mexicans. The negative predictive value was 8% for Americans and 27% for Mexicans.
Conclusions: Mexican adults are more susceptible than American adults to varicella. Health care providers should be aware of these differences to prevent future varicella cases and outbreaks in this population.

Corresponding Author: M.C. Holliday, M.D., Visiting Fellow, National Immunization Program, CDC, 1600 Clifton Rd., MS: E-61, Atlanta, GA 30333, USA