Varicella seroepidemiology in United States air force recruits: A retrospective cohort study comparing immunogenicity of varicella vaccination and natural infection.

BACKGROUND/OBJECTIVES: Infection with varicella zoster virus (VZV) produces lifelong immunity, but duration of post-vaccination immunity has not been established. The purpose of this study is to determine if a difference exists in the long-term seropositivity of anti-VZV antibodies in a cohort of young adults who were vaccinated against varicella as compared to a similar cohort with a history of chickenpox disease, and to determine which variables best predict waning seropositivity following varicella vaccination. METHODS: This retrospective cohort study captures immunization and serology data from approximately 10,000 recruits who entered basic military training between January 1, 2008, and December 31, 2015, and who have childhood immunization records in the Air Force Aeromedical Services Information Management System. Varicella vaccine immunogenicity was determined relative to the immunogenicity of chickenpox disease, as measured by multiplex flow immunoassay. Among vaccine recipients, waning seroimmunity was modeled and adjusted for several important covariates. RESULTS: Basic military trainees who received varicella vaccine in childhood were 24% less likely to be seropositive to VZV than trainees who were exempt from vaccine due to a history of chickenpox disease. There was no significant difference in seropositivity between male and female trainees. The odds of a vaccinated trainee being seropositive to VZV decreased by 8% with each year elapsed since vaccination. Seroprevalence declined below estimated herd immunity thresholds in vaccinated trainees born after 1994, and in the cohort as a whole for trainees born after 1995. CONCLUSION: Despite prior vaccination, seroimmunity in a large cohort of young adults unexposed to wild-type VZV failed to meet the estimated threshold for herd immunity. If vaccination in accordance with the current US VZV vaccination schedule is inadequate to maintain herd immunity, young adults not previously exposed to wild-type VZV may be at increased risk for varicella outbreaks.